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This is to certify that the

thesis entitled

THE INFLUENCE OF COLD TREATMENT, PHOTOPERIOD, AND FORCING
TEMPERATURE ON THE DORMANCY, GROWTH, AND
FLOWERING OF HOSTA

presented by

Beth Anne Fausey

has been accepted towards fulﬁllment
of the requirements for

M. S . degree in Borgignlnure

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/Major professor

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1/98 cICIHCJDmDmpGS-ptu

 

THE INFLUENCE OF COLD TREATMENT, PHOTOPERIOD, AND FORCING
TEMPERATURE ON THE DORMANCY, GROWTH, AND
FLOWERING OF HOSTA
By

Beth Anne Fausey

A THESIS
Submitted to
Michigan State University
in partial fulﬁllment of the requirements
for the degree of
MASTER OF SCIENCE
Department of Horticulture

1999

ABSTRACT
THE INFLUENCE OF COLD TREATMENT, PHOTOPERIOD, AND FORCING
TEMPERATURE ON THE DORMANCY, GROWTH, AND
FLOWERING OF HOS TA
BY

Beth Anne Fausey

Studies were conducted to determine the inﬂuence of cold treatment, photoperiod,
and forcing temperature on the dormancy, growth, and ﬂowering of Hosta. Plants
consisting of single-eye divisions were used in 1997-1998 resulting in small shoots with
low ﬂowering percentages. Larger plants used in 1998-1999 were more uniform and
yielded higher ﬂowering percentages. Each hosta clone was cooled for 0, 3, 6, 9, 12, or
15 weeks at 5 °C then forced in a 20 °C greenhouse under a short day (9-h) or long day
(NI) photoperiod. The duration of cold required to break crown dormancy varied with
genotype. Long days were required for continual vegetative growth followed by
ﬂowering, and short days induced dormancy of all clones. The response of Hosta to
photoperiod duration was further evaluated by growing plants having received 0 or 15
weeks of cold under 10, 12, 13, 14, 15, 16, 24-h photoperiods or NI lighting. Cooled
plants grown under 513 h emerged yet developed only a single ﬂush of leaves and
became dormant. Mature plants of all clones actively grew and ﬂowered under
photoperiods 214 h and NI. In a separate experiment, cold-treated plants were grown in
greenhouse sections set at 14, 17, 20, 23, 26, or 29°C with 16-h day-extension lighting.
Time to ﬂower decreased as temperature increased. Plant height, average leaf size, and
leaf color were adversely affected by high temperatures, and plant quality was greatest for

plants grown at 523°C.

ACKNOWLEDGMENTS

I would like to begin by thanking my major professor, Dr. Royal Heins, for his
encouragement and belief in my ability to succeed. Royal’s generosity, understanding,
and direction are greatly appreciated. I would also like to thank the other members of my
guidance committee: Dr. Art Cameron for his enthusiasm and insight over the past two
years, and Dr. Rick Ward for sharing his time and knowledge with me.

I am fortunate to have developed many lasting friendships here at MSU. I want to
thank Suzanne Downey and Leslie Finical for welcoming me with open arms and for
inspiring me always. I must also thank Bridget Behe, John Biernbaum, Will Carlson,
Joaquin Chong, Emily Clough, Alison F rane, Hongwen Gao, Kathy Kelley, Bin Liu,
Mary-Slade Morrison, Genhua Niu, Erik Runkle, Shiying Wang, and Cathy Whitman for
their help, advice, and moral support. The assistance of Dan Tschirhart, David Joeright,
Kevin Kern, Dave F reville, Ron Wik, and all the undergraduate students who supported
this research project is greatly appreciated.

Finally I would like to thank my family - my parents, Tom and Kathy Coon and
Chuck and Kay Fausey; and most importantly my husband Jason, a true blessing and
inspiration in my life - for their love, support, and continual encouragement. I could not

have done this without you all!

iii

TABLE OF CONTENTS

LIST OF TABLES .................................................... vii

LIST OF FIGURES .................................................... ix
SECTION I

LITERATURE REVIEW ................................................ 1

Evolution ........................................................ 2

Genome ......................................................... 3

Historic Overview. ................................................. 4

Nomenclature and Classiﬁcation. ..................................... 5

Phylogeny. ................................................. 6

Species Classiﬁcation. ........................................ 6

Reproduction ..................................................... 7

Hybridization. .............................................. 7

Self-Fertilization. ............................................ 8

Genetics .......................................................... 9

Variegation. ..................................................... 10

Plastid Inheritance .......................................... 11

Asexual Propagation. .............................................. 12

Hosta Morphology ................................................ 13

Form ..................................................... 13

Leaf Characteristics .......................................... 13

Flower Stalk ............................................... 15

Reproductive Structures ...................................... 15

Hosta in Thesis Experiments ........................................ 17

Subgenus Hosta ............................................ 17

Subgenus Bryocles .......................................... 18

Subgenus Giboshi .......................................... 18

Perennial Growth Habit ............................................ 20

The Axillary Bud ................................................. 21

Correlative Inhibition of Lateral Buds ................................. 22

Inhibition by the Apex ....................................... 22

Inhibition by the Shoot and Stern ............................... 23

Inhibition by the Stem, Leaf, and Inﬂorescence ................... 23

Inhibition Scale Leaves ...................................... 25

Environmental Effects ............................................. 26

Light Quantity ............................................. 26

Light Quality .............................................. 27

Effects of Water, Relative Humidity, and Nutrition ................ 27

Methods to Increase Division of Hosta ................................ 28

iv

Mechanical Methods ........................................ 29

Chemical Branching Agents .................................. 29
Cytokinins ................................................ 29

References ...................................................... 3 1

SECTION II

THE INFLUENCE OF COLD-TREATMENT DURATION AND PHOTOPERIOD

ON DORMANCY, GROWTH, AND FLOWERING OF HOSTA ............... 36
Introduction ..................................................... 39
Materials and Methods ............................................. 43
Results ......................................................... 45
Discussion ...................................................... 50
References ...................................................... 57

SECTION III

THE INFLUENCE OF COLD TREATMENT AND PHOTOPERIOD DURATION

ON GROWTH AND FLOWERING OF HOSTA. ........................... 71
Introduction ..................................................... 74
Materials and Methods ............................................. 77
Results ......................................................... 80
Discussion ...................................................... 87
References ...................................................... 92

SECTION IV

THE EFFECTS OF F ORCING TEMPERATURE FOLLOWING COLD

TREATMENT ON GROWTH AND FLOWERING OF HOSTA ............. 119
Introduction .................................................... 122
Materials and Methods ............................................ 123
Results and Discussion ........................................... 126
References ..................................................... 1 3 3

APPENDIX A. DEVELOPMENT OF HOS TA ‘HALCYON’ LATERAL BUDS..154

Objective ...................................................... 1 5 5
Plant Material and Culture ......................................... 155
Treatments ..................................................... 155
Data Collection and Analysis ....................................... 156
Results and Discussion ........................................... 156
References ..................................................... l 59

APPENDIX B. FALL-INDUCED DORMANCY OF HOS TA ‘ROYAL

STANDARD’. ....................................................... 160
Objective ...................................................... 161
Plant Material and Culture ......................................... 161
Treatments ..................................................... 161
Data Collection and Analysis ....................................... 162
Results and Discussion ........................................... 162
References ..................................................... 1 62

vi

LIST OF TABLES

SECTION II

Table I . Plant material characteristics for 1997 and 1998 cold-treatment duration
experiments. All plants in 1998-1999 were grown in containers and originated from 1997
experimental plant material, except for H. montana and ‘Royal Standard’ ............ 59

Table 2. Average daily temperatures and daily light integrals from date of forcing to
average date of ﬂowering for hosta clones in 1998-1999. ........................ 60

Table 3. Differences in characteristics of hosta clones between 1997-1998 and 1998-
1999. ................................................................ 61

Table 4. Differences in ﬂowering characteristics of hosta clones between 1997-1998 and
1998-1999. ........................................................... 62

Table 5. Average days to visible bud and average days from visible bud to ﬂower for
hosta clones in 1998-1999. ............................................... 63

Table 6. Effect of cold duration on subsequent plant height, inﬂorescence height, and
average leaf number per ﬂowering shoot of hosta clones in 1998-1999. Plants were
grown at 20°C for 15 weeks under a 9-h natural day plus a 4-h night interruption
photoperiod following cold treatment ........................................ 64

Table 7. Cold temperature and greenhouse forcing requirements for hosta clones in
1998-1999 . ........................................................... 56

SECTION III

Table 1. Plant material characteristics for 1997 and 1998 photoperiod experiments. All
plants in 1998-1999 were grown in containers and originated from 1997 experimental
plant material except for H. montana and ‘Royal Standard’ ....................... 94

Table 2. Average air temperatures and average daily light integrals from start of forcing
to the average date of ﬂowering for cooled and noncooled Hosta clones under various
photoperiods in 1998-1999 ................................................ 95

Table 3. Cold treatment and photoperiod effects on time to ﬂower of Hosta clones in
1998-1999 ............................................................. 96

Table 4. Average days to visible bud and from visible bud to ﬂower of Hosta clones in
1998-1999 having received zero or ﬁfteen weeks of 5°C cold followed by various

photoperiod treatments .................................................... 97

vii

Table 5. Cold treatment and photoperiod effects on vegetative and reproductive
characteristics of Hosta clones in 1998-1999 .................................. 98

SECTION IV
Table 1. Plant material characteristics for 1997 and 1998 temperature experiments. All

plants in 1998-1999 were grown in containers and originated from 1997 experimental
plant material except for H. montana and ‘Royal Standard’ ...................... 135

Table 2. Average number of days to ﬂower and average daily temperatures ﬁ'om date of
forcing to average date of ﬂowering of Hosta clones grown at various temperatures in
1998-1999 ............................................................ 136

Table 3. Effect of forcing temperature on ﬂowering percentage, plant height, and average
leaf size of single-eye divisions of Hosta clones in 1997-1998. .................. 137

Table 4. Effect of forcing temperature on ﬂowering percentage, ﬂower number, and
inﬂorescence height of Hosta clones in 1998-1999 ............................. 138

Table 5. Effect of forcing temperature on plant height, average leaf number, and average
leaf size of Hosta clones in 1998-1999 ...................................... 139

Table 6. Effect of forcing temperature for the average number of days to emergence, days
to visible bud, and days from visible bud to ﬂower of Hosta clones in 1998-1999.. . . 140

Table 7. Parameters of linear regression analysis relating forcing temperature to rate of
progress to visible bud (VB), to ﬂowering (F LW), and ﬁ'om VB to F LW for Hosta clones
in 1998-1999. The base temperature (Tb) and degree days (°days) were calculated from
the intercept and slope ................................................... 141

APPENDIX A

Table 1. Effect of benzyladenine, plant orientation, and photoperiod on development of
Hosta ‘Halcyon’ lateral buds. Plants used in this experiment were dug in mid-August,
1997, foliage removed to 10 cm above the crown, and all lateral buds _>_ 5 mm in length
removed .............................................................. 158

APPENDIX B

Table 1. Fall-induced dormancy of Hosta ‘Royal Standard’. Plants were grown outdoors
and in a 20°C greenhouse and were exposed to SD, ND, or LD photoperiods beginning
August 22, 1997 for six weeks. Foliage was removed from plants at two-week intervals
from the start of the experiment, and plants were transferred to a 20°C greenhouse with
LD lighting for six weeks. The number of shoots and buds, and the weight of the buds
were determined for each plant. .......................................... 164

viii

LIST OF FIGURES

SECTION I

Figure 1. Classiﬁcation of Hosta (Grenfell, 1996) ............................. 35
Figure 2. Benedict’s Cross (Schmid, 1991) ................................... 11
Figure 3. Flowering season of Hosta species in native sites (Fujita, 1976b) .......... 17
SECTION 11

Figure 1. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta plantaginea and 'Royal Standard’ after 0, 3, 6, 9, 12, or
15 weeks of 5°C cold under a 9-hour short day in year one (C) or a 9-hour day plus a
4-hour night interruption in year one (O)and year two ( D). Error bars are 95%
conﬁdence intervals. L=linear; Q=quadratic trends. NS, *, **, ***Nonsigniﬁcant or
signiﬁcant at P < 0.05, 0.01, or 0.001, respectively. ............................ 65

Figure 2. Percent emergence (A), percent ﬂowering (B), days to ﬂower (C), and

average leaf size (D) of Hosta ‘Lancifolia’ after 0, 3, 6, 9, 12, or 15 weeks of 5°C cold
under a 9-hour short day in year one (C) or a 9-hour day plus a 4-hour night interruption
in year one (Q) and year two ( CI). Error bars are 95% conﬁdence intervals. L=linear;
Q=quadratic trends. NS, *, **, ***Nonsigniﬁcant or signiﬁcant at P < 0.05, 0.01, or
0.001, respectively ....................................................... 66

Figure 3. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta montana and 'Undulata' after 0, 3, 6, 9, 12, or 15 weeks
of 5°C cold under a 9-hour short day in year one (C) or a 9-hour day plus a 4-hour night
interruption in year one (Q) and year two ( CI). Error bars are 95% conﬁdence intervals.
L=linear; Q=quadratic trends. NS, *, **, ***Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively. ............................................... 67

Figure 4. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta 'Golden Scepter' and 'Golden Tiara' after 0, 3, 6, 9, 12,
or 15 weeks of 5°C cold under a 9-hour short day in year one (C) or a 9-hour day plus a
4-hour night interruption in year one (Q) and year two ( CI). Error bars are 95%
conﬁdence intervals. L=linear; Q=quadratic trends. NS, *, **, ***Nonsigniﬁcant or
signiﬁcant at P < 0.05, 0.01, or 0.001, respectively. ............................ 68

Figure 5. Percent emergence (A), percent ﬂowering (B), days to ﬂower (C), and average
leaf size (D) of Hosta ‘Hyacinthina’ after 0, 3, 6, 9, 12, or 15 weeks of 5°C cold under a
9-hour short day in year one (C) or a 9-hour day plus a 4-hour night interruption in year
one (Q) and year two ( Cl). Error bars are 95% conﬁdence intervals. L=linear;
Q=quadratic trends. NS, *, **, ***Nonsigniﬁcant or signiﬁcant at P < 0.05, 0.01, or

ix

0.001, respectively. .................................................... 69.

Figure 6. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta 'Tokudama' gold and 'Tokudama' green after 0, 3, 6, 9,
12, or 15 weeks of 5°C cold under a 9-hour short day in year one (C) or a 9-hour day plus
a 4-hour night interruption in year one (Q) and year two ( D). Error bars are 95%
conﬁdence intervals. L=linear; Q=quadratic trends. NS, *, **, ***Nonsigniﬁcant or
signiﬁcant at P < 0.05, 0.01 , or 0.001 , respectively. ............................ 70

SECTION 111

Figure 1. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta plantaginea under various
photoperiods after 0 (I, I) or 15 (0, CI) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively .......................................... 99

Figure 2. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Royal Standard’ under various
photoperiods after 0 (C, I) or 15 (O, 0) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 100

Figure 3. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Lancifolia’ under various
photoperiods after 0 (I, I) or 15 (O, D) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 101

Figure 4. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Hyacinthina’ under various
photoperiods after 0 (O, I) or 15 (0, CI) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 102

Figure 5. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Golden Scepter’ under various
photoperiods after 0 (O, I) or 15 (O, [1) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 103

Figure 6. Percent emergence (A, E), plant height (B, F), average number of leaves per

shoot (C, G), and average leaf size (D, H) of Hosta montana under various photoperiods
after 0 (O, I) or 15 (O, D) weeks of 5°C cold in year one and year two. Error bars are
95% conﬁdence intervals and are offset to the right of year two data points for clarity.
L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively ............................................... 104

Figure 7. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Undulata’ under various
photoperiods after 0 (O, I) or 15 (0, CI) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 105

Figure 8. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Golden Tiara’ under various
photoperiods after 0 (O, I) or 15 (0, CI) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 106

Figure.9. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Tokudama’ gold under various
photoperiods after 0 (C, I) or 15 (0, Cl) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 107

Figure 10. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Tokudama’ green under various
photoperiods after 0 (O, I) or 15 (O, D) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 108

Figure 11. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta plantaginea under various photoperiods
after 0 (C, I) or 15 (0, CI) weeks of 5°C cold in year one and year two. Error bars are
95% conﬁdence intervals and are offset to the right of year two data points for clarity.
L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively ............................................... 109

Figure 12. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Royal Standard’ under various
photoperiods after 0 (O, I) or 15 (0, CI) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P

xi

< 0.05, 0.01, or 0.001, respectively ......................................... 110

Figure 13. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Lancifolia’ various photoperiods after 0
(C, I) or 15 (0, CI) weeks of 5°C cold in year one and year two. Error bars are 95%
conﬁdence intervals and are offset to the right of year two data points for clarity.
L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively ............................................... 111

Figure 14. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Hyacinthina’ under various photoperiods
after 0 (O, I) or 15 (O, 0) weeks of 5°C cold in year one and year two. Error bars are
95% conﬁdence intervals and are offset to the right of year two data points for clarity.
L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively ............................................... 112

Figure 15. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Golden Scepter’ under various
photoperiods after 0 (O, I) or 15 (O, D) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 113

Figure 16. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta montana under various photoperiods after 0
(C, I) or 15 (O, D) weeks of 5°C cold in year one and year two. Error bars are 95%
conﬁdence intervals and are offset to the right of year two data points for clarity.
L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively ............................................... 114

Figure 17. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Undulata’ under various photoperiods after
0 (I, I) or 15 (0, CI) weeks of 5°C cold in year one and year two. Error bars are 95%
conﬁdence intervals and are offset to the right of year two data points for clarity.
L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P < 0.05 ,
0.01, or 0.001, respectively ............................................... 115

Figure 18. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Golden Tiara’ under various photoperiods
after 0 (O, I) or 15 (0, CI) weeks of 5°C cold in year one and year two. Error bars are
95% conﬁdence intervals and are offset to the right of year two data points for clarity.
L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively ............................................... 116

Figure 19. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Tokudama’ gold under various

xii

photoperiods after 0 (O, I) or 15 (0, CI) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 117

Figure 20. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C,
G), and inﬂorescence height (D, H) of Hosta ‘Tokudama’ green under various
photoperiods after 0 (O, I) or 15 (0, Cl) weeks of 5°C cold in year one and year two.
Error bars are 95% conﬁdence intervals and are offset to the right of year two data points
for clarity. L=linear; Q=quadratic trends. NS, *, **,***Nonsigniﬁcant or signiﬁcant at P
< 0.05, 0.01, or 0.001, respectively ......................................... 118

SECTION IV

Figure 1. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta montana in 1998-1999. The parameters of linear regression lines are presented
in Table 7. The regression lines in graphs A, B, and C are the reciprocals of correlated
linear regression lines in graphs D, E, and F. Error bars represent 95% conﬁdence
intervals ............................................................. 143

Figure 2. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Tokudama’ green in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%
conﬁdence intervals ..................................................... 144

Figure 3. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Francee’ in 1998-1999. The parameters of linear regression lines are presented
in Table 7. The regression lines in graphs A, B, and C are the reciprocals of correlated
linear regression lines in graphs D, E, and F. Error bars represent 95% conﬁdence
intervals ............................................................. 145

Figure 4. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Golden Scepter’ in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%
conﬁdence intervals .................................................... 146

Figure 5. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Golden Tiara’ in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%

xiii

conﬁdence intervals .................................................... 147

Figure 6. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Hyacinthina’ in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%
conﬁdence intervals. ................................................... 148

Figure 7. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Lancifolia’ in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%
conﬁdence intervals . ................................................... 149

Figure 8. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Tokudama’ gold in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%
conﬁdence intervals. ................................................... 150

Figure 9. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Undulata’ in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%
conﬁdence intervals ..................................................... 1 51

Figure 10. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta plantaginea in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%
conﬁdence intervals ..................................................... 152

Figure 10. The effects of temperature on time and the rate of progress from forcing to
visible bud (A, D), from visible bud to anthesis (B, E), and from forcing to anthesis (C, F)
for Hosta ‘Royal Standard’ in 1998-1999. The parameters of linear regression lines are
presented in Table 7. The regression lines in graphs A, B, and C are the reciprocals of
correlated linear regression lines in graphs D, E, and F. Error bars represent 95%

conﬁdence intervals ..................................................... 1 53
APPENDIX B
Figure 1. Natural air temperatures from 08/31/97 to 09/30/97, East Lansing, MI ...... 166

xiv

SECTION I
LITERATURE REVIEW

Hosta originated in the eastern Asian countries of China, Korea, and Japan.
Collectively, 25 to 40 species evolved in these countries, with the greatest differentiation
occurring in Japan (Chung et al., 1991; Grenfell, 1996). Mountain valleys, forest
margins, grasslands, and rocky soils and slopes are home to different Hosta species
(F ujita, 1976a). The diversity of plants within Hosta makes them premier perennials for
any location in the garden or landscape. Their versatility and adaptability to different soil
moisture content allow survival under adverse conditions. Culturally, hostas thrive in
varying degrees of sun and shade, require minimal care, and endure vast climatological
differences. Combined, these characteristics have made Hosta the most popular

herbaceous perennial for decades.

Evolution

The progenitors of Hosta are thought to be lilylike ancestors from which Hosta
plantaginea, the most primitive species, evolved (Schmid, 1991). These predecessors
likely migrated from the Chinese mainland south through Korea to southern Japan and
north through southeastern Russia to northern Japan. Speciation then occurred in the
diverse climate and ecology of Japan. Chung et a1. (1991) hypothesize that, based on
morphology and plant distribution, H. venusta is a recent derivative of H. minor. Hosta
venusta may have originated from a population of H. minor that moved to Cheju Island,
Japan from southeastern Korea after the last ice age. Hostajonesii and H. tsushimensis
may have developed from elements of H. minor as well. Evidence suggests Tsushima
Island, Japan and mainland Korea were connected during the Pleistocene Age (Chung et

al., 1991). The progenitor of H. tsushimensis may have migrated to Tsushima Island

during the ice age where it adapted and differentiated into an endemic species.
Cytological studies by Kaneko (1968) suggest that H. ventricosa and H. clausa descended
from a common prototype. Morphological divergence studies by Chung et al. (1991)
indicate the Korean species, H. minor, H. clausa, H. capitata, and H. yingeri have been
isolated reproductively for long periods.

Investigations into pollen morphology show Hosta pollen has evolved into ﬁve
distinct types that aid in species delimitation (Schmid, 1991). Hosta plantaginea and H.
ventricosa can be identiﬁed by a unique pollen type not found in other hosta species.
Korean and Japanese hosta, however, have similar pollen types; thus, differentiation of

Korean and Japanese taxa by pollen type is difﬁcult.

Genome

A genome is the monoploid set of chromosomes (x) for a species that contains
one of each chromosome (Poehlman and Sleper, 1995). The haploid, or gametic,
chromosome number (n) and the basic genome, or chromosome set (x), of hostas is n = x
= 30 (Kaneko, 1968; Yasui, 1935). Five of these chromosomes are large and 25 are small
(Yasui, 1935). However, work by Yasui (1935) revealed that hosta chromosome sizes are
not absolute, and gradations do occur. The somatic or diploid number (2n) of most hostas
species is 2n = 2x = 60 (Kaneko, 1968; Yasui, 1935).

Kaneko (1968) found evidence of polyploidy in Hosta. The somatic cells of
polyploids possess multiples of the plant genome in excess of the diploid number
(Poehlman and Sleper, 1995). The haploid chromosome number in a polyploid series is a

multiple of x and increases in an arithmetic ratio (Jones and Luchsinger, 1986). Hosta

clausa, a triploid species in which 2n = 3x = 90, and H. ventricosa, a tetraploid species in
which 2n = 4x = 120, have been identiﬁed (Chung et al., 1991; Kaneko and Maekawa,
1968).

Evidence suggests that hostas are allopolyploids (Kaneko, 1968). Allopolyploid
species originate from two or more genome combinations of distinctly separate species,
whereas autopolyploids form by gene duplication of a single species (Poehlman and
Sleper, 1995). Polyploids with an uneven genome number are infertile, as is H. clausa
(Poehlman and Sleper, 1995; Yasui, 193 5). Infertile pollen grains, a degeneration of the
embryo sac, and an inability of the corolla to open contribute to sterility of H. clausa

(Yasui, 1935).

Historical Overview

Chinese and Korean plant exchanges with Japan enabled the Japanese cultivation
of many nonnative hosta species (Schmid, 1991). The Edo Period lasted from1603
tol867 and opened trade between Japan and the West, allowing European scientists,
botanists, and plant explorers to acquire plant material (Schmid, 1991). Englebert
Kaempfer was the ﬁrst Westemer to describe and draw the likeness of a hosta; his
illustrations were published in Amoenitates Exoticae in 1712 (Grenfell, 1996; Schmid,
1991). Following Kaempfer, Carl Thunberg ﬁrst assigned species names to hosta
specimens by using the Linnaeus system of binomial nomenclature and published Flora
Japonica in 1784 (Bailey, 1930).

The actual introduction of plant material to Europe from Asia did not begin until

the latter part of the eighteenth century. Seeds of the Chinese species H. plantaginea

arrived in France between 1784 and 1789 (Grenfell, 1996; Schmid, 1991). Live
specimens of another Chinese species, H. ventricosa, along with H. plantaginea entered
Europe in 1790 through the aid of Thunberg (Grenfell, 1996; Schmid, 1991). Later in
1829, Phillip von Siebold imported Japanese species to Holland (Grenfell, 1996).
According to plant listings from early nineteenth-century garden directories,
hostas entered the United States around 1839 (Schmid, 1991). By 1850 the United States
gained access to trade with Japan, and by 1861, direct shipments of hostas from the
Japanese archipelago had occurred (Schmid, 1991). Plant exchanges with Europe
enabled the selection of hostas in America to rival that of European countries by 1900.
The use of the hosta in American gardens and landscapes increased after 1930 as more
nurseries offered and specialized in hosta plant material (Schmid, 1991). During the
19605 and 19703, an extensive group of enthusiasts, hybridizers, growers, and gardeners
avidly collected plant material and began the introduction of new hosta cultivars (Schmid,
1991). The formation of the American Hosta Society in 1968 nationally promoted the
versatility and appeal of hostas. The American Hosta Society also provided an
authoritative means for proper cultivar registration with the objective of preventing name
misuse and duplication (Jones and Luchsinger, 1986). New species and cultivar
introductions continually renew interest in hostas and increase the popularity of these

versatile plants.

Nomenclature and Classiﬁcation
The classiﬁcation of plants within Hosta has undergone numerous changes since

the introduction of hostas to Europe. Botanists initially employed names that lacked

uniformity, which led to incoherent classiﬁcations and a complex synonymy. Hosta was
the ﬁrst unique genus name pr0posed by Leopold Trattinick in 1812 to honor botanist
Nicholas Thomas Host (Bailey, 1930). Five years later the name F unkia proposed by
Kurt Sprengel was embraced by several European countries as the common name for the
genus (Grenfell, 1996; Schmid, 1991). The International Botanical Congress in Vienna

eventually restored the name Hosta to the genus in 1905 (Schmid, 1991).

Phylogeny. The phylogenetic placement of Hosta has also undergone several revisions.
Originally, Hosta was placed with Hemerocallis in Liliaceae (Bailey, 1930).
Taxonomists, noting karyotypic similarities with Agave, later transferred the genus to
Agavaceae. After further review, these ﬁndings were considered inconclusive, and Hosta
was taxonomically classiﬁed in Funkiaceae (Dahlgren etal., 1985). Recently, Mathew

placed Hosta in the monotypic Hostaceae (Watson and Dallwitz, 1992).

Species Classiﬁcation. A biological species typically has been deﬁned as a
reproductively isolated natural population of plants with distinct morphological
boundaries (Jones and Luchsinger, 1986; Schmid, 1991). With Hosta, however, a proper
species concept has not been deﬁned, thus complicating the classiﬁcation process (Chung
et al., 1991; Jones, 1989; Schmid, 1991). Furthermore, existing herbarium specimens
often provide few diagnostic characters for proper species identiﬁcation (Jones, 1989).
However, Schmid’s (1991) redeﬁned species criterion recently declared many species to
be cultivars. Figure 1 presents the current classiﬁcation of Hosta species and their

present habitat.

Reproduction

Hostas, except for H. plantaginea, are receptive to pollen early in the morning
(Schmid, 1991). Because of morphological differences, the nocturnal-blooming H.
plantaginea is naturally reproductively isolated from the day-blooming Korean and
Japanese hostas. Japanese and Korean taxa readily hybridize unless geographical,
ecological, or seasonal reproductive barriers prevent pollination (Jones and Luchsinger,
1986; Schmid, 1991). However, no absolute barriers prevent the movement and

exchange of genes within Hosta species (Schmid, 1991).

Hybridization. Breeding systems are associated with levels of genetic variability in

plant groups, and several mating systems allow for gene ﬂow in Hosta. Both intraspeciﬁc
and interspeciﬁc hosta hybrids are produced through hybridization, which involves the
cross-pollination and subsequent fertilization of two taxa (Schmid, 1991). The female
parent is the pod parent and produces seed, while the male parent provides the pollen for
pollination. Generally, uncontrolled hybridization is accomplished through random insect
or wind pollination (Schmid, 1991). High levels of phenotypic variation within Hosta
result from a predominantly outcrossing breeding system, gene duplications, and high
haploid chromosome numbers (Chung et al.,1991).

Hybrids also can be produced by hand through controlled hybridization. Hand-
pollination is useful on reproductively isolated plants that do not ﬂower simultaneously
(Janick, 1986). Plants are artiﬁcially pollinated by mechanically transferring pollen from
one plant to the stamen of another. Breeders attempt to incorporate fragrance, leaf

substance, heat and sun tolerance, slug resistance, compactness, leaf and ﬂower color,

variegation, and increased vigor into existing hosta genomes through controlled
hybridization (Crockett, 1996). By manipulating the photoperiod, Hosta species and
cultivars that do not naturally ﬂower simultaneously can be induced to do so, thus making
hybridization possible. High-pressure sodium (HPS) or incandescent lighting can extend
the natural photoperiod to provide the day length requirement (214 hours) needed for
hosta ﬂower induction (Fausey, 1998; Finical et al., 1997). Night interruption (NI)
lighting (3 to 5 umol m'2 s'1 from 2200 to 0200) with incandescent bulbs in addition to a
nine-hour natural day also induces ﬂowering of hosta.

Hosta species overlap, interbreed, and hybridize in the wild to create self-
sustaining natural populations (Schmid, 1991). These populations interbreed and
commonly backcross to a parental type (Jones and Luchsinger, 1986). This process,
called introgressive hybridization, occurs as genes from one hosta species mingle with
another’s, thereby creating an intermediate type (Jones and Luchsinger, 1986; Schmid,
1991). The abundance of variable intermediate types increases the complexity and

difﬁculty of classifying true hosta species (Chung et al., 1991; Schmid, 1991).

Self-fertilization. Most hosta species are self-compatible, permitting fertilization after
self-pollination occurs (Schmid, 1991). Isolated hosta populations in the wild perpetuate
through self-pollination and fertilization aided by open insect pollination (Schmid, 1991).
Self-pollination is the process by where pollen from an anther is transferred to a stigma
within the same ﬂower or on the same plant (Poehlman and Sleper, 1995). Self-
fertilization occurs when a sperm and an egg gamete produced on the same plant unite to

form a zygote (Poehlman and Sleper, 1995). Self-fertilization of wild populations

increases their homozygous state, yields uniform individuals, and can decrease plant
vigor over time (Jones and Luchsinger, 1986; Schmid, 1991). Natural populations of the
Korean species H. capitata are predominantly self-pollinated because of the lack of
surrounding pollinators (Schmid, 1991). These populations display a lack of vigor and
adapt poorly to adverse environmental conditions.

Over an extended time, homozygosity appears to affect the distance, or spatial
separation, between the anther and stigma of most Korean and Japanese species (Chung
et al., 1991; Schmid, 1991). This separation in highly self-pollinated and self-fertilized
hostas is quite small. In some cases, pollination and fertilization occur before the ﬂower
opens. However, a pronounced distance between the stigma and the anthers may impede

self-pollination and promote outcrossing (Chung et al., 1991).

Genetics

The morphological characteristics of hostas, except for variegation, follow the
general rules of Mendelian genetics (Schmid, 1991); both parents equally contribute
chromosomal DNA to offspring (Vaughn, 1982). Recessive traits in the FI generation
can be expressed by selﬁng or backcrossing the F1 progeny to the recessive parent, thus
expanding the range of possible gene combinations in the F2 progeny (Crockett, 1996).

Vaughn (1982) found that ﬂower color, color intensity, and the size and shape of
blossoms and leaves are controlled by multiple genes. In particular, two complementary
genes control the production of anthocyanin, which determines ﬂower color, in Hosta.
The production of lavender ﬂowers requires two dominant genes, yet only one recessive

gene is required to produce white ﬂowers. Other dominant traits include fragrance and

lavender petiole color.

Variegation

The occurrence of variegated hostas in the wild is uncommon and nonperpetuating
without human intervention (Schmid, 1997). Variegation, however, is common among
cultivated hostas. Variegated hostas are often periclinal chimeras in which tissue of one
genetic type is surrounded by that of another. Variegation arises in somatic cells near the
apical dome and results from plastid or nuclear DNA mutations that prevent normal
chlorophyll synthesis (Vaughn, 1979; Schmid, 1991). These mutations are fairly unstable
and eventually will rearrange to a more stable form. The Benedict’s Cross (Figure 1)
illustrates four stable chimera] forms of hosta arising from unstable or streaky variegated
types (Schmid, 1991). Plants having unstable variegation will eventually revert to forms
having monochromatic dark leaves, monochromatic light leaves, dark-margined leaves
with a light center, or light-margined leaves with a dark center. However, unstable
variegation can be maintained by removing shoots with stable variegation, thus
preventing the more vigorous, stable shoots from dominating the plant’s grth (Nash,

1998; Schmid, 1991).

10

 

Monochromatic Dark

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stable Chimera
Dark Margin/Light Splashed/Streaked Light Margin/Dark
Center Variegation Center
Stable Chimera Unstable Chimera Stable Chimera
Monochromatic Light
Stable Chimera

 

 

 

Figure 2. Benedict’s Cross (Schmid, 1991).

Plastid Inheritance. Variegation is transferred maternally by plastids in the cytoplasm.
The transfer process of cytoplasm and chloroplasts is a source of nonnuclear inheritance
and is controlled by the female pod parent (Yasui, 1929). Plastid destruction occurs
during pollen development in young pollen grain cells but does not occur in the female
egg cell (Vaughn, 1979). Thus, the genetic composition of the plastid is transferred to
progeny via the female pod parent. .

Vaughn (1979) identiﬁed ﬁve unique classes of plastid mutants in Hosta, the
marginata, mediovariegata, aurea, ‘Snow Flurry’, and mosaic. Marginata mutants have
green leaves with white (albo) or yellow (aureo) leaf margins. When used as a pod
parent, marginata mutants’ progeny are green. Mediovariegated mutants have yellow or
white central leaf tissue, and the resulting progeny are primarily yellow or white,
depending on the original central leaf color. Seedlings of aurea (Chartreuse to yellow)

mutants are all aurea in color. Seedlings of selfed ‘Snow Flurry’ (white with green

11

ﬂecks) emerge nearly all white and develop green sectors on the leaves. Backcrossing to
‘Snow Flurry’ provides white seedlings with green ﬂecks or all-green plants. Mosaic
mutants have no discernable color pattern, and self-fertilization gives a wide segregation

of normal, variegated and mutant types.

Asexual Propagation

Today, over 2,500 named hosta cultivars exist (Chung and Kim, 1991). The term
culton, or cultivar, applies to any member of a systematic group of cultivated plants
whose origin or selection is due to the activities of mankind and is maintained for
deliberate and continuous propagation (Schmid, 1996). A cultivar is distinguished by one
or more characteristics and retains these distinguishing characteristics when reproduced
sexually or asexually (Schmid, 1996; Zonneveld, 1997). Most Hosta cultivars and
cultivated species do not come true from seed, except for the clonal seed of the apomictic
H. ventricosa (Schmid, 1991). Neither variegation nor morphological characteristics are
transferred reliably through sexual means. Therefore, hosta cultivars are propagated
asexually or vegetatively to perpetuate their existence (Schmid, 1991). The most
common forms of asexual propagation are rhizome divisions and tissue culture
propagation. Interestingly, H. clausa reproduces asexually by rhizomes in the wild to
compensate for inefﬁcient sexual reproduction caused by unstable environmental

conditions at ﬂowering and seed set (Chung et al., 1991; Yasui, 193 5).

l2

Hosta Morphology

Form. Shape, height, and diameter combine to create a plant’s overall form. Hostas
typically have a mounding habit with ﬂeshy, rhizomatous roots that form clumps
(Schmid, 1991). A hosta clump can be classiﬁed into size categories by diameter and
height. The diameter is measured from one lower leaf tip to another on the opposite side
of the clump (Schmid, 1991). Diameters can range from fewer than four inches to more
than several feet. Height also varies tremendously and can range from several inches to

several feet.

Leaf Characteristics. A Hosta species cannot be identiﬁed solely by leaf characteristics;
therefore, both vegetative and reproductive characteristics are used for proper Hosta
identiﬁcation and classiﬁcation (Schmid, 1991; Schmid, 1997). Leaf characteristics are
derived from mature hosta plantings to avoid developmental discrepancies found between
juvenile and adult plants (Henson, 1984). According to Schmid (1991), a hosta is mature
after six complete growing seasons.

Overall leaf shape is based upon the ratio of leaf blade length to width and
includes the shape of the leaf base and tip. Leaves can be straplike (12:1), lance-shaped
(6:1 to 3:1), ovate (2:1 to 3:2), heart-shaped (6:5), or round (1:1) (Schmid, 1991). Some
hostas produce several leaf ﬂushes within a year, while others such as H. ‘Tokudama’
produce only one (Schmid, 1991). Three different leaf types, vernal, juvenile, and
summer, may originate from the same growing point of hostas that produce multiple
ﬂushes of growth (Schmid, 1991). Vernal leaves emerge in the spring following

dormancy and are the ﬁrst to mature. These leaves are used to determine leaf area, color,

13

texture, or shape (Schmid, 1991). Juvenile leaves emerge in late spring and, for some
Hosta, are followed by a ﬂush of summer leaves. Both juvenile and summer leaves are
atypical of the mature leaf type (Schmid, 1991).

The positioning of veins within the leaf blade is important in the identiﬁcation
process. Hostas are monocotyledonous plants with parallel veins. Veins within the leaf
blade curve outward from the base of the leaf as the blade widens and then curve inward
as the blade narrows to the tip (Grenfell, 1996; Schmid, 1991). The number of vein pairs
ranges from two to twenty and varies for leaves of different cultivars and species, and
even for leaves on an individual plant (Grenfell, 1996; Schmid, 1991). An average
number of veins is taken from several mature leaves on a single plant to determine the
most accurate number of vein pairs present.

Hostas are identiﬁed according to primary and secondary leaf color. The primary
leaf color covers at least 60% of the total leaf area, while the secondary color covers 40%
or less and includes descriptions of marginal or streaky variegation (Schmid, 1991).
Nonvariegated leaves do not have secondary coloration. The blue color of some hostas
results from the bloom, a waxy, chalklike substance on the leaf epidermis (Schmid,
1991)

Leaf colors can be unstable and may change as a plant ages. Viridescence is a
leaf’ 5 color change from white or yellow to green (Schmid, 1991). Heat-affected
Viridescence primarily occurs when daytime temperatures exceed 95°F (Pollack, 1997).
Lutescence means green or Chartreuse leaves change to yellow or whitish yellow, and
albescence occurs when yellow or green leaves become white (Schmid, 1991). Blue

leaves turn green when the leaf epidermal wax is lost through rain or when an increase in

14

the day and night temperature differential occurs (Hensen, 1984; Schmid, 1991).

Leaf surface describes texture and can be smooth and ﬂat or rugose (Schmid,
1991). Rugose leaves have an uneven texture expressed through dimpling, puckering,
pleating, and crinkling of the leaf surface and also may show cupping as the leaf edges
turn upward (Schmid, 1991). Other variations in surface texture include wavy

undulations, contortions, or piecrust and furrowed margins (Schmid, 1991).

Flower Stalk. The ﬂower stalk, also called the scape, is another important means for
species identiﬁcation. Botanically, the ﬂower stalk is a stem and not a scape because of
the presence of modiﬁed leaves known as bracts (Pollack, 1997). The stalk of some
species is horizontal, therefore, the stern length rather than height is measured (Schmid,
1991). The degree of stem foliation varies with species and cultivars. Some bracts are
inconspicuous and tightly wrap about the stem, while others take on the appearance of
true leaves. A hosta ﬂower bud forms in a leaf axil where the leaf bract connects to the
stem (Pollack, 1997). A fertile bract subtends a ﬂower bud, whereas a sterile bract does

not (Pollack, 1997).

Reproductive Structures. Typically, the reproductive features of a plant, not the
vegetative features, are essential for proper identiﬁcation and classiﬁcation. The
reproductive structures of the ﬂower, the fruit, and the seed provide the basis for much of
the classiﬁcation of plants, especially that of Hosta (Jones and Luchsinger, 1986; Schmid,
1997). Hosta ﬂowers are aggregated in an inﬂorescence called a raceme (Jones and

Luchsinger, 1986; Watson and Dallwitz, 1992). A raceme is an arrangement of ﬂowers

15

composed on a single main axis with pedicles, or short stalks, attached to each ﬂower
(Jones and Luchsinger, 1986; Watson and Dallwitz, 1992). The hosta inﬂorescence is
indeterminate, meaning the ﬂowering sequence begins at the base of the raceme and
proceeds upward (Jones and Luchsinger, 1986).

Most ﬂowers are funnel-shaped, bell-shaped, or possess a spiderlike form. For
some Hosta such as H. clausa, the collective calyx and corolla, also known as the
perianth, remain closed (Schmid, 1991; Yasui, 1935). Flower color ranges from the pure
white of H. plantaginea to lavender and deep purple. These colors intensify in cooler
climates and fade under warm conditions (Grenfell, 1996). The color of the anther of
unopened ﬂowers before pollen shed is yellow or purple for true species and bicolored for
hybrids (Grenfell, 1996; Henson, 1984).

Hosta ﬂowering occurs from early summer to late fall in North America and is
species and cultivar dependent. Hostas can be grouped into general ﬂowering categories:
early season (before June 1), midseason (June 1 to July 15), mid- to late season (July 15
to September 1), and late season (after September 1) (Grenfell, 1996). Figure 3 illustrates
the diverse ﬂowering times of some Japanese Hosta in their native sites (Fujita, 1976b).

The hosta fruit contains many mature ovules, and seeds are borne in dehiscent,
nonﬂeshy capsules (Schmid, 1991; Watson and Dallwitz, 1992). Hosta seeds are ovate

and black when fertile, white when sterile (Schmid, 1991; Zonneveld, 1998).

16

 

June July August September October

 

 

H. sieboldiana
H. kiyosumiensis

 

 

H. kikutii
H. pycnophylla

 

 

H. hypole uca

 

H. longipes

 

H. albomarginata

 

H. longissima

 

H. alismifolia
H. pulchella

 

 

H. tardiva
H. tsushimensis
H. tibai

 

 

H. shikokiana

 

 

H. capitata

 

Figure 3. Flowering season of Hosta species in native sites (F ujita, 1976b).

Hostas in Thesis Experiments

In order to determine the effect of photoperiod, vemalization, and temperature on
the development and ﬂowering of Hosta, species and cultivars were selected to reﬂect the
geographical distribution of their native habitat while acknowledging those plants
important to the horticultural industry. All three subgenera, Hosta, Bryocles, and
Giboshi, are represented. (A subgenus includes plant species that originated in the same

geographical region [Jones and Luchsinger, 1986; Schmid, 1991]).

Subgenus Hosta. The ﬁrst subgenus represented is that of Hosta, which solely includes
H. plantaginea (Grenfell, 1996; Schmid, 1991). This hosta originated in southeastern

China and has the most southerly native habit of any species (Schmid, 1991). Hosta

17

 

plantaginea was the ﬁrst hosta to reach Europe, the ﬁrst to receive taxonomic
classiﬁcation, and is a typic species--a specimen for which the genus name is permanently
associated (Jones and Luchsinger, 1986; Schmid, 1991). It is referred to as the August
lily and the plantain lily. Interestingly, it is prized as the only nocturnal ﬂowering hosta
with heavily fragrant, pure white blooms that produce abundant seed during warm, long

summers (Grenfell, 1996).

Subgenus Bryocles. The subgenus Bryocles is represented by H. ‘Golden Tiara’ and H.
‘Golden Scepter’ (Schmid, 1991). Hosta ‘Golden Tiara’ is a member of the Tiara group,
which includes hybrids of H. nakaiana made by Robert Savory (Grenfell, 1996). Hosta
nakaiana, the “ornamental hairpin hosta,” evolved on the mainland of Korea. It is most
closely related to and almost indistinguishable from another Korean hosta, H. capitata
(Schmid, 1991). Hosta ‘Golden Scepter’ is a 1983 Chartreuse sport of H. ‘Golden Tiara’,

also made by Savory (Grenfell, 1996).

Subgenus Giboshi. The third subgenus, Giboshi, encompasses plant material from which
many of today’s most important hosta cultivars are derived (Schmid, 1991). ‘Giboshi’,
the Japanese equivalent of the word ‘hosta’, also is subdivided into three groups (Group
I, Group II, and Group 111), each with section divisions (Schmid, 1991). (A section
division assembles closely related species.)

Hostas in Group I are closely related to those in the Section Helipteroides

originating in central to northern Japan (Grenfell, 1996). Members of this section include

18

H. montana, H. sieboldiana, H. ‘F ortunei’ and H. ‘Tokudama’ (Schmid, 1991). Hosta
montana and H. sieboldiana are true species. H. ‘Fortunei’ and. H. ‘Tokudama’ have
been reduced from species classiﬁcation to cultivar status (Schmid, 1991). Hosta
montana, meaning “large-leaved hosta,” is native to woodlands and forest margins and
exhibits variable leaf form, variegation, and ﬂower morphology (Grenfell, 1996; Schmid,
1997). Many plants offered as true H. montana are thought to be hybrids (Schmid, 1991).

Hosta ‘Halcyon’, a classic blue—grey hybrid, resulted from a cross made by Eric
Smith and involved a late-ﬂowering H. ‘Elegans’ and an early-ﬂowering H. ‘Tardiﬂora’
(Grenfell, 1996). The ﬁrst leaves to emerge from young clumps are characteristically
lance-shaped and soon are followed by oval and eventually heart-shaped leaves as the
clump matures (Grenfell, 1996).

Hosta ‘F ortunei’ encompasses a large group of sports and hybrids formerly
cultivated in Europe (Schmid, 1991). Hosta ‘Fortunei Hyancinthina’ developed in von
Siebold’s garden and is considered to be a clone originally propagated in Holland
(Schmid, 1991). Hyacinthinus, meaning violet, describes both the color of the leaves in
early spring and the ﬂower in summer (Schmid, 1991). Plant material is generally
uniform and may produce fertile seed, though most seed is sterile (Grenfell, 1996). A
white line traces the leaf edge of this cultivar, and plants are prone to bud mutations that
produce variegated forms (Schmid, 1991).

The last hosta in the Giboshi subgenus is H. ‘Tokudama’. The name means “well-
rounded hosta” and describes the typical rounded leaf of the plant (Schmid, 1991 ). Hosta
‘Tokudama’ arrived in Europe around 1860 from plants Fortune received from von

Siebold in Japan (Schmid, 1991). Tokudamas have been hybridized extensively in

19

cultivation, and most are considered similar clones with minor macromorphological
differences (Schmid, 1991). They grow extremely slowly and are prone to variegation
mutations.

Group III of the subgenus Giboshi includes two important hostas: H. ‘Undulata’,
belonging to the Section Nipponosta A, and H. ‘Lancifolia’, belonging to the Section
Tardanthae (Grenfell, 1996; Schmid, 1991). Hosta ‘Lancifolia’, meaning “little hosta,”
was reportedly the ﬁrst hosta introduced into the United States (Schmid, 1991). Live
plants ﬁrst were sent to Holland by von Siebold in 1829 (Schmid, 1991). This ancient
hybrid is sterile (Schmid, 1991). Live plants of H. ‘Undulata,’ meaning “striped hosta,”
also were imported by von Siebold around 1829 (Schmid, 1991). Clones of this pod-
sterile hybrid of cultivated origin have unstable variegation, leaf forms, and ﬂower
scapes, especially on recently disturbed clumps (Schmid, 1991). The ratio of green to
white tissue of H. ‘Undulata’ is 1:4. Hosta ‘Undulata’ eventually reverts to the all-green
form, H. ‘Undulata Erromena’, after passing through the transitional H. ‘Undulata

Univittatal’ (Vaughn, 1979).

Perennial Growth Habit

Hostas are perennial herbs with ﬂeshy, rhizomatous root systems. The rhizome is
a thickened underground stem composed of nodes and intemodes from which root and
shoot buds arise (Schmid, 1991). The rhizome functions as a perennating organ and
enables a plant to survive unfavorable environmental conditions such as extreme
temperature or severe water stress (Schmid, 1991). Hostas exhibit three rhizomatous

growth types: stoloniferous and spreading, horizontal and tuberlike, or nearly vertical

20

(Schmid, 1991). The transitional area from the stem to the root system, including the
rhizome, is called the crown. A division or section of the crown includes a ﬂeshy bud
plus the associated roots from which a new plant may arise. Hosta shoots emerge from

buds that were formed in the leaf axils of the crown the previous growth season.

The Axillary Bud

The latent axillary bud of a hosta is an embryonic shoot consisting of an apical
meristem, nodes, intemodes, and leaf primordia enclosed within bud scales. Bud scales
prevent desiccation, provide insulation, and restrict oxygen movement into the bud
(Raven etal., 1992). Buds remain vegetative and slowly form leaf primordia while in this
quiescent state.

Environmentally, the survival of perennial plants with rhizomatous growth is
controlled by their ability to produce lateral shoots from existing axillary buds called
reserve meristems (Stafstrom, 1995). Reserve meristems partially develop into shoots
after a period of climate-induced dormancy. These meristems supplement existing shoots
within a growing season by replacing shoots lost to herbivory, disease, or damage
(Stafstrom, 1995).

Many buds, however, remain latent and are inhibited from emerging while the
plant actively grows. This within-season dormancy is known as correlative inhibition
(Stafstrom, 1995). By limiting shoot production through the inhibition of lateral buds,
adequate food reserves stored in the rhizome allow future regenerative growth (McIntyre,
1990)

When plant foliage is destroyed, the resulting loss of leaf area may affect

21

carbohydrate reserves. Carbohydrate levels may decline, depending upon the time
defoliation occurs within the growth season (Lubbers and Lechowicz, 1989). Plants can
increase their photosynthetic rate to compensate for a loss in carbohydrate reserves.
Additional compensatory growth by the parent shoot or previously latent buds may ensue;
however, increases in leaf production in response to defoliation are at the expense of
nutrients and carbohydrates stored in the rhizome (Archer and Tieszen, 1983).
Consecutive defoliations of the graminoid Eriophorum vaginatum L. depleted the storage

structure’s reserves, resulting in a general decline in plant growth (Archer and Tieszen,

1983).

Correlative Inhibition of Lateral Buds

What factors promote or delay the outgrowth of hosta lateral buds? Shoot
emergence from axillary buds is a correlative event and in Hosta is inhibited strongly by
leaves and the ﬂower stalk. Correlative inhibition involves the control of one plant part
by another and is inﬂuenced directly or indirectly by leaves, shoots, inﬂorescences, or

apices (Rubinstein and Nagao, 1976).

Inhibition by the Apex. The growing apex and apical portions of the shoot are partly
responsible for inhibition of the outgrowth of axillary buds (Cline, 1991). This
phenomenon is referred to as apical dominance, which controls a plant’s growth and
form; however, the degree of control varies among plant species.

The direct auxin theory of apical dominance contends auxin, as indole acetic acid

(1AA), migrates from the apex down the stem and into the lateral buds, where it inhibits

22

growth (Cline, 1994). Excision of the growing point removes the source of auxin and
growth of lateral buds ensues. Auxin’s direct role in inhibition, however, remains

unknown (Cline, 1991).

Inhibition by the Shoot and Stem. Smith and Rogan (1980) showed main shoot growth
of quackgrass and that of axillary shoots, or tillers, are reciprocally suppressed. Axillary
shoots impose a stress that inhibits main shoot growth and promotes tiller growth. In
contrast, the removal of lateral buds or axillary shoots, also known as detillering, results
in an increase in stem height, leaf number, and overall size of the main shoot.

A study by Clifford (1977) suggests that tiller buds of ryegrass, Lolium
multiﬂorum Lam. cv. Westerwoldicum, are suppressed by auxin levels from adjacent
intemodes. A close source-sink relationship was identiﬁed between a leaf, the elongating

intemode below the leaf, and the tiller bud located in the leaf axil.

Inhibition by the Stem, Leaf, and Inﬂorescence. Immature, cotyledonary, and mature
leaves inhibit lateral bud growth in dicotyledonous species during a plant’s vegetative
phase, while the inﬂorescence inhibits bud outgrowth during a plant’s reproductive phase
(Laidlaw and Berrie, 1974; Weiss and Shillo, 1988). Little research evaluating
correlative inhibition by the leaves or inﬂorescence has been conducted with monocot
species. It is suspected, however, that the mechanisms are similar (Smith and Rogan,
1980). For example, apical dominance is maintained in L. multiﬂorum by the apex and
expanding leaves when the plant is vegetative (Laidlaw and Berrie, 1974). Upon

ﬂowering, apical dominance is released by excision of the young inﬂorescence. I

23

observed this phenomenon for Hosta as well (Fausey, 1998).

Weiss and Shillo (1988) evaluated the inﬂuence of the apex and young leaves on
the inhibition of Euphorbia pulcherrima Willd. ‘Brilliant Diamond’ axillary buds. In this
study, the apical bud or the immature, expanding leaves were removed (decapitation or
defoliation, respectively) from a plant over a six-week period. Rapid lateral bud break
occurred three to four days after defoliation. However, lateral bud break of decapitated
plants was not visible until ﬁfteen days after apical bud removal. The rate of shoot
elongation also increased for defoliated plants compared with decapitated plants.

Weiss and Shillo (1988) also found that apical dominance in poinsettia is
weakened upon transition to the ﬂoral phase. The three uppermost buds are released from
inhibition, initiate ﬂoral buds, and develop into the inﬂorescence, while lower buds
subtending the inﬂorescence remain inhibited. Removal of the poinsettia bract, cyathia,
or both was compared to assess the source of axillary bud inhibition. The rate of lateral
bud break depended on the organ removed. Rapid bud break followed by shoot
elongation resulted with bract plus cyathia removal. Bract removal alone resulted in a
similar yet slower response. Cyathia removal, however, resulted in fewer bud breaks that
did not elongate. The authors concluded bracts and cyathia are the primary and secondary
inhibitors of axillary bud outgrowth in poinsettia, respectively. Hosokawa et a1. (1990)
examined the inhibitory effects of upper shoot tissues, the apex, expanding leaves, and
the stem segment of Ipomoea nil L. on lateral bud outgrowth. The apical region inhibited
bud growth more than the basal portion, while the stem segment had as strong an
inﬂuence on apical dominance as the upper leaves.

A synergistic effect on lateral bud growth also was identiﬁed between apex

24

removal and defoliation of 1. ml Glosokawa et al., 1990). The apical stem segments of all
plants were removed. Subsequent defoliation resulted in a six- to seven-fold increase in
lateral bud outgrowth after seven days compared with that of nondefoliated plants. The
presence of leaves fewer than 5 cm in length inhibited lateral bud outgrowth, with smaller
leaves having a greater control over growth. Results suggested the ability for small leaves
to expand may be associated with their ability to inhibit bud growth.

Similarly, McIntyre and Hsiao (1990) showed fully expanded leaves inhibited
growth of axillary buds located on the root and the shoot of common milkweed, Asclepias
syriaca L. Stem decapitation and defoliation synergistically increased bud length
compared with either used alone. The total bud lengths for decapitated, defoliated, and
decapitated plus defoliated plants were 22.1 mm, 48.2 mm, and 154 mm, respectively.

Theron et a1. (1987) identiﬁed two primary sources of bud inhibition in Malus
domestica Borkh ‘Granny Smith’ trees. They reported buds were inhibited by either a
decrease in the age of a subtending leaf or an increase in bud age. In contrast, axillary
buds of Rosa are correlatively inhibited by mature leaves and stem tissue located above
the bud (Zieslin and Halevy, 1976). Zieslin and Halevy (1976) pruned seven-node Rosa
hybrida ‘Baccara’ branches to the fourth node. Buds located at the fourth node sprouted
following pruning and removal of their subtending leaf. In contrast, stem segments with

attached leaves above the fourth node did not sprout.

Inhibition by Scale Leaves. Quackgrass bud-scale leaves suppress bud development by
producing an inhibitory substance (Robertson et al., 1989). Robertson et a1. (1989)

reported bud suppression limits the competition for nutrients between the bud and the

25

apex. Denudation of rhizome buds, however, removed the inhibitory growth factor and

temporarily allowed bud growth.

I Environmental Effects

Lateral bud development ultimately depends upon the environmental conditions
imposed upon a plant. In Hosta, heavy fertilization, increased light levels, and copious
amounts of water promote new ﬂushes of growth from reserve meristems (Pollack, 1998).
The degree of lateral bud outgrowth is inﬂuenced by interactions involving light quality

and quantity, moisture availability, and nutritional levels.

Light Quantity. Light duration affects many aspects of plant development, such as
ﬂower initiation and dormancy. Plants are classiﬁed as long-day, short-day, or day—
neutral according to their light and dark requirements within a 24-hour period. Short-day
plants require longer dark periods, while long-day plants require shorter ones to achieve a
particular response. Day—neutral plants are not affected by day length.

Climate-induced dormancy of many woody species and herbaceous perennials is
promoted by short photoperiods (Stafstrom, 1995). Exposure to short photoperiods alters
the developmental pathway of shoot meristems, resulting in a shift from production of
vegetative leaves to bud scales (Villiers, 1975). Therefore, many herbaceous perennials
require long day lengths to promote vegetative growth and induce ﬂowering. In Hosta,
ﬂower induction occurs under photoperiods longer than or equal to fourteen hours, and
vegetative leaf production terminates as the apex differentiates into a ﬂower. Once

ﬂowering ensues, surrounding buds at the base of the ﬂower stalk or on the crown are

26

released from apical control and are capable of growing under favorable conditions

(Pollack, 1998).

Light Quality. Plants respond not only to light duration, but also to its spectrum (Thomas
and Vince-Prue, 1997). Plant responses to the far-red to red (FR/R) ratio affect apical
dominance with red light (R) from 600 to 700 nm, weakening dominance, and far-red
light (FR) from 700 to 770 nm, strengthening dominance (Cline, 1991). Thus, a high
FR/R ratio reduces lateral branching and promotes stem elongation (Smith, 1994).
Kasperbauer and Karlen (1986) investigated the effect of the F R/R ratio on the
tillering of wheat, T riticum aestivum L. cv. Coker. F ield-grown wheat at high densities
averaged 2.9 tillers compared to that of widely spaced plants, which averaged 14.
Kasperbauer and Karlen (1986) hypothesized that the reduction in tillering of closely
spaced plants resulted from a higher percentage of FR light within the plant canopy.
Further studies examined the effects of FR/R on tillering, leaf length, and the root-to-
shoot biomass of wheat (Kasperbauer and Karlen, 1986). Exposure to FR light reduced
the number of tillers per plant and increased the leaf length and the root-to-shoot biomass

of wheat compared with the R light treatment.

Effects of Water, Relative Humidity, and Nutrition. Hosta cultivars commonly have
several ﬂushes of leaf growth or division increases within a single growing season.
Midseason growth ﬂushes in plants can be attributed to water regulation and starch
breakdown by the roots (Stafstrom, 1995). Water potential also may affect lateral bud

outgrowth since an increase in water supply often coincides with bud extension growth.

27

McIntyre (1990) hypothesizes the reduction in bud inhibition at high relative humidity
results from a reduction in transpiration, which increases the water potential of the parent
shoot. This theory is supported by the fact that lateral bud inhibition was prevented in
quackgrass rhizomes grown under 100% relative humidity (McIntyre, 1990). However,
reducing relative humidity to 98% resulted in the inhibition of the ﬁrst ﬁve buds on the
rhizome. Inhibition was completely eliminated when water was supplied freely to the
rhizome.

Another theory postulates that correlative inhibition is based on an internal
competition for nutrients between plant organs (Cline, 1991). The growing apex
commands dominance by acting as a sink for nutrients that are diverted from other plant
organs. After apex decapitation, lateral buds become new sites for nutrient accumulation.
The theory holds the primary requirement for bud outgrowth is nutrient availability in the
vicinity of the bud.

The most critical inorganic nutrient contributing to the growth and morphogenesis
of axillary buds is nitrogen. Exposure to high nitrogen concentrations oﬁen promotes
outgrowth of axillary buds in many species (McIntyre, 1990). McIntyre and Hsiao (1990)
reported the inhibition of common milkweed axillary buds was released by increasing the

nitrogen supply from 21 to 210 mg'L".

Methods to Increase Division of Hosta
Principles of correlative inhibition and apical dominance can be applied to
existing production methods within the horticultural industry. Both chemical and

mechanical methods to increase the number of plant divisions can be implemented by

28

growers and plant producers.

Mechanical Methods. A current method to increase shoot growth of Hosta is to remove
the foliage and ﬂower stalk when the ﬂower stalk emerges (Schmid, 1991). This
technique promotes lateral bud break and allows additional shoot growth. Alternatively,
hosta leaves can be mowed 1/4 to 1/2 inch above the ground to elicit lateral shoot growth
from latent buds (Grenfell, 1996). This bulking technique increases the number of shoots
per plant and can be performed two times during a single growing season (Grenfell,
1996). Unlike mowing, the Ross method does not inhibit plant growth by removing
actively photosynthesizing leaves. Instead, incisions in the rhizome induce division
formation (Schmid, 1991). This technique is performed by inserting a sharp knife into
the crown of a hosta plant and is favored by hosta gardeners because of its simplicity and

effectiveness (Grenfell, 1996).

Chemical Branching Agents. Many chemical branching agents successfully promote
lateral branching. Maleic hydrozide, ﬂuorenols, fatty acid esters, and ethephon release
axillary buds from inhibition by inhibiting the terminal bud (Cline, 1991). I observed the
ethylene-releasing substance Florel effectively promoted lateral bud break of Hosta

(Fausey, 1998).

Cytokinins. Cytokinin and cytokinin /gibberellin combinations can promote lateral
branching of Hosta successfully. Cytokinins are synthesized primarily in root tips and

are involved in cell division, promotion of shoot formation, delay of leaf senescence, and

29

release of apical dominance (Cline, 1991; Letham, 1994). Cytokinins interact with auxin
to promote lateral bud growth when apical dominance is lessened or broken and auxin
levels in the bud decline (Cline, 1994). Exogenous applications of cytokinin can induce
endogenous cytokinin synthesis (Letham, 1994). King and van Staden (1988) suggest
lateral bud response to exogenous cytokinin depends upon the capacity of a bud to use
cytokinin.

Keever (1994) applied different rates of benzyladenine (BA) to H. sieboldiana to
promote growth from axillary and rhizomic buds. No offsets formed on untreated plants;
however, offset production increased with increasing rates of BA and was similar
between foliar and drench applications. Additional work determined the optimum drench
and foliar spray rate for plant growth was 40 mg BA/pot and 3000 ppm, respectively
(Keever, 1994).

Garner et a1. (1997) further examined the effect of foliar BA applications on ten
Hosta cultivars and found that the cultivars responded differently in their ability to form
offsets. Control plants of several cultivars readily formed offshoots without a BA
application, yet others relied heavily on exogenous BA.

Garner et a1. (1998) also evaluated the effects of multiple BA applications and
repeated removal of offsets on single-eye divisions of H. ‘Francee’ and H. ‘Frances
Williams’ at thirty-day intervals. The authors found that offset yields increased linearly
with subsequent BA applications and repeated applications were necessary to achieve a

continual increase in offset number.

30

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Villiers, T. A. 1975. Dorrnancy and the survival of plants. Butler and Tanner, London.

Watson, L., and M. J. Dallwitz. 1992. ‘The Families of Flowering Plants: Descriptions,
Illustrations, Identiﬁcation, and Information Retrieval.’ Version: 16 March 1997.

Weiss, D., and R. Shillo. 1988. Axillary bud inhibition induced by young leaves or bract
in Euphorbia pulcherrima Willd. Annals of Botany 62:43 5-440.

Yasui, K. 1929. Studies on the maternal inheritance of plastid characters in Hosta
japonica Ashers. et Graeben. f. albomarginata Mak. and its derivitives. Cytologia
11192-215.
Yasui, K. 1935. Cytological studies in diploid and triploid Hosta. Cytologia 62484-490.
Zieslin, N., and A. H. Halevy. 1976. Components of axillary bud inhibition in rose
plants: I. The effect of different plant parts (correlative inhibition). Bot. Gaz.
137(4):291-296.

Zonneveld, B. J. M. 1997. Cultivar and cultivar-group deﬁnitions applied to Hosta. The
Hosta Journal 28 (1):37-41.

Zonneveld, B. J. M. 1998. Timetable in the hosta garden. The Hosta Journal 29(1):90.

34

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35

SECTION II
THE INFLUENCE OF COLD-TREATMENT DURATION AND PHOTOPERIOD

ON DORMANCY, GROWTH, AND FLOWERING OF HOS TA

36

The influence of cold-treatment duration and photoperiod on dormancy, growth,

and ﬂowering of Hosta.

Beth A. Fausey‘, Royal D. Heins’, and Arthur C. Cameron2

Department of Horticulture, Michigan State University, East Lansing, MI 48824-1325

Received for pulication . Accepted for publication . We gratefully
acknowledge the ﬁnancial support of the Agricultural Experiment Station of Michigan
State University and funding by hosta producers and greenhouse growers supportive of
Michigan State University ﬂoricultural research.

' Graduate Student

2 Professor

Production and Culture

37

The inﬂuence of cold-treatrnent duration and photoperiod on dormancy, growth, and

ﬂowering of Hosta.

Additional index words. Maturity, long-day plant, plantain lily, herbaceous perennial.

Abstract. Many dormant plants require a cold treatment to break vegetative dormancy
and continue growth. Plants may also require cold for ﬂower induction or to improve
plant vigor and ﬂowering characteristics. The cold requirement to break dormancy of H.
montana, H. plantaginea, H. ’Golden Scepter’, H. ‘Golden Tiara’, H. ‘Hyacinthina’, H.
‘Lancifolia’, H. ‘Royal Standard’, H. ‘Tokudama’ gold, H. ‘Tokudama’ green, and H.
‘Undulata’ clones was determined by exposing plants to 5°C cold for 0, 3, 6, 9, 12, or 15
weeks. Following cold treatment, plants were grown at 20°C under a 9-h photoperiod
(short days) or with a 4-h night interruption from 2200 to 0200 (long days) in 1997-1998
and only under long days in 1998-1999. Plants consisting of single-eye divisions were
used in 1997-1998 resulting in small shoots with low ﬂowering percentages. Larger
plants used in 1998-1999 were more uniform with higher ﬂowering percentages. Hosta
clones required 0, 3, or 6 weeks of cold for 100% emergence of all plants in both years.
Noncooled and cooled plants grown under short days emerged and went dormant
irrespective of cold-treatment duration. Cold was not required for ﬂowering of hosta
clones, and noncooled and cooled plants grown under long days actively grew and

ﬂowered.

38

INTRODUCTION

Herbaceous and woody perennial plants persist from season to season by
experiencing a resting period known as dormancy during the winter. Perennials enter a
dormant state when physiological or environmental factors temporarily suspend visible
growth of plant structures containing meristems (Lang et al., 1987). Three speciﬁc
dormancy types exist (Lang et al., 1987). Ecodormancy is regulated by unsuitable
environmental factors that directly prevent growth. Paradormancy occurs when physical
or biochemical factors are produced in the plant but are external to the dormant tissue.
Endodormancy, often referred to as rest, results from physiological factors produced
inside the dormant structure.

Dormancy is often accompanied by the formation of specialized structures in
woody and herbaceous plants. Many herbaceous perennials form underground storage
tissues that enable survival following exposure to extreme environmental conditions such
as cold, heat, or drought when in a dormant state. The resting structure of hosta is called
the crown and enables the plant to survive to USDA hardiness zone 3 where winter air
temperatures may reach - 30 to -40°F ( -34 to -40°C). The crown also enables hosta to
survive summer temperatures above 95°F by entering a state of heat dormancy (Solberg,
1997)

Plants synchronize and optimize their growth and development according to
environmental signals. The onset of dormancy and the formation of storage organs are
inductive processes triggered primarily by seasonal ﬂuctuations in daylength (Jones,
1992). The critical daylength necessary to induce these processes can vary between

broadly distributed species and ecotypes of species as daylength varies considerably with

39

latitude. A plant’s local environment is impacted by temperature and water availability
which also play a role in initiating these processes.

Continuous exposure to inductive photoperiods are required by many plants to
enter a state of true dormancy (Thomas and Vince-Prue, 1997). Many plant species enter
a transitional state when initially exposed to a short series of inductive photoperiods.
This transitional state can be reversed by noninductive photoperiods until continued
exposure to inductive photoperiods renders the plant fully dormant. Long days may
prevent or delay dormancy whereas short days hasten dormancy of alpine and woody
temperate plants. In contrast, long photoperiods interact with high temperatures to induce
summer dormancy of many herbaceous perennials including Anemone coronaria, a
geophyte native to hot, dry Mediterranean regions (Ben-Hod et al., 1988). In some cases
when plants prematurely enter dormancy before an inductive photoperiod is perceived
endogenous factors override the need for an appropriate daylength (Thomas and Vince-
Prue, 1997).

Periods of low temperatures may be required by some plants to continue growth
and development. The therrnoninductive temperatures required for ﬂowering are often
species and cultivar-speciﬁc. These temperatures range from below freezing to 16°C
with an optimum range of 1 to 7°C for most cold-requiring plants (Lang, 1965; Roberts
and Summerﬁeld, 1987).

Exposure to cold temperatures affects plant development in several ways. Many
dormant woody and herbaceous perennials require sufﬁcient periods of low temperatures
to break dormancy of vegetative or reproductive buds to resume growth or ﬂowering.

When dormancy is fully broken, the resumption of growth is usually independent of

40

photoperiod (Thomas and Vince-Prue, 1997). For example, six weeks of cold were
required to break crown dormancy of Platycodon grandiflorus ‘Mariesii’, and 12 weeks
of cold were required to break rhizome dormancy and achieve complete emergence of
Lysimachia clethroides under 8, 12, and 16 h photoperiods (Iversen and Weiler, 1994).
Cold exposure also increased the rate of emergence of both species. Although cold was
not required for long day ﬂower induction of Lysimachia clethroides under 12 and 16 h
photoperiods, six weeks of cold were required for ﬂowering of Platycodon under all
photoperiods.

Cold temperatures may also be required by plants for vemalization, the induction
of ﬂowering by low temperatures. The requirement for vemalization is commonly found
in long-day plants (LDP) and is followed by a requirement for long-day photoperiods
(N app-Zinn, 1984). For plants with an obligate vemalization requirement, ﬂower initials
form only after vemalization is complete and differentiate into ﬂoral organs when the
plant is exposed to warmer growing conditions (Thomas and Vince-Prue, 1997).
Campanula persicifolia and Lavandula angustifolia did not require cold for vegetative
growth but required cold for inﬂorescence development (Iversen and Weiler, 1994;
Whitman et al., 1996). Cold temperatures may also reduce the critical photoperiod of a
species or eliminate it’s photoperiodic requirement for ﬂowering (Thomas and Vince-
Prue, 1997). The critical photoperiod for Rudbeckiafulgida ‘Goldsturm’ shifted from 14
h prior to cold to 13 h following cold treatment (Runkle et al., 1999). Leucanthemum
xsuperbum ‘Snowcap’ performed as a qualitative LDP prior to cold and a quantitative
LDP following cold (Runkle et al., 1998a).

Finally, plants may exhibit a facultative response to cold. In this case, cold is not

41

absolutely required but accelerates growth and improves plant quality and uniformity.
Cold was not required to break vegetative dormancy or for ﬂoral development of Phlox
‘F airy’s Petticoat’ and Phlox paniculata ‘Eva Cullum’; yet exposure to cold accelerated
ﬂower development, increased plant height, and improved overall vigor (Iversen and
Weiler, 1994; Runkle et al., 1998b).

Few researchers have investigated the effect of cold temperatures and photoperiod
on Hosta dormancy, growth, and development. The regrowth of ﬁeld-grown Hosta
‘Honeybells’ crowns stored at -10, -5, -2, 2, or 5°C for six months varied among the
temperatures examined (Maqbool and Cameron, 1994). Plants stored at -10°C for six
months failed to regrow, and regrowth was poor with reduced survival and plant height
for plants stored at -5°C. However, plant growth after storage at -2, 2, or 5°C was not
impacted by exposure to cold temperatures. Improved emergence, growth, and ﬂowering
of Hosta ‘Francee’ occurred when dormant divisions received 15 weeks of 5°C compared
with noncooled divisions (Finical et al., 1997).

Hosta is comprised of 40 species native to China, Korea, and Japan and contains
over 2,500 named cultivars (Chung and Kim, 1991). Because of the large diversity
within Hosta, the objectives of this research were to determine the amount of cold
necessary to break vegetative dormancy of a diverse group of hosta clones and to
determine the effects of cold temperature on vegetative and reproductive growth

characteristics.

42

MATERIALS AND METHODS

Plant material. 1997-1998. Hostas were received from commercial producers (Table 1)
in fall 1997 and were separated into single-eye divisions. Divisions were placed upright
into 23 x 15 x 7 cm (6.8 L) bulb crates or planted individually in 13-cm (1.1 L) square
containers ﬁlled with a commercial soilless medium composed of composted pine bark,
vermiculite, Canadian Sphagnum peat, and coarse perlite with a wetting agent, lime, and
starter fertilizer charge (High Porosity Mix, Strong-Lite Products, Pine Bluff, AR).
Potted divisions were placed directly in a glass greenhouse, while bulb crates were placed
in a cooler set at 5°C. Plants were watered as required with well water acidiﬁed with
citric acid to a pH of 6.0. Divisions were removed from the bulb crates after 3, 6, 9, 12,
or 15 weeks of cold, planted into 13-cm square containers as described above, and placed
in the greenhouse.
1998-1999. Hostas from the 1997 experiments were grown outdoors from May 15 to
October 16, 1998 in l3-cm (1.1 L) square containers under 50% shade created by
alternate strips of wood lath at the Michigan State University Horticultural Teaching and
Research Center, East Lansing, MI. An exception was Hosta ‘Royal Standard’ where
single-eye divisions were taken from 3 year-old crowns grown in 8-cm (350 ml)
containers. Plants showed visible signs of dormancy (leaf senescence) and had the
foliage removed prior to ﬁrst frost on October 16, 1998. Pots were placed in a 20°C glass
greenhouse or in a cooler at 5°C for 0, 3, 6, 9, 12, or 15 weeks.

Following cold treatment, plants were placed under a 9-hour short day (SD) or a
9-hour plus 4-hour night interruption (N1) in 1997, but only under N1 in 1998. Plants

were covered with opaque black cloth from 1700 to 0800, and NI lighting was provided

43

from 2200 to 0200 with 60-W incandescent lights delivering 3 to 5 umol m'zs".

General Procedures. Plants were fertilized at every irrigation with a nutrient solution of
well water (EC of 0.70 mS'cm'1 and 105, 35, and 23 mg°L’l Ca, Mg, and S, respectively)
acidiﬁed with H2SO4 to a titratable alkalinity of 130 mg°L'l CaCO3 and water soluble
fertilizer providing 125-12-125-13 N-P-K-Ca mg°L‘l (30% ammonical N) plus 1.0-0.5-
0.5-0.5-0.1-0.1 (Fe, Mn, Zn, Cu, B, Mo) mg'L" (MSU Special, Greencare Fertilizers,
Chicago, IL).

Four-hundred-watt high-pressure sodium (HPS) lamps provided a photosynthetic
photon ﬂux (PPF) of 100 umol'm'2 s'I when the ambient greenhouse PPF dropped below
200 umol'm'z's'l from 800 to 1700. Supplemental lighting was terminated when PPF
exceeded 400 umol'm'2 s". In 1998, the average daily light integral was measured with a
quantum sensor (LI-COR) connected to a CR-10 datalogger (Campbell, Scientiﬁc, Logan,
UT). Greenhouse air temperature was monitored on each bench with 36-gauge type E
thermocouples connected to a CR-10 datalogger (Campbell, Scientiﬁc, Logan, UT).
Temperatures and light measurements were collected every 10 seconds and the hourly
average recorded. Supplemental heat was provided at night as needed to maintain 20°C
by 1500-W electric heaters (Model T771, Rival Manufacturing Co, Sedalia, MO) located
under each bench. In 1998, the average daily temperature and daily light integral from
force to ﬂower for each species and cultivar were calculated for each cold treatment
(Table 2).

Experiments were conducted in the Plant and Soil Sciences Research Greenhouses

at Michigan State University, East Lansing, MI. The experiment was a factorial design

44

study with two factors, photoperiod and cold duration, in 1997-1998. In 1998-1999, the
experiment only examined one factor, cold duration. Each treatment was completely
randomized with ten replications in both years. Unless otherwise indicated, the
greenhouse air temperature was a constant 20°C. All plants were forced under the

speciﬁed treatments for ﬁfteen weeks.

Data collection and analysis. Emergence and ﬂowering percentages were calculated for
each hosta clone in both years. Flowering percentages were calculated as the number of
ﬂowering plants divided by the number of emerged plants in each treatment. The date of
visible ﬂower bud was collected for all reproductive plants; the date of ﬂower anthesis,
plant height (cm), inﬂorescence height (cm), ﬂower number, scape leaf number, leaf
number, and shoot number were collected when the ﬁrst ﬂower opened. Plant height, leaf
number, and shoot number were collected for nonreproductive plants ﬁfteen weeks after
forcing. Leaf area was taken on all plants ﬁfteen weeks after the start of forcing with a
LI-300 portable leaf area meter (LI-COR, Lincoln, NE).

Days to visible bud, days to ﬂower, and days from visible bud to ﬂower were
calculated for all reproductive plants. Data were analyzed using SAS’s (SAS Institute,

Cary, NC) analysis of variance (ANOVA) and general linear models (GLM) procedures.

RESULTS
General responses. Emergence of dormant plants when grown under short-day
photoperiods depended upon the cold-treatrnent duration and genotype. Of those that did

emerge following cold, plants developed only one ﬂush of leaves and then became

45

dormant irrespective of cold treatment. Growth under long-day photoperiods was more
vigorous and, depending upon cold duration, led to ﬂowering.

Plants grown under long-day photoperiods in 1997-1998 were signiﬁcantly
smaller with lower ﬂowering percentages compared to plants grown in 1998-1999
(Figures 18; 2B, F; 3B; 48, F; 68, F). Plant height, average leaf size, and ﬂower number
were generally greater for plants grown in 1998-1999 than in 1997-1998 (Table 3). Time
to visible bud and time from visible bud to ﬂower were generally less in 1998-1999 when
compared to 1997-1998 (Table 4). Time to ﬂower for all hosta clones decreased or did
not change between years (Figures 1C; 2C, G; 3C; 4C, G; 6C, G). The following results
and discussion are based upon growth and development of plants grown under the long-
day photoperiod in 1998-1999 because plants under these experimental conditions were

larger and more typical of established hostas.

Cold requirement for emergence. Each hosta clone evaluated in this study was placed
in one of three categories based on the cold requirement for 100% of the plants to emerge
from dormancy. These categories were 0, 3, or 6 weeks of cold (Figure 1A, E; 2A, E; 3A;
4A, B; 5A; 6A, E). A smaller percentage of plants in each category emerged with less
cold.

H. plantaginea, ‘Royal Standard’, and ‘Lancifolia’ did not require a cold
treatment to break dormancy. Essentially all plants emerged and actively grew after
exposure to any cold duration, including none (Figure 1A, B; 2A). Both H. plantaginea
and ‘Royal Standard’ plants receiving no cold displayed more vigorous growth than

cooled plants as evidenced by maximum plant height and average leaf size (Figure 1D, H;

46

Table 4).

A diverse group of hosta clones required at least three weeks of cold to achieve
complete emergence of all plants. This group included ‘Golden Scepter’, ‘Golden Tiara’,
‘Hyacinthina’, H. montana, and ‘Undulata’. Greater than 50% of H. montana and
’Undulata’ plants (Figure 3A, E), a smaller percentage of ‘Golden Scepter’ and ‘Golden
Tiara’ plants (Figure 4A, E), and no ‘Hyacinthina’ plants (Figure 5A) emerged without
exposure to cold temperatures. However, each clone displayed increased vigor following
3 weeks of cold.

H. ‘Tokudama’ gold and ‘Tokudama’ green required six weeks of cold for 100%
emergence (Figure 6A, E). One ‘Tokudama’ gold and one ‘Tokudama’ green plant did
emerge without cold, and emergence rates for both cultivars were 40% after exposure to 3

weeks of cold.

Vegetative and reproductive characteristics. The ﬂowering characteristics of noncold-
requiring hosta clones varied with cold treatment. Flowering percentage of H.
plantaginea was less than or equal to 50% for each cold treatment (Figure 18).

Flowering of H. plantaginea did not occur when plants were exposed to 3 weeks of cold
although some plants ﬂowered when exposed to 0 or 6 weeks of cold. Maximum
ﬂowering occurred after plants were exposed to 9 or more weeks of cold. Ninety percent
of ‘Royal Standard’ plants ﬂowered with six weeks of cold, yet other cold treatments
yielded lower ﬂowering percentages (540%) (Figure 1F). All ‘Lancifolia’ plants
ﬂowered after 9 or more weeks of cold (Figure 2A). Time to ﬂower increased 10 days for

‘Lancifolia’ (Figure 2C) after 3 weeks of cold then declined slightly with further cold.

47

Days to visible bud decreased and days from visible bud to ﬂowering increased for
‘Lancifolia’ with increasing cold duration (Table 5). Cold duration did not affect days to
visible bud, days to ﬂower, or days from visible bud to ﬂower for either H. plantaginea or
‘Royal Standard’ (Figure 1C, G; Table 5).

Cold-treatment duration affected the average height and leaf size of ‘Lancifolia’,
H. plantaginea, and ‘Royal Standard’ plants. Plant height and average leaf size increased
33 and 36%, respectively for ‘Lancifolia’ plants exposed to 15 weeks of cold (Figure 2D;
Table 4). However, H. plantaginea and ‘Royal Standard’ plant height decreased 6 to 8
cm from a maximum for noncooled plants to a minimum after 15 weeks of cold (Table
6). Average leaf size of H. plantaginea varied considerably for noncooled and cooled
plants (Figure 1D), and ‘Royal Standard’ average leaf size declined after 15 weeks of cold
(Figure 1H). Potted ‘Royal Standard’ crowns were dry when removed from the cooler
after 9, 12, and 15 weeks of cold and were slow to initiate growth when placed in the
greenhouse. This may account for the unusually low vigor and ﬂowering percentage of
‘Royal Standard’ plants compared with H. plantaginea plants exposed to 9, 12, and 15
weeks of cold.

The ﬂowering percentages of hosta clones requiring 3 weeks of cold for complete
emergence varied with cold duration. The ﬂowering percentage of H. montana generally
increased with cold but never exceeded 60% (Figure 3A). Flowering of noncooled
‘Undulata’ plants was 14% and increased with cold but never exceeded 80% (Figure 3E).
Flowering of all ‘Hyacinthina’ plants occurred after 12 or more weeks of cold (Figure
5A). Flowering percentage of emerged ‘Golden Scepter’ plants ranged from 60 to 100%,

while ﬂowering of ‘Golden Tiara’ reached 100% for plants in all treatments (Figure

48

4A,E). In 1998-1999, 40% of ‘Golden Scepter’ plants failed to ﬂower after ﬁfteen weeks
of cold. These plants appeared to be in a vegetatively dormant state for much of the
forcing duration and did not produce a second ﬂush of growth commonly observed in
plants belonging to the ‘Tiara’ series (Pollock, 1997).

Time to ﬂower generally decreased with increasing cold for most clones.
However, days to visible bud, days to ﬂower, and days from visible bud to ﬂower were
not affected by cold duration for the H. montana plants that ﬂowered (Figure 3C, Table
5). Time to visible bud and ﬂower increased for ‘Undulata’ (Figure 3G) after 3 weeks of
cold then declined slightly with further cold. Time to ﬂower decreased 5 weeks for
‘Hyacinthina’, 2 weeks for ‘Golden Tiara’, and 6 weeks for ‘Golden Scepter’ as cold
duration increased from 0 to 15 weeks (Figure 4C, G; 5C). The decrease in time to
ﬂower is primarily attributed to a decrease in time to visible bud and a decrease in time
from visible bud to ﬂower (Table 5) as forcing temperature only increased slightly as cold
duration increased (Table 2). The average daily light integral may have contributed to the
decrease in time to ﬂower as light levels more than doubled for plants having received 15
weeks of cold versus noncooled plants (Table 2). .

Plant height and average leaf size of plants requiring 3 weeks of cold also varied
with cold treatment. ‘Undulata’ and ‘Golden Scepter’ average leaf size were not affected
by cold duration (Figures 3H; 4D). The average leaf size of ‘Golden Tiara’ and
‘Hyacinthina’ plants increased as cold duration increased (Figure 4H, 5D), and the
average leaf size of H. montana plants more than doubled with increasing cold (Figure
3D). Plant height was greatest for ‘Golden Scepter’, ‘Hyacinthina’, and H. montana

plants following nine weeks of cold, although absolute differences were only 4 to 6 cm

49

(Table 4). ‘Undulata’ plant height was not affected by cold duration (Figure 4H; Table 6).
The responses of ‘Golden Scepter’ and ‘Golden Tiara’ to cold treatments were less
uniform than anticipated despite their nearly identical genetic background as ‘Golden
Scepter’ is a Chartreuse sport of the proliﬁc green and gold variegated ‘Golden Tiara’
(Savory, 1985).

‘Tokudma’ gold and ‘Tokudama’ green plants varied in their response to cold-
treatment duration. Complete ﬂowering of ‘Tokudama’ gold did not occur under any
cold treatment (Figure 6B). ‘Tokudama’ green reached 100% ﬂowering following 15
weeks of cold (Figure 6F). Days to ﬂower declined 3 weeks for ‘Tokudama’ gold and 5
to 6 weeks for ‘Tokudama’ green, respectively with increasing cold (Figure 6C, G).
Inﬂorescence height of ‘Tokudama’ gold was not affected by length of cold treatment
(Table 4). However, inﬂorescence height of ‘Tokudama’ green plants increased as cold
duration increased.

Although plant height increased as the length of cold treatment increased for both
hosta clones, ‘Tokudama’ gold plants were 4 to 9 cm shorter than ‘Tokudama’ green
plants in all treatments (Table 4). Cold did not affect ‘Tokudama’ green average leaf size
(Figure 6D) while the average leaf size of ‘Tokudama’ gold plants exposed to cold

temperatures increased over 50% after 15 weeks of cold (Figure 6H).

DISCUSSION
This study provides evidence that plant size, photoperiod, exposure to cold
temperatures, and genotype interact to impact growth, development, and ﬂowering of

Hosta. Flowering is controlled in part by plant size and crown maturity. Flowering

50

percentages of single-eye divisions were low for noncooled and cooled plants in 1997-
1998. Not all bulked plants ﬂowered in 1998-1999 despite larger shoot sizes and a
greater number of divisions per container. Hostas are considered mature and attain a
mature leaf size and shape when they have completed six growing seasons after being
started from one- or two-year-old divisions (Schmid, 1991). Thus, the plants that did not
ﬂower under inductive conditions are considered juvenile and may be physiologically
incapable of ﬂowering. Complete ﬂowering percentages would be expected of mature
plants.

The foliage of experimental plants was removed prior to ﬁrst frost but after leaf
senescence began in early-to-mid autumn. However, some or all the plants may not have
entered a fully dormant state as the foliage had not died back to the crown. For many of
the hosta clones examined, a small percentage of noncooled plants emerged irrespective
of photoperiod and exhibited low vigor. Developmentally these plants may not have
completed the transition into dormancy when outdoors under natural short days in the lath
house. Artiﬁcial long days provided in the greenhouse apparently promoted emergence
and subsequent growth. Therefore, the cold duration necessary to break dormancy for
plants experiencing deep dormancy induced in late-autumn or winter may be longer than
for the plants examined in this experiment.

Short-day photoperiods alone induced vegetative dormancy of hosta clones.
Plants grown under short-day photoperiods with and without a cold treatment produced
an initial ﬂush of leaves, then entered a state of dormancy where vegetative growth
ceased and no ﬂowering occurred. Leaf senescence only occurred in the greenhouse

under short-day photoperiods for ‘Golden Scepter’ and ‘Golden Tiara’. This suggests

51

that cold temperatures may be required by other genotypes to promote the leaf senescence
that is observed outdoors. Exposure to cold temperatures breaks crown dormancy of
hosta clones, yet long-day photoperiods must follow the cold treatment to promote
continued growth and subsequently induce ﬂowering (Figures 13; 2B, F; 3B; 43, F; 6B,
F).

Growth of H. plantaginea and ‘Royal Standard’ plants appears to be inhibited
solely by unfavorable environmental factors (short days), a form of ecodormancy. H.
plantaginea and ‘Royal Standard’ plants produced an initial ﬂush of leaves under short-
day photoperiods then entered a dormant state. When transferred to long-day
photoperiods after 15 weeks of short days, H. plantaginea and ‘Royal Standard’ plants
resumed growth (personal observation). For the remaining hosta clones, growth was not
observed upon transfer from short-day to long-day photoperiods and must be inhibited by
physiological factors within the crown (paradorrnancy or endodormancy).

The range of effective low temperatures required to break vegetative dormancy of
hosta clones and their duration are unknown. All clones examined had cold requirements
of 0, 3, or 6 weeks at 5°C to break vegetative dormancy and achieve complete emergence
in both years. In the natural environment, hostas are remarkably hardy plants and tolerate
long durations of cold temperatures (e. g. up to ﬁve months of temperatures at 5°C or
below in climates similar to Michigan) when dormant. Plants having longer cold
requirements may have a selective advantage in areas where warm temperatures favorable
for growth follow periods of cold temperatures in autumn and early winter. The longer
cold requirement would prevent plants from emerging prematurely and dying when

conditions for growth turn unfavorable. Longer durations of cold generally decreased the

52

time to ﬂower and improved percent ﬂowering under long-day photoperiods (Figures 2B,
C, F, G; 38, C; 4B, C, F, G; 63, C, F, G). For most hosta clones, time to ﬂower of plants
exposed to long days did not vary for photoperiods 214 h and NI (personal observation).
These characteristics would ensure emergence in areas with long winters followed by
short growing seasons and enable plants to complete their life cycle from emergence to
seed set within the shortest amount of time possible. Thus, a decrease in time to ﬂower
would be advantageous for the survival of seedling progeny and existing plants.

Cold temperatures were not absolutely required by ‘Lancifolia’, H. plantaginea, or
it’s seedling derivitive ‘Royal Standard’ (Pollock, 1989) to break dormancy or for ﬂower
initiation as ﬂowering of these clones occurred without a cold treatment. The disparity in
size between noncooled and cooled plants in 1998-1999 suggests that H. plantaginea was
not dormant and continued growth from the summer season when brought into the
greenhouse. H. plantaginea is presumably native to Zheijing province or provinces
further south in China (Schmid, 1991). These areas are located between 20°N and 30°N
latitude (similar to Houston, TX and New Orleans, LA) and experience annual average
low temperatures of -6.6 to 44°C (20 to 40°F) (Widrlechner, 1997). Temperatures below
5°C in these areas would not persist for long durations, therefore it is not surprising these
hosta clones do not require cold periods to initiate or maximize growth. ‘Lancifolia’, H.
plantaginea and ‘Royal Standard’ plants naturally bloom in late summer (Solberg, 1988),
and a greater percentage of ﬂowering of these plants might be achieved with forcing
periods longer than 15 weeks or with larger, more mature plant material.

The remaining hosta clones beneﬁtted from cold exposure with improved

emergence and percent ﬂowering, reduced time to ﬂower, and increased vigor as

53

evidenced by greater plant height and average leaf size. Many of these clones are
cultivars of unknown origin, therefore the native locations of their parents are difﬁcult to
determine and mostly hypothesized. However, most Hosta species are native to Korea
and the Japanese archipelago which span from 35°N to 45°N latitude (similar latitudes as
Memphis, TN and Minneapolis, MN) (Graves, 1992) and experience a temperate
continental climate much similar to North America (Schmid, 1991). The parental species
of ‘Golden Tiara’ and ‘Golden Scepter’ is H. nakaiana, a species native to central and
southern Korea (Schmid, 1991). ‘Tokudama’ gold and green are hypothesized hybrids of
H. sieboldiana, a species native to Japan (Schmid, 1991).

In a comparative study of Hosta across the United States, the emergence patterns
of several genotypes closely parallel their cold requirements as established in this study
(Solberg, 198 8). H. plantaginea emerged earlier than other hostas in southern gardens
and later than most in northern gardens; ‘Lancifolia’, H. montana, and ‘Undulata’ were
ﬁrst to emerge in all gardens; H. nakaiana and ‘Fortunei’ emerged mid-to-late in all
gardens; and ‘Tokudama’ emerged late in all gardens. Early emergence would be
expected for hostas with a minimum cold requirement, such as H. plantaginea,
‘Lancifolia’, H. montana, and ‘Undulata’. Presumably these hostas are native to areas
that experience short durations of cool or cold temperatures. H. montana and ‘Undulata’
exhibited 250% emergence without cold and may require fewer than 3 weeks of cold to
saturate the cold requirement for emergence. ‘Golden Scepter’, ‘Golden Tiara’ (a H.
nakaiana hybrid), and ‘Hyacinthina’ (a ‘Fortunei’) appear to require an absolute
minimum of 3 weeks of cold, as evidenced by poor emergence without cold and mid-to-

late emergence in relation to other hostas. The ‘Tokudamas’ are set apart from other

54

hosta clones by having the longest cold requirement (6 weeks). The parental species of
‘Golden Scepter’, ‘Golden Tiara’, ‘Hyacinthina’, and ‘Tokudama’ originated in areas of
Japan and Korea that presumably experience a longer duration of cold temperatures.

Long periods of cold appeared to alter the emergence pattern and delay emergence
of H plantaginea (Solberg, 1988). Emergence of hostas such as H. plantaginea which do
not have a cold requirement to break dormancy may be controlled by temperature. These
plants may emerge only when the root zone temperature warms above a clonal-speciﬁc
base temperature. This would explain early emergence in southern climates with warm
soil temperatures and late emergence in cooler climates where the soil takes a longer
period of time to warm in the spring. However, this hypothesis needs to be tested as the
base soil temperature was not determined in this experiment.

Most hosta clones required or beneﬁtted from a period of cold temperatures
although the optimum temperature and it’s required duration remain unknown. These
experiments indicate that clonal divisions of hosta harvested in early to late autumn
should be cooled up to 6 weeks at 5°C to maximize vigor and growth. The cold
requirement for divisions harvested in winter and early spring should be naturally
fulﬁlled, and no artiﬁcial cold would be required. Emergence of cooled divisions
occurred irrespective of photoperiod in 1 to 3 weeks at 20°C, depending upon genotype.
When continuously grown under short-day photoperiods, hostas entered a vegetatively
dormant state and further growth ceased. Therefore, it is recommended that long-day
photoperiods _>_14 h be provided to promote vegetative and reproductive development.
When forcing for foliage, hostas reach a saleable size in approximately six weeks at

20°C. It is recommended that large, mature plants be used when forcing plants to ﬂower

55

as incomplete ﬂowering will result with small plants. Flower buds form in 8 to 13 weeks
for mature plants given the minimum amount of cold required for emergence (Table 7).
Greater durations of cold generally decreased time to ﬂower and improved percent
ﬂowering.

Because many commercially available hosta clones are closely related,
approximations for the cold requirement of other genotypes not examined in this study
can be made based upon their growth characteristics and genetic background. For
example, members of the ‘Tiara’ series share a close relationship with ‘Golden Scepter’
and ‘Golden Tiara.’ It can be inferred that these hostas would require at least three weeks
of cold for maximum emergence. However, the vegetative and ﬂowering responses to

cold temperature duration of plants not examined warrants further investigation.

Table 7. Cold temperature and greenhouse forcing requirements for hosta clones in 1998-
1999.

 

 

Weeks of 5°C cold Weeks to ﬂower at

Clone for 100% emergence 20°Cz
‘Golden Scepter’ 3 10-12
‘Golden Tiara’ 3 10-12
‘Hyacinthina’ 3 1 1-13
‘Lancifolia’ 0 13-15
H. montana 3 10-12
H. plantaginea 0 14-16
‘Royal Standard’ 0 16-18
‘Tokudama’ gold 6 8-9

‘Tokudama’ green 6 9-11

‘Undulata’ 3 10-12

 

‘ With NI lighting provided from 2200 to 0200 h.

56

REFERENCES

Ben-Hod, G., J. Kigel, and B. Steinitz. 1988. Dormancy and ﬂowering in Anemone
coronaria L. as affected by photoperiod and temperature. Ann. Bot. 61 :623-633.

Chung, M. G. and J. W. Kim. 1991. The Genus Hosta Tratt. (Liliaceae) in Korea. Sida
14(3):411-420.

Finical, L., A. Cameron, R. Heins, W. Carlson, and C. Whitman. 1997. Inﬂuence of cold
treatment and photoperiod on development and ﬂowering of Hosta. The Hosta
Journal 28(2):88-89.

Graves, W. 1992. National Geographic Atlas of the World, Revised, 6‘h edition.
National Geographic Society, Washington, DC.

Iversen, R. R. and T. C. Weiler. 1994. Strategies to force ﬂowering of six herbaceous
garden perennials. HortTechnology 4(1):61-65.

Jones, H. G. 1992. Plants and microclimate. Cambridge University Press. Cambridge,
UK.

Lang, G. A., J. D. Early, G. c. Martin, and R. L. Darnell. 1987. Endo-, para-, and
ecodorrnancy: physiological terminology and classiﬁcation for dormancy research.
HortScience 22(3):371-377.

Lang, A. 1965. Physiology of ﬂower initiation in: Encyclopedia of Plant Physiology,
Editor: W. Ruhland. Springer-Verlag, Berlin.

Maqbool, M. and A. C. Cameron. 1994. Regrowth performance of ﬁeld-grown
herbaceous perennials following bare-root storage: The effect of storage

temperature. Hortscience 29(9): 1039-1041 .

Napp-Zinn, K. 1984. Light and Vemalization. In: Light and the Flowering Process.
Academic Press, London, UK.

Pollock, W. I. 1989. H. ‘Royal Standard’ - The only patented Hosta. The Hosta Journal
20(1):52-54.

Pollock, W. I. 1997. Second and more leaf ﬂushes. The Hosta Journal. 28(2):33-36.
Roberts, E. and R. Summerﬁeld. 1987. Measurement and prediction of ﬂowering in
annual crops. In: J. G. Atherton (ed.). Manipulation of Flowering.

Butterworth’s, London.

Runkle, E. S., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1998a. Flowering of

57

Leucanthemum xsuperbum ‘Snowcap’ in response to photoperiod and cold
treatment. HortScience 33(6): 1003-1006.

Runkle, E. S., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1998b. Flowering of
Phlox paniculata is inﬂuenced by photoperiod and cold treatment. HortScience
33(7):]172-1174.

Runkle, E. S., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1999. Photoperiod and
cold treatment regulate ﬂowering of Rudbeckia fulgida ‘Goldsturm’. HortScience
34(1):55-58.

Savory, R. P. 1985. H. ‘Golden Tiara’ and ‘Golden Scepter.’ The Hosta Journal. 16:35.

Schmid, W. G. 1991. The Genus Hosta -- Giboshi Zoku. Timber Press, Portland, OR.

Solberg, R. M. 1988. The emergence pattern of Hosta: A semi-scientiﬁc survey. The
Hosta Journal 19(1):24-26.

Solberg, R. M. 1997. Hosta Physiology. The Hosta Journal 28(2):111-113.

Thomas, B. and D. Vince-Prue. 1997. Photoperiodism in plants. Academic Press, San
Diego, CA.

Whitman, C. M., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1996. Cold
treatments, photoperiod, and forcing temperature inﬂuence ﬂowering of

Lavandula angustifolia. HortScience 31(7):1150-1153.

Widrlechner, M. 1997. Hardiness Zones in China. USDA Agriculture Research Service,
Ames, IA.

58

 

 

 

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Weeks at 5°C Weeks at 5°C

Figure 1. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta plantaginea and 'Royal Standard' after 0, 3, 6, 9, 12, or 15
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65

 

 

 

 

 

 

 

 

 

 

 

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66

 

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Figure 3. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta montana and 'Und ulata' after 0, 3, 6, 9, 12, or 15 weeks of
5°C cold under a 9-hour short day in year one (C) or a 9—hour day plus a 4-hour night
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0.001, respectively.

67

 

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Figure 4. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta 'Golden Scepter’ and 'Golden Tiara' after 0, 3, 6, 9, 12, or
15 weeks of 5°C cold under a 9-hour short day in year one (C) or a 9-hourday plus a 4-hour
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intervals. L=linear, Q=quadratic trends. NS, *, **, “*Nonsigniﬁcant or signiﬁcant at P < 0.05,
0.01, or 0.001, respectively.

Average leaf size (cmz)

  

 

 

 

 

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69

 

 

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(I)
O

. .0.— LD Year .1
+ SD Year 1
—0- ,LD Year 2.

  

A
O

'Tokudama' gold 1 ' ' 'TokUdama' green

Emergence percentage
N O)
O O

O

 

.—
p—
_—
db-
—(r—
—11—-
‘l
_4
ﬂy—

.4
N-bCDCDO
OOOOO

Flowering percentage
of emerged plants

 

 

O

160 «

1201

80 ~ .
L

 

Days to ﬂower

4o.

 

 

160 J

120 , . LNS QNS
Lt. QNS

40J

Average leaf size (cm2)
m
C

  

  

 

 

 

 

O 3 6 9 12 15 0 3 6 9 12 15
Weeks at 5°C Weeks at 5°C

Figure 6. Percent emergence (A, E), percent ﬂowering (B, F), days to ﬂower (C, G), and
average leaf size (D, H) of Hosta 'Tokudama' gold and 'Tokudama' green after 0, 3, 6, 9, 12, or
15 weeks of 5°C cold under a 9-hour short day in year one 9) or a 9—hour day plus a 4—hour
night interruption in year one (0) and year two (3). Error bars are 95% conﬁdence intervals.
L=linear; Q=quadratic trends. NS, *, **, *“Norisigiiﬁcant or signiﬁcant at P< 0.05, 0.01, or
0.001, respectively.

70

SECTION III

THE INFLUENCE OF COLD TREATMENT AND PHOTOPERIOD DURATION

ON GROWTH AND FLOWERING OF HOS TA.

71

The influence of cold treatment and photoperiod duration on growth and ﬂowering

of Hosta.

Beth A. Fausey', Royal D. Heins’, and Arthur C. Cameron2

Department of Horticulture, Michigan State University, East Lansing, MI 48824-1325

Received for pulication . Accepted for publication . We gratefully
acknowledge the ﬁnancial support of the Agricultural Experiment Station of Michigan
State University and funding by hosta producers and greenhouse growers supportive of
Michigan State University ﬂoricultural research.

' Graduate Student

2 Professor

Production and Culture

72

The inﬂuence of cold treatment and photoperiod duration on growth and ﬂowering of

Hosta.

Additional index words. Herbaceous perennial, critical photoperiod, plantain lily.

Abstract. Single-eye divisions of H. montana, H. plantaginea, H. ‘Golden Scepter’, H.
‘Golden Tiara’, H. ’Hyacinthina’, H. ‘Lancifolia’, H. ‘Royal Standard’, H. ‘Tokudama’
gold, H. ‘Tokudama’ green, and H. ‘Undulata’ were grown under seven photoperiods
following 0 or 15 weeks of 5°C to determine the effects of cold treatment and
photoperiod duration on growth and ﬂowering in 1997-1998. Plants were bulked during
the summer, and the experiment was repeated in 1998-1999. Photoperiods were a 9-h
natural day extended with incandescent bulbs to 10, 12, 13, 14, 15, 16, or 24 hours. An
additional night interruption treatment (N I) was a 9-h natural day with a 4-h night break
from 2200 to 0200. Larger plants used in 1998-1999 were more uniform with higher
ﬂowering percentages than single-eye divisions, suggesting that plant size and crown
maturity inﬂuence ﬂowering of hosta clones. Not all plants emerged from a dormant
state and grew without exposure to cold. A small percentage of noncooled plants
emerged under photoperiods 513 h then went dormant without ﬂowering. A greater
percentage of noncooled plants emerged under photoperiods 214 h or with a 4-h N1 and
ﬂowered. All plants emerged following cold and were more vigorous. Cooled plants
under _<_13 h emerged yet developed only a single ﬂush of leaves and became dormant.
Vegetative growth led to ﬂowering of mature plants of all clones under photoperiods 214

h and NI. Plant height was greater for plants under photoperiods 214 h and NI. Average

73

leaf size and leaf number generally did not vary with photoperiod following cold. Time
to ﬂower decreased as photoperiod duration increased, but differences between

photoperiods 2 16 h and NI were not signiﬁcant.

INTRODUCTION

The speciﬁc vemalization, forcing temperature, and photoperiodic requirements
for growth and ﬂowering of herbaceous perennials must be determined in order to devise
production schedules to force plants for sale on a speciﬁc date. Although hosta are
primarily grown for their foliage, the scheduling of hosta in ﬂower is of interest to
producers wanting to force plants for retail sale or show, for breeders wishing to cross
plants with incongruent ﬂowering habits, and for propagators wishing to increase shoot
production by releasing lateral buds from apical dominance. Many questions concerning
the physiology of Hosta remain, and it is necessary to determine the environmental
conditions required for growth, ﬂoral induction, and development.

Plants optimize growth and adapt to seasonal changes by synchronizing their
development to environmental signals (Rees, 1987). Many physiological processes, such
as ﬂower initiation, are determined or inﬂuenced by a plant’s interaction with temperature
and photoperiod. The photoperiod duration and effective low temperature requirements
for ﬂower induction are often species- and cultivar-speciﬁc. Effective temperatures for
ﬂower induction range from below freezing to 16°C with an optimum range of 1 to 7°C
for most cold-requiring plants (Lang, 1965; Roberts and Summerﬁeld, 1987). Plants are
categorized according to their ﬂowering response to photoperiod. Short day plants (SDP)

ﬂower or ﬂower faster when the daylength is less than a critical duration; long day plants

74

(LDP) ﬂower or ﬂower faster when the daylength is greater than a critical duration; and
day neutral plants (DNP) ﬂower irrespective of daylength.

Exposure to cold temperatures affects plant development in several ways. Many
dormant woody and herbaceous perennials require sufﬁcient periods of low temperatures
to break dormancy of vegetative or reproductive buds to resume growth or ﬂowering.
When dormancy is fully broken, the resumption of growth is usually independent of
photoperiod (Iversen and Weiler, 1994). Plants may also require vemalization, the
induction of ﬂowering by low temperatures, to continue growth and ﬂower. For plants
with an obligate vemalization requirement, ﬂower initials form only after vemalization is
complete and differentiate into ﬂoral organs when the plant is exposed to warmer
growing conditions (Thomas and Vince-Prue, 1997). Finally, plants may exhibit a
facultative response to cold where cold is not absolutely required but accelerates growth
and ﬂowering and improves plant quality and uniformity.

The cold requirement for ﬂowering is often coupled with a photoperiodic
requirement. Many herbaceous perennials are LDP native to temperate regions and
require cold temperatures followed by long daylengths for spring and summer ﬂowering
(Roberts and Summerﬁeld, 1987). Long day plants are induced to ﬂower when the
daylength is greater than a critical length, also known as the critical photoperiod. This
critical photoperiod marks the transition from vegetative growth to reproductive growth
in obligate plants. For some plants such as Gaura lindheimeri and Geranium
dalmaticum, the response to daylength is facultative, and ﬂowering occurs under all
photoperiods but more rapidly when plants are exposed to long days (Finical et al., 1998a;

Finical et al., 1998b). In this case, the critical photoperiod would be the photoperiod

75

where time to ﬂower is minimal and not affected by further increases in photoperiod, and
below which ﬂowering is delayed (Roberts and Summerﬁeld,1987). The critical
photoperiod can also be deﬁned as the photoperiod required for 50% ﬂowering of a
population, or the photoperiod that induces complete, rapid, and uniform ﬂowering when
met or exceeded (Runkle et al., 1998b; Thomas and Vince-Prue, 1997).

Cold and photoperiod interact to impact several aspects of ﬂowering. Cold
temperatures may reduce the critical photoperiod for particular species or cultivar. The
critical photoperiod for Rudbeckiafulgida ‘Goldsturm’ shifted from 14 hours without
cold to 13 hours following 15 weeks of cold (Runkle et al., 1999). Cold temperatures
may also eliminate a plant’s photoperiodic requirement for ﬂowering (Runkle et al.,
1998a; Thomas and Vince-Prue, 1997). Lavandula angustifolia ‘Munstead’ ﬂowered
only under LD prior to cold yet ﬂowered under LD and SD following 10 or more weeks
of cold temperatures (Whitman et al., 1996). Finally, short photoperiods followed by
long photoperiods can replace or partially replace the cold requirement for some LDP
(Thomas and Vince-Prue, 1997).

Few studies have examined the inﬂuence of cold treatment and photoperiod on
ﬂowering of Hosta. Improved emergence, growth, and ﬂowering of Hosta ‘F rancee’
occurred when dormant divisions received 15 weeks of 5°C compared with noncooled
divisions (F inical et al., 1997). All plants failed to ﬂower without cold, and ﬂowering
only occurred with photoperiods 2 14 h following cold. Because of the diversity within
commercially available Hosta, the objectives of this research were to examine the effects
of photoperiod duration on vegetative and reproductive development and determine the

critical photoperiod necessary for ﬂower induction of a diverse group of hosta clones.

76

MATERIALS AND METHODS

Plant material. 1997-1998. Hostas were received from commercial producers (Table 1)
in fall 1997 and were separated into single-eye divisions. Divisions were placed upright
into 23 x 15 x7 cm (6.8 L) bulb crates or planted individually in 13-cm(1.1 L) square
containers ﬁlled with a commercial soilless medium composed of composted pine bark,
vermiculite, Canadian Sphagnum peat, and coarse perlite with a wetting agent, lime, and
starter fertilizer charge (High Porosity Mix, Strong-Lite Products, Pine Bluff, AR).
Potted divisions were placed directly in a glass greenhouse, while bulb crates were placed
in a cooler set at 5°C. Plants were watered as required with well water acidiﬁed with
citric acid to a pH of 6.0. Divisions were removed from the bulb crates after 15 weeks of
cold, planted into 13-cm square containers as described above, and placed in the
greenhouse.
1998-1999. Hostas from the 1997 experiments were grown outdoors from May 15 to
October 16, 1998 in 13-cm (1.1 L) square containers under 50% shade created by
alternate strips of wood lath at the Michigan State University Horticultural Teaching and
Research Center, East Lansing, MI. An exception was Hosta ‘Royal Standard’ where
single-eye divisions were taken from 3 year-old crowns previously grown in 8-cm (350
ml) containers, potted in the spring of 1998, and bulked in the lath house until fall. Plants
showed visible signs of dormancy (leaf senescence) and had the foliage removed prior to
ﬁrst frost on October 16, 1998. Pots were placed in a 20°C glass greenhouse or in a
cooler at 5°C without supplemental lighting for 15 weeks prior to photoperiod treatments.

Plants were placed under 10, 12, l3, 14, 15, 16, or 24 hours of continual light or

under a 9-h day plus 4-h night interruption (N I) from 2200 to 0200. Photoperiods were

77

established by covering benches with opaque black cloth from 1700 to 0800 to eliminate
natural light. The 9-h natural daylength was then extended from 1700 until photoperiod
completion with 60-W incandescent lights delivering 3 to 5 umol'm'zs" under the opaque

black cloth.

General Procedures. Plants were fertilized in the greenhouse at every irrigation with a
nutrient solution of well water (EC of 0.70 mS'cm" and 105, 35, and 23 mg'L'l Ca, Mg,
and S, respectively) acidiﬁed with H2804 to a titratable alkalinity of 130 mg°L" CaCO3
and water soluble fertilizer providing 125-12-125-13 N-P-K-Ca mg°L" (30% ammonical
N) plus l.0-0.5-0.5-0.5-0.l-0.1 (Fe, Mn, Zn, Cu, B, Mo) mg°L'l (MSU Special, Greencare
Fertilizers, Chicago, IL).

Four-hundred-watt high-pressure sodium (HPS) lamps provided a photosynthetic
photon ﬂux (PPF) of 100 umolm‘2 s'l starting at 0800 and continuing until the outside
PPF exceeded 400 pmol'm'2 s". If the outside PPF then dropped below 200 pmol'm'z's'l
lamps were again turned on until 1700. In 1998, the average daily light integral was also
measured from the start of forcing to the average date of ﬂowering with a quantum sensor
(LI-COR) connected to a CR-lO datalogger (Campbell, Scientiﬁc, Logan, UT) (Table 2).
Greenhouse air temperature was monitored on each bench with 36-gauge type E
thermocouples connected to a CR-lO datalogger (Campbell, Scientiﬁc, Logan, UT).
Temperatures and light measurements were collected every 10 seconds and the hourly
average recorded. Supplemental heat was provided by 1500—W electric heaters (Model
T771, Rival Manufacturing Co, Sedalia, MO) located under each bench at night as

needed to maintain a 20°C air temperature under the black cloth. In 1998, the average

78

daily temperature and daily light integral from the start of greenhouse forcing to ﬂower
for each clone was calculated for each cold and photoperiod treatment (Table 2).
Experiments were conducted in the Plant and Soil Sciences Research Greenhouses
at Michigan State University, East Lansing, MI. The experiment was a factorial design
study with two factors, photoperiod and cold duration. Each treatment was completely
randomized with ten replications. Unless otherwise indicated, the greenhouse air
temperature was a constant 20°C. All plants were forced under the speciﬁed treatments

for ﬁfteen weeks.

Data collection and analysis. Emergence and ﬂowering percentages were calculated for
each hosta clone. Flowering percentages werecalculated as the number of ﬂowering
plants divided by the number of emerged plants in each treatment. The date of visible
ﬂower bud was collected for all reproductive plants; the date of ﬂower anthesis, plant
height (cm), inﬂorescence height (cm), ﬂower number, scape leaf number, leaf number,
and shoot number were collected when the ﬁrst ﬂower opened. Plant height, leaf number,
and shoot number were collected for nonreproductive plants ﬁﬁeen weeks aﬁer forcing.
Leaf area was taken on all plants ﬁfteen weeks after the start of forcing with a LI-3 00
portable leaf area meter (LI-COR, Lincoln, NB).

Days to visible bud, days to ﬂower, and days from visible bud to ﬂower were
calculated for all reproductive plants and no data were available for these parameters on
nonﬂowering plants. Data were analyzed using SAS’s (SAS Institute, Cary, NC) analysis

of variance (ANOVA) and general linear models (GLM) procedures.

79

RESULTS
General responses. In both years, not all plants emerged from a dormant state and grew
under the photoperiod treatments without prior exposure to cold. A small percentage of
plants emerged under photoperiods 513 h but then went dormant without ﬂowering. The
greatest percentage of noncooled plants emerged under photoperiods 214 h or with a 4-h
NI, and many of these plants ﬂowered. The actual emergence and ﬂowering percentages
of noncooled plants varied with genotype. Essentially all plants, except several
‘Lancifolia’ plants in 1997-1998, emerged in both years under all photoperiods following
cold. Cooled plants grown under 10-h and 12-h photoperiods developed a single ﬂush of
leaves and then became dormant. Irregular ﬂowering of some plants occurred under 10-h
and 12-h photoperiods following cold treatment in 1998-1999. ‘Golden Scepter’ and
‘Golden Tiara’ plants ﬂowered uniformly under 13 h in 1998-1999, but other hosta clones
went dormant. Vegetative growth led to ﬂowering of all clones under photoperiods 214 h

in both years.

Emergence. Emergence rates of depended upon genotype, cold treatment, and
photoperiod in both years. Trends in emergence were generally the same between years
although absolute percentages differed. Emergence percentage of noncooled plants
increased as photoperiod increased from 10 h to 24 h without exception. Emergence
under 10-h and 12-h photoperiods was dramatically different between years for noncooled
H. plantaginea and ‘Lancifolia’ plants with both having little emergence in 1998-1999

compared to 1997-1998. Following cold treatment, essentially all plants emerged and

formed leaves.

80

Noncooled clones were compared for emergence under long-day photoperiods by
averaging the percent emergence for the 16, 24, and NI photoperiod treatments in 1998-
1999. The NI treatment was included because emergence under N1 was often similar to
emergence under the 16 and 24-h photoperiods. Ninety to 100% of H. plantaginea,
‘Royal Standard’, and ‘Lancifolia’ plants (Figures 1A, 2A, 3A); ﬁﬁry to 90% of
‘Hyacinthina’, ‘Golden Scepter’, H. montana, and ‘Undulata’ plants (Figures 4A, 5A, 6A,
7A); and less than 50% of ‘Golden Tiara’, ‘Tokudama’ green, and ‘Tokudama’ gold
emerged under the 16-h, 24-h, and NI photoperiod treatments prior to cold (Figures 8A,

9A, 10A).

Vegetative Characteristics. Noncooled plants of all hosta clones under 10, 12, and 13 h
entered a dormant state after emergence in both years. Plants grown under 214-h and NI
photoperiods produced new leaves, and some ﬂowered. Plants developed similarly under
the photoperiod treatments following cold although cooled plants under 10, 12, and 13 h
grew to a taller height, were more robust than noncooled plants under the same
photoperiods, and eventually became dormant. Cooled plants of all clones under 214 h
emerged and actively produced new leaves followed by ﬂowering.

Several vegetative characteristics of Hosta were inﬂuenced by plant size, cold
treatment, and photoperiod. Average leaf size and plant height were generally greater for
plants grown in 1998-1999 than in 1997-1998. Noncooled plants grown under 10, 12,
and 13-h photoperiods had fewer and smaller leaves and were shorter compared with
plants grown under 214-h and NI photoperiods (Figures 1-10B, C, D, F, G, H).

Statistically, leaf size of ‘Golden Scepter’, H. montana, H. plantaginea, ‘Royal Standard’,

81

and ‘Undulata’ plants increased with increasing photoperiod duration prior to cold
(Figures 5D, 6D, 1D, 2D, 7D). However, leaf size of all hosta clones except ‘Golden
Scepter’ did not vary with photoperiod following cold in 1998-1999 (Figure 5D). Leaf
size of cooled H plantaginea was smaller under photoperiods 214 h and NI compared
with noncooled plants (Figures 1D, H). Average leaf size of cooled ‘Lancifolia’ was
similar to noncooled plants (Figures 3D, H). Cooled ‘Hyacinthina’, H. montana,
‘Tokudama’ gold and ‘Tokudama’ green plants had larger leaves under all photoperiods
than noncooled plants (Figures 4H, 6H, 9H, 10H).

The effects of photoperiod and cold treatment on plant height varied for hosta
clones. Cooled ‘Hyacinthina’, ‘Tokudama’ gold, and ‘Tokudama’ green plants under
photoperiods 214 h were taller than noncooled plants in 1998-1999 (Figures 4B, F; 9B, F;
10B, F). Plant heights were similar for noncooled and cooled ‘Golden Scepter’, ‘Golden
Tiara’, ‘Lancifolia’, H. montana, and ‘Undulata’ under 16-h, 24-h, and NI photoperiods
(Figures 5B, F; 8B, F; 38, F; 68, F; 7B, F). However, cooled H. plantaginea and ‘Royal
Standard’ plants grown under photoperiods 214 h were shorter than noncooled plants
only in 1998-1999 (Figures 18, F; 2B, F).

The average number of leaves per shoot was generally similar between years. The
leaf number for noncooled plants in 1998-1999 progressively increased as photoperiod
increased and was greatest under 16 h, 24 h, or NI (Figures 1C-10C). Plants under 10,
12, and 13 h produced an initial ﬂush of leaves and went dormant. Plants under 214 h
continued to produce leaves until ﬂowering occurred. Following cold, the average
number of leaves per shoot did not vary signiﬁcantly with photoperiod except for ‘Golden

Scepter’, ‘Lancifolia’, H. plantaginea, and ‘Royal Standard‘ plants(Figures lG-lOG).

82

‘Golden Scepter’, H. plantaginea, and ‘Royal Standard‘ leaf number increased with
increasing photoperiod while ‘Lancifolia’ leaf number was inconsistent in 1998-1999

(Figures 5G, 1G, 2G, 3G).

Flowering. Flowering was inﬂuenced by genotype, plant size, photoperiod, and cold
treatment. Without exception, more plants of each clone ﬂowered in 1998-1999 than in
1997-1998, and ﬂowering percentages of many hosta clones increased dramatically
following cold (e.g. ‘Golden Scepter’ increased from O to 100% following cold under a
13-h photoperiod). Noncooled and cooled plants in 1998-1999 generally required less
time to ﬂower than plants in 1997-1998.

Photoperiod was the primary determinate of ﬂowering. Only 3 plants of 800
ﬂowered under 512-h photoperiods in 1998-1999 (Figures 16E, 19E). Several clones
ﬂowered to varying degrees under the 13-h photoperiod following cold (Figures 16E,
20E, 15E, 18E). The 13-h and 14-h photoperiods appear to be transitional photoperiods
for growth and ﬂowering. Maximum ﬂowering percentages of several clones occurred
under 21 S-h and NI photoperiods. Flowering percentages for ‘Hyacinthina’, H.
plantaginea, ‘Tokudama’ gold, and ‘Tokudama’ green were lower under continuous light
than the 16-h treatment; while ﬂowering percentages under N1 were often as high or
higher than the continuous photoperiod treatment.

No H. montana plants ﬂowered without exposure to cold (Figure 16A). No
ﬂowering occurred under 12 h, and incomplete ﬂowering occurred under all other
photoperiods for cooled plants (Figure 16E). Time to ﬂower ranged from 60 to 86 days

and was not affected by cold or photoperiod. However, ﬂowering of one cooled plant

83

occurred in 45 days under the 10-h photoperiod, thus suggesting that the ﬂower bud was
initiated prior to cooling (Figure 16F; Table 3).

No ‘Hyacinthina’ plants ﬂowered in 1997-1998, and only noncooled plants
ﬂowered in 1998-1999 under the 24-h photoperiod. After cold, ‘Hyacinthina’ required
photoperiods 214 h and NI for ﬂowering (Figure 14A, E). Time to ﬂower decreased
following cold but did not vary with photoperiod (Figures 14B, F; Table 3).

Flowering of ‘Tokudama’ gold improved in 1998-1999 although complete
ﬂowering was never achieved under any photoperiod treatment. ‘Tokudama’ gold plants
ﬂowered without cold under 24 h in 1997-1998 and under 16 h in 1998-1999 (Figure
19A, E). Following cold treatment, ﬂowering percentages were 270% under 214-h
photoperiods and NI, except for the 24-h photoperiod (Figure 19E). One noncooled plant
and all cooled plants grown in 1998-1999 ﬂowered in 8 to 9 weeks irrespective of
photoperiod (Figures 19B, F; Table 3). Flower buds formed on several ‘Tokudama’ gold
plants grown under 10-, 12-, and 13-h photoperiods in 1998-1999; however, the buds of
lO-h and 13-h plants aborted prior to anthesis while the buds of 12-h plants developed to
anthesis (Figure 19E).

Noncooled ‘Tokudama’ green plants did not ﬂower in 1997-1998, and only 30%
ﬂowered under 16-h and 24-h photoperiods in 1998-1999 (Figure 20A, E). Following
cold, ﬂowering in 1998-1999 occurred under 213-h photoperiods with 100% ﬂowering
under 16 h (Figure 20E). Time to ﬂower decreased from 105 days for noncooled plants
to 64 days for cooled plants (Figure 15F). The reduction in time to ﬂower resulted from a
decrease in days to visible bud (Table 4).

‘Lancifolia’, H. plantaginea, and ‘Royal Standard’ plants ﬂowered to varying

84

degrees under photoperiods 214 h prior to and following cold in 1998-1999. Flowering
of ‘Lancifolia’ was greater in 1998-1999 than the previous year. Essentially all
noncooled ‘Lancifolia’ plants ﬂowered under 24-h and NI photoperiods, and all cooled
plants ﬂowered under 214 h and NI (Figures 13A, E). Time from forcing to visible bud
was two weeks longer under the 14-h and 15-h photoperiods than for photoperiods 216-h
(Figure 13B, F; Table 4). H. plantaginea did not ﬂower in 1997-1998, and incomplete
ﬂowering of noncooled and cooled plants reached a maximum under l6-h and 24-h
photoperiods in 1998-1999 (Figure 11A, B). All ‘Royal Standard’ plants ﬂowered under
l6-h photoperiods prior to cold and under N1 following cold (Figures 12A, E). Flowering
of noncooled and cooled H. plantaginea and ‘Royal Standard’ plants occurred in 14 to 15
weeks and did not vary with photoperiod (Figures 12B, F; 13B, F, Table 3).

No treatment combination resulted in 100% ﬂowering of ‘Undulata’, yet
ﬂowering percentages were generally greater for larger plants in 1998-1999 (Figures 17A,
E). Flowering occurred under 21 S-h photoperiods prior to cold and under 213-h
photoperiods following cold with some exceptions. Maximum ﬂowering occurred under
24 h in 1998-1999 (Figures 17A, E). Time to ﬂower decreased following cold in 1998-
1999 but did not vary with photoperiod treatment (Figure 17B, F; Table 3). The
reduction in time to ﬂower resulted from a 2 to 5 week decrease in time to visible bud
(Table 4). The most rapid uniform ﬂowering of ‘Undulata’ occurred in 10 weeks under
continuous light following cold treatment.

Flowering percentages for ‘Golden Scepter’ and ‘Golden Tiara’ increased in
1998-1999. Flowering of both clones occurred under 214-h photoperiods prior to cold

and under 213-h photoperiods following cold (Figures 15A, E; 18A, E). Essentially all

85

 

noncooled and cooled ‘Golden Scepter’ plants ﬂowered under 216 h and NI (Figures
15A, E). Time to ﬂower of cooled ‘Golden Scepter’ plants decreased 2 to 3 weeks under
these photoperiods due to a decrease in time to visible bud (Table 4). Flowering
percentage of noncooled ‘Golden Tiara’ plants varied for photoperiods 214 h and N1, and
essentially all cooled plants ﬂowered under photoperiods 2 13 h and NI (Figure 17E).
Time to ﬂower of ‘Golden Tiara’ plants decreased 6 to 7 weeks under photoperiods 216 h
following cold (Figure 18B, F). Uniform ﬂowering of cooled ‘Golden Tiara’ plants

occurred in 9 to 10 weeks for 213-h photoperiods (Figure 18E; Table 4).

Reproductive characteristics. Neither cold nor photoperiod consistently affected ﬂower
number or inﬂorescence height (Table 5). Flower number and inﬂorescence height were
highly variable for H. montana, H, plantaginea, ‘Tokudama’ gold, and ‘Tokudama’ green
plants following cold in 1998-1999. Flower number and inﬂorescence height of H
plantaginea decreased following cold in 1998-1999 (Figures 11C, D, G, H) while ‘Royal
Standard’ ﬂower number and inﬂorescence height were slightly greater for cooled plants
than for noncooled plants (Figures 12C, D, G, H). Flower number and inﬂorescence
height did not vary with photoperiod for H. plantaginea or ‘Royal Standard’ plants (Table
5). Flower number and inﬂorescence height of noncooled ‘Lancifolia’ plants were greater
than cooled plants and decreased from 14 h to 24 h following cold in 1998-1999 (Figure
13C, D, G, H). Flower number and inﬂorescence height of ‘Golden Tiara’,

‘Hyacinthina’, H. montana, and ‘Undulata’ plants were not affected by cold treatment or

photoperiod in either year (Table 5).

86

DISCUSSION

Hosta species are herbaceous perennials native to the temperate regions of China,
Japan, and Korea (Schmid, 1991). They are long-day plants that naturally ﬂower as the
daylength increases in early-to-mid summer. Most hostas require a period of cold
temperatures to break dormancy and long-day photoperiods for ﬂowering. Each hosta
clone in this study was previously determined to require 0, 3, or 6 weeks of cold for
complete emergence (Fausey, 1999). The minimum duration of cold for emergence was
required for uniform growth of all plants followed by ﬂowering under long-day
photoperiods.

Emergence of dormant buds was low under all photoperiod treatments prior to
cold, and ﬂowering responses of emerged plants were variable. Noncooled plants that
emerged in both years may not have been fully dormant at the start of forcing. Hosta
leaves were cut back to the crown prior to ﬁrst frost after plants exhibited signs of leaf
senescence. Further exposure to natural fall and winter conditions might have driven
plants into a more-fully-dormant state where emergence would only occur following
exposure to cold temperatures. Fifteen weeks of 5°C generally decreased time to ﬂower
and improved ﬂowering percentage of hostas in this study. Cooled plants were taller,
more vigorous, and typically had larger leaves than noncooled plants.

Average leaf size and leaf number were smaller under SD than LD for noncooled
plants but did not vary appreciably following cold. Cooled plants emerged in response to
the greenhouse temperature, and plants produced an initial ﬂush of leaves irrespective of
photoperiod following cold. Plants were not initially receptive to photoperiodic stimuli

following emergence although prolonged exposure (2 6 weeks) to short-day photoperiods

87

appeared to induce vegetative dormancy of all selections. Emergence in response to
temperature is important for long-day plants that emerge early in the spring when short
photoperiods might inhibit growth. Hostas naturally emerge in mid-to-late March and
April in the southern United States (approximately 33°N) and in late April and May in the
northern United States (approximately 42°N) (Solberg, 1988). The daylength including
civil twilight as perceived by most plants ranges from 10 to 16 h at 40°N and from 11 to
15 h at 30°N throughout the year (Roberts and Summerﬁeld, 1987). These latitudes
closely correspond to the native habitat range of Hosta species. Consequently, hostas
may be exposed to photoperiods _<_13 h upon emergence, yet the daylength would be
sufﬁciently long to promote leaf production and induce ﬂowering when plants become
receptive to photoperiodic stimuli.

Some H. montana and ‘Undulata’ plants exhibited episodic growth patterns under
long—day photoperiods. Leaf production ceased for an undetermined period of time. In
many cases, these plants did not ﬂower in the 105-day forcing time but might have
ﬂowered if grown for longer durations under long days. The mechanisms underlying this
with-in season dormancy are unclear, although many hostas experience growth ﬂushes in
the natural environment (Pollock, 1997).

All hosta clones responded as obligate long-day-plants for ﬂowering before and
following cold in both years. There was a clear division between photoperiods which
induced dormancy and those which allowed for continual growth and ﬂowering.
Essentially all clones remained vegetative and then went dormant under short-day
photoperiods (_<_13 h) with or without a cold treatment. Several clones had irregular

ﬂowering of single plants under photoperiods 513 h following cold, but the remaining

88

plants in the treatment went dormant. Flower buds may have been initiated in these
plants prior to the start of the experiment. Only ‘Golden Scepter’ and ‘Golden Tiara’ had
a high percentage of ﬂowering plants under 13 h in 1998-1999. Emerged plants grown
under 214 h and NI generally produced new vegetative growth and ﬂowered with or
without a cold treatment, although 100% ﬂowering of each clone was not always
achieved during the 15-week forcing period.

Complete ﬂowering of many clones did not occur as ﬂowering of Hosta appears
to be inﬂuenced by plant size and crown maturity. Flowering percentages of single-eye
divisions were low for noncooled and cooled plants in 1997-1998 and increased in 1998-
1999. All plants had larger leaves in the second year. However, not all bulked plants
ﬂowered in 1998-1999 despite larger shoot sizes and a generally larger plant with
multiple growing points. One- or two-year-old hosta divisions are considered immature
and attain a mature leaf size and shape after completing six growing seasons (Schmid,
1991). Thus, nonﬂowering plants grown under inductive conditions in this experiment
would be considered juvenile and physiologically incapable of ﬂowering. Photoperiods
which induced ﬂowering of mature plants would be expected to induce ﬂowering of
juvenile plants upon reaching maturity.

For obligate photoperiodic responses, the photoperiod can be described as a base,
transitional, or critical photoperiod according to it’s effect on plant growth and
development. The base photoperiod is the photoperiod where plants remain vegetative
and at which progress towards ﬂowering is zero (Roberts and Summerﬁeld, 1987).
Hostas clones grown under base photoperiods produced an initial ﬂush of leaves then

entered a vegetatively dormant state. Photoperiods that induce only a percentage of

89

mature plants to ﬂower while other plants in the population remain vegetative are called
transitional photoperiods (Runkle et al., 1998). The critical photoperiod marks the
transition from vegetative growth to reproductive growth in obligate plants. The critical
photoperiod for hosta clones in this experiment is identiﬁed as the photoperiod where
mature plants are induced to ﬂower and juvenile plants remain actively vegetative and not
dormant.

The response of Hosta to cold and photoperiod treatments varied between years
with larger plants yielding more uniform growth and ﬂowering following cold in 1998-
1999. Therefore, the base and critical photoperiods are deﬁned for cooled plants in 1998-
1999 only. The base photoperiod was 12 h for ‘Golden Tiara’, ‘Golden Scepter’, and
‘Tokudama’ green plants; and 13 h for ‘Hyacinthina’, ‘Lancifolia’, H. montana, H.
plantaginea, ‘Royal Standard’, Tokudama’ gold, and ‘Undulata’ plants following cold in
1998-1999.

The critical photoperiod varied between 13 and 14 h for all cooled hosta clones in
1998-1999. The critical photoperiod was 213 h for cooled ‘Golden Scepter’, ‘Golden
Tiara’, and ‘Tokudama’ green plants with a high percentage of ﬂowering in 9 to 10
weeks. H. montana, ‘Tokudama’ gold, and ‘Undulata’ plants also ﬂowered in 9 to 10
weeks; ‘Hyacinthina’ plants ﬂowered in 11 to 12 weeks; ‘Lancifolia’ and H. plantaginea
plants ﬂowered in 14 to 15 weeks; and ‘Royal Standard’ plants ﬂowered in 15 tol6 weeks
under 214 h following cold. The highest ﬂowering percentages and lowest time to ﬂower
generally occurred under photoperiods 216 h and NI. Time to visible bud generally did
not change with increasing photoperiod.

Flowering of Hosta varies with latitude as photoperiod varies. Consequently, the

90

ﬂowering season for hostas is generally divided into early, mid, mid-to-late, and late
ﬂowering periods (Grenfell, 1996). H. montana and ‘Tokudama’ naturally ﬂower in
early-to-mid summer; ‘Golden Scepter’, ‘Golden Tiara’, ‘Hyacinthina’, and ‘Undulata’
naturally ﬂower in mid summer; and ‘Lancifolia’, ‘Royal Standard’, and H. plantaginea
naturally ﬂower in mid-to-late summer (Grenfell, 1996). The early-to-mid and mid
season ﬂowering clones are induced to ﬂower under 213-h or 214-h photoperiods .1

ﬂowered in 10 to 12 weeks. However, the mid-to-late season clones ﬂowered under 214-

 

h photoperiods, and ﬂowering of these plants took 3 to 6 weeks longer.

tWAﬁ‘M '6‘“. - ._1 8A--

This experiment indicates that ﬂowering of small hosta plants is undesirable as
complete, rapid, and uniform ﬂowering of these plants may not occur. Cooled plants will
produce a vigorous ﬂush of growth under all photoperiods. However, hosta clones
should be grown under long-day (214 h and NI) photoperiods because plants will
eventually go dormant under short-day (513 h) photoperiods. Photoperiods 513 h may
limit the growth of Hosta in lower latitudes where short days during the growing season
would induce dormancy and a lack of cold winter temperatures would ultimately prevent
the breaking of crown dormancy and long-term plant survival.

For greenhouse production of hostas, long-day lighting 214 h should be provided
when the natural daylength is 514 h in late winter or early spring. The photoperiod for
complete, rapid ﬂowering varies with genotype, but is generally 216 h or NI. Foliage
plants of salable size can be produced in approximately six weeks at 20°C. The forcing

of hostas to ﬂower will take 3 to 12 weeks longer depending upon the genotype grown.

91

REFERENCES

Fausey, B. A. 1999. The inﬂuence of cold treatment, photoperiod, and forcing
temperature on the dormancy, growth, and ﬂowering physiology of Hosta. MS
Thesis, Michigan State University, East Lansing.

F inical, L., A. Cameron, R. Heins, W. Carlson, and C. Whitman. 1997. Inﬂuence of cold
treatment and photoperiod on development and ﬂowering of Hosta. The Hosta
Journal 28(2):88-89.

F inical, L. A., A. Franc, P. Koreman, A. Cameron, R. D. Heins, and W. Carlson. 1998a.
Forcing Perennials: Geranium dalmaticum. Greenhouse Grower. June: 69-72.

Finical, L. A., A. Cameron, R. D. Heins, W. Carlson, and K. Kern. 1998b. Forcing
Perennials: Gaura lindheimeri ‘Whirling Butterﬂies’. Greenhouse Grower. July:
121-124.

Grenfell, D. 1996. The gardener’s guide to growing hostas. Timber Press, Portland, OR.

Iversen, R. R. and T. C. Weiler. 1994. Strategies to force ﬂowering of six herbaceous
garden perennials. HortTechnology. 4(1):61-65.

Lang, A. 1965. Physiology of ﬂower initiation in: Encyclopedia of Plant Physiology,
Editor: W. Ruhland. Springer-Verlag, Berlin.

Pollock, W. I. 1997. Second and more leaf ﬂushes. The Hosta Journal. 28(2):33-36.

Rees, A. R. 1987. Environmental and genetic regulation of photoperiodism- a review.
In: J. G. Atherton (ed.). Manipulation of Flowering. Butterworth’s, London.

Roberts, E. and R. Summerﬁeld. 1987. Measurement and prediction of ﬂowering in
annual crops. In: J. G. Atherton (ed.). Manipulation of Flowering.
Butterworth’s, London.

Runkle, E. S., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1999. Photoperiod and
cold treatment regulate ﬂowering of Rudbeckiafulgida ‘Goldsturm.’ HortScience
34(1):55-58.

Runkle, E. S., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1998a. Flowering of
Leucanthemum xsuperbum ‘Snowcap’ in response to photoperiod and cold
treatment. HortScience 33(6): 1003-1006.

Runkle, E. S., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1998b. Flowering of

Phlox paniculata is inﬂuenced by photoperiod and cold treatment. HortScience
33(7): 1 172-1 174.

92

 

Schmid, W. G. 1991. The Genus Hosta -— Giboshi Zoku. Timber Press, Portland, OR.

Solberg, R. M. 1988. The emergence pattern of Hosta: A semi-scientiﬁc survey. The
Hosta Journal: 19(1):24-26.

Thomas, B. and D. Vince-Prue. 1997. Photoperiodism in plants. Academic Press, San
Diego, CA.

Whitman, C. M., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1996. Cold

treatments, photoperiod, and forcing temperature inﬂuence ﬂowering of
Lavandula angustifolia. HortScience 31(7):]150-1153.

93

 

 

 

 

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Figure 1. Percent emergence (A, E), plant height (B, F), average nunber of leaves per shoot
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Figure 2. Percent emergence (A, E), plant height (B, F), average number of leaves per
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Figure 3. Percent emergence (A, E), plant height (B, F), average number of leaves per shoot
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Figure 4. Percent emergence (A, E), plant height (B, F), average number of leaves per shoot
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(O, I) or 15 (O. 0) weeks of 5°C cold in year one and year two. Error bars are 95%
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Figure 5. Percent emergence (A, E), plant height (B, F), average number of leaves per shoot
(C, G), and average leaf size (D, H) of Hosta ‘Golden Scepter' under various photoperiods
after 0 (O, I) or 15 (O, 0) weeks of 5°C cold in year one and year two. Error bars are 95%
conﬁdence intervals and are offset to the right of yeartwo data points for clarity. L=inear,
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Figure 6. Percent emergence (A, E), plant height (B, F), average number of leaves per shoot
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Figure 7. Percent emergence (A, E), plant height (B, F), average number of leaves per
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Figure 8. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘Golden Tiara' under various
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Figure 9. Percent emergence (A, E), plant height (B, F), average number of leaves per shoot
(C, G), and average leaf size (D, H) of Hosta ‘Tokudama’ gold under various photoperiods
after 0 (O, I) or 15 (O, 0) weeks of 5°C cold in year one and year two. Error bars are 95%
conﬁdence intervals and are offset to the right of yeartwo data points for clarity. L=inear,
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Figure 10. Percent emergence (A, E), plant height (B, F), average number of leaves per
shoot (C, G), and average leaf size (D, H) of Hosta ‘T okudama’ green under various
photoperiods alter 0 (O, I) or 15 (O, D) weeks of 5°C cold in year one and yeartwo. Error
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Figure 11. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C, G),
and inﬂorescence height (D, H) of Hosta plantaginea under various photoperiods after 0 (O,
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Figure 12. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C, G),
and inﬂorescence height (D, H) of Hosta ‘Royal Standard’under various photoperiods aﬂer 0
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Figure 13. Percent flowering (A, E), number of days to ﬂower (8, F), ﬂower number (C, G),
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and are offset to the right of year two data points for clarity. L=linear; Q=quadratic trends.
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Figure 14. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C, G),
and inﬂorescence height (D, H) of Hosta‘Hyacinthina’ under various photoperiods after 0 (O,
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Figure 15. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C, G),
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Figure 16. Percent ﬂowering (A, E), number of days to ﬂower (B, F), flower number (C, G),
and inﬂorescence height (D, H) of Hosta montana under various photoperiods after 0 (O, I)
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Photoperiod

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G), and inflorescence height (D, H) of Hosta ‘Undulata’ under various photoperiodaﬁer O
(O, I) or 15 (O, D) weeks of BC cold in year one and year two. Error bars are 95%
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115

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116

 

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Figure 19. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C, G),
and inﬂorescence height (D, H) of Hosta ‘Tokudama’ gold under various photopeIiods after
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Figure 20. Percent ﬂowering (A, E), number of days to ﬂower (B, F), ﬂower number (C, G),
and inﬂorescence height (D, H) of Hosta ‘Tokudama’ green under various photoperiods after
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respectively .

118

 

SECTION IV

THE EFFECTS OF F ORCING TEMPERATURE FOLLOWING COLD

TREATMENT ON GROWTH AND FLOWERING OF HOS TA.

119

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The effects of forcing temperature following cold treatment on growth and

ﬂowering of Hosta.

Beth A. Fausey‘, Royal D. Heins’, and Arthur C. Cameron2

Department of Horticulture, Michigan State University, East Lansing, MI 48824-1325

Received for pulication . Accepted for publication . We gratefully
acknowledge the ﬁnancial support of the Agricultural Experiment Station of Michigan
State University and funding by hosta producers and greenhouse growers supportive of
Michigan State University ﬂoricultural research.

' Graduate Student

2 Professor

Production and Culture

120

.1“. a?

 

 

The effects of forcing temperature following cold treatment on growth and ﬂowering of

Hosta.

Additional index words. Herbaceous perennial, base temperature, optimum temperature,

plantain lily.

Abstract. The effects of forcing temperature following cold treatment on plant
appearance and time to ﬂower were evaluated for Hosta clones. H. montana, H.
plantaginea, H. ‘Francee’, H. ‘Golden Scepter’, H. ‘Golden Tiara’, H. ’Hyacinthina’, H.
‘Lancifolia’, H. ‘Royal Standard’, H. ‘Tokudama’ gold, H. ‘Tokudama’ green, and H.
‘Undulata’ plants were exposed to 5°C for 15 weeks. Plants were grown in greenhouse
sections set at 14, 17, 20, 23, 26, or 29°C for 15 weeks with l6-h day-extension lighting.
Days to visible bud W B), days to anthesis (FLW), and days from VB to FLW decreased
as temperature increased. The rate of progress toward visible bud and ﬂowering
increased as temperature increased for each clone. The base temperature and cumulative
growing degree days for each hosta clone were calculated for each developmental stage.
Plant height and average leaf size generally decreased as temperature increased from 14

to 29°C. Plant quality was greatest for plants grown at 523°C.

121

 

 

INTRODUCTION
Although photoperiod and vemalization typically control ﬂower induction in
herbaceous perennials, many developmental processes such as ﬂower timing are
temperature-dependent. Temperature directly determines the timing of ﬂoriculture crops

by affecting the rate of plant development. The rate of development generally increases

 

linearly with temperature until a maximum rate is achieved at an optimum temperature
(Roberts and Summerﬁeld, 1987). The general relationship between temperature and the

rate of development toward ﬂowering is measured by taking the reciprocal of days to

 

ﬂower (l/DTF). The linear relationship between the rate of progress towards ﬂowering 1:
and the mean temperature T(°C) can be described as follows:
1/DTF=bo+b,*T (1)
where b0 (intercept) and bI (slope) are species-speciﬁc constants. Both the base
temperature and the cumulative thermal time (°days) required for ﬂowering can be

calculated from the constants in Equation 1:

 

Tb = -bo/b, (2)

°days = 1/bl (3)
The base temperature (Tb) is the temperature at, or below which, the rate of progress
toward ﬂowering is zero (Roberts and Summerﬁeld, 1987). Cumulative thermal time
represents the number of thermal units above the base temperature required for ﬂowering.

Forcing temperature not only affects ﬂower timing but inﬂuences plant quality

and appearance. Temperatures above 26°C reduced ﬂower quality of Echinacea purpurea
‘Bravado’ and Campanula ‘Birch Hybrid’ (Finical et al., 1998a; Finical et al., 1998b).

Flower-bud number for Rudbeckiafulgida ‘Goldsturm’ decreased 75% and ﬂower

122

diameter decreased 2.7 cm as temperature increased from 16°C to 26°C (Yuan et al.,

 

1998). Temperature also inﬂuenced plant height ofAchiIlea millefolium ‘Summer
Pastels’, Coreopsis grandiﬂora ‘Sunray’, and Leucanthemum xsuperbum ‘Snowcap’ as
plants grown at 26°C were shorter than plants grown at 18°C (Zhang et al., 1996; Yuan et
al,1998)

Hostas are perennial shade plants commercially grown for their ornamental
foliage. Limited information exists on the response of Hosta to temperature. Nau (1996)

reported that divisions grown at 14°C night temperatures and 22 to 29°C day

 

temperatures will have 8 leaves and be ready for sale in 7 to 9 weeks. Temperatures

III.‘ 2' 7‘
13

above 35°C reportedly promote heat-dormancy of hosta and may be responsible for heat-
effected Viridescence, the abrupt change of white or gold leaves to green in the summer
(Pollack, 1997; Solberg, 1997). Therefore, the objective of this study was to characterize
the vegetative responses of hosta clones to different temperatures and identify the

relationship between temperature and time to ﬂower.

MATERIALS AND METHODS
Plant material. 1997-1998. Hostas were received from commercial producers (Table 1)
in fall 1997 and were separated into single-eye divisions. Divisions were placed upright
into 23 x 15 x 7 cm (6.8 L) bulb crates ﬁlled with a commercial soilless medium
composed of composted pine bark, vermiculite, Canadian sphagnurn peat, and coarse
perlite with a wetting agent, lime, and starter fertilizer charge (High Porosity Mix, Strong-
Lite Products, Pine Bluff, AR). Bulb crates were placed in a cooler set at 5°C for 12

weeks. Bulb crates were watered as needed with well water acidiﬁed with citric acid to a

123

pH of 6.0. Divisions were removed from the bulb crates after 12 weeks of cold, planted
into l3-cm square containers, and placed in one of 6 greenhouses set to 14, 17, 20, 23, 26,
or 29°C. Plants received a 16—h photoperiod provided by four-hundred-watt high-
pressure sodium (HPS) lamps from 0500 to 0700 and from 1700 to 2300 that delivered
approximately 90 umolm‘2 s". HPS lamps provided a photosynthetic photon ﬂux (PPF)
of 100 umol'm'2 s‘l when the ambient greenhouse PPF dropped below 200 1.1mol-m‘2°s‘1
from 800 to 1700. Supplemental lighting was terminated when PPF exceeded 400
umol-m'2 3". Greenhouse benches were covered with 50% aluminum shade cloth in
1997-1998 (LS Americas, Charlotte, NC). A vapor pressure deﬁcit (V PD) of ~ 0.7 kPa
was maintained by steam injection into greenhouse sections.

1998-1999. Hostas from the 1997 experiments were grown outdoors from May 15 to
October 16, 1998 in l3-cm (1.1 L) square containers under 50% shade created by
alternate strips of wood lath at the Michigan State University Horticultural Teaching and
Research Center, East Lansing, MI. Plants displayed visible signs of dormancy (leaf
senescence) and had the foliage removed prior to ﬁrst frost on October 16, 1998. Pots
were immediately placed in a cooler set at 5°C without supplemental lighting for 15
weeks. Multiple-eye ‘Golden Scepter’ divisions were received in the fall of 1998 and
placed in bulb crates as previously described in 1997-1998. Single-eye Hosta ‘Royal
Standard’ divisions were taken from 3 year-old crowns previously grown in 8-cm (350
ml) containers following cold treatment in the spring of 1998. Experimental conditions
were as previously described in 1997-1998 although no shade cloth was provided in

1998-1999.

124

 

 

 

General Procedures. Plants were fertilized at every irrigation with a nutrient solution of
well water (EC of 0.70 mS°cm‘l and 105, 35, and 23 mg‘L" Ca, Mg, and S, respectively)
acidiﬁed with H280, to a titratable alkalinity of 130 mg'L'l CaCO3 and water soluble
fertilizer providing 125-12-125-13 N-P-K-Ca mg'L'l (30% ammonical N) plus 1.0-0.5-

0.5-0.5-O.1-0.1 (Fe, Mn, Zn, Cu, B, Mo) mg'L" (MSU Special, Greencare Fertilizers,

 

Chicago, IL). 1
Greenhouse air temperature was monitored on each bench with 36-gauge type B
thermocouples connected to a CR-lO datalogger (Campbell, Scientiﬁc, Logan, UT).

Temperatures were collected every 10 seconds and the hourly average recorded. In 1998,

 

the average daily temperature from start of the greenhouse forcing to ﬂower for each
clone was calculated for each forcing temperature (Table 2).

Experiments were conducted in the Plant and Soil Sciences Research Greenhouses
at Michigan State University, East Lansing, MI. Each experiment was a completely
randomized design with ten replications. All plants were forced under the speciﬁed

treatments for ﬁfteen weeks.

Data collection and analysis. Plant responses are reported in accordance with the
treatment setpoint temperature as the actual average temperature from forcing to ﬂower
varied for each hosta clone (Table 2). Emergence and ﬂowering percentages were
calculated for each clone in both years. Flowering percentages were calculated as the
number of ﬂowering plants divided by the number of emerged plants in each treatment.
The date of visible ﬂower bud was collected for all reproductive plants; the date of ﬂower

anthesis, plant height (cm), inﬂorescence height (cm), ﬂower number, scape leaf number,

125

leaf number, and shoot number were collected when the ﬁrst ﬂower opened. Plant height,
leaf number, and shoot number were collected for nonreproductive plants ﬁfteen weeks
after forcing. Leaf area was taken on all plants ﬁﬁeen weeks after the start of forcing
with a LI-300 portable leaf area meter (LI-COR, Lincoln, NB).

Days to visible bud, days to ﬂower, and days from visible bud to ﬂower were
calculated for all reproductive plants, and no data were available for these parameters on
nonﬂowering plants. Data were analyzed using SAS’s (SAS Institute, Cary, NC) analysis
of variance (ANOVA) and general linear models (GLM) procedures.

All hostas exhibited a linear relationship between temperature and the rate of
progress toward VB, from VB to FLW, and toward F LW up to an optimum temperature
(Tom) (F iguresl -1 lD-F). The optimum temperature for ﬂowering of Hosta appeared to be
23°C, except for H. montana (20°C), ‘Tokudama’ green (20°C), H. plantaginea (26°C),
and ‘Royal Standard’(26°C). The decision to include data points beyond these
temperatures became arbitrary, and they were excluded from regression analysis. Slopes
and intercepts of regression equations for the different developmental stages were
calculated using SAS REG procedure. Base temperatures and cumulative thermal time
were calculated for all clones according to Equations 2 and 3 (Roberts and Summerﬁeld,

1987)

RESULTS AND DISCUSSION
To accurately predict ﬂower timing, all plants in a population must be of a similar
developmental stage at the start of forcing (Heins et al., 1997). Flowering of Hosta was

previously demonstrated to be inﬂuenced by plant size and crown maturity (Fausey,

126

 

 

 

1999). Hostas forced under temperature treatments in 1998-1999 were larger and yielded
more uniform vegetative and ﬂowering responses than single-eye divisions forced in
1997-1998 (Table 3, 4, 5). Therefore, rates of development and plant characteristics in
response to temperature are based upon 1998-1999 experimental data.

Flowering percentages of all clones under forcing temperatures were generally
higher in 1998-1999 than in 1997-1998 (Table 3, 4). The lowest ﬂowering percentage for
each clone in 1998-1999 occurred at the low (14°C) or high (29°C) temperature extreme.
However, plants grown at 14°C developed at a slower rate and may have eventually
ﬂowered if grown beyond the 105-day forcing period of this experiment. Flower
development of plants grown at 29°C is unlikely beyond the 15-week forcing period as
growth was reduced by high temperatures. Essentially all ‘Francee’, ‘Golden Scepter’,
‘Golden Tiara’, ‘Hyacinthina’, ‘Lancifolia’, ‘Tokudama’ gold, and ‘Undulata’ plants
ﬂowered under temperatures between 17°C and 26°C (Table 4). Flowering of H
montana and ‘Tokudama’ green plants decreased as temperatures increased above 20°C.
Flowering of H. plantaginea and ‘Royal Standard’ plants was highest at 23 and 26°C but
varied greatly for other temperatures.

H. plantaginea requires long growing seasons in temperate climates as it is A
presumably native to the Zheij ing province or provinces further south in China (Schmid,
1991). These areas are located between 20°N and 30°N latitude (similar to Houston, TX
and New Orleans, LA) and have humid, subtropical climates (Graves, 1992). No H.
plantaginea plants ﬂowered under 14°C after 15 weeks of forcing, but all plants ﬂowered
when grown at 26°C (Table 4).

Hosta shoots emerge in response to the root zone temperature, and emergence

127

 

 

 

may only occur when a clonal-speciﬁc minimum base temperature is exceeded. The base
temperature for emergence could not be identiﬁed for the range of temperatures examined
in this experiment. The number of days to emergence signiﬁcantly decreased as
temperature increased for about half of the clones (Table 6). Time to emergence
decreased 10 days for ‘Tokudama’ gold plants, 6 days for ‘Tokudama’ green and
‘F rancee’ plants, and 5 days for ‘Hyacinthina’ plants as temperature increased from 14 to F -
26°C (Table 6).

Days to visible bud (VB), days to ﬂower (F LW), and days from VB to FLW for

all clones decreased as temperature increased from 14°C to _>_23°C (Table 2, 6).

 

Flowering accelerated more rapidly when temperature increased from 14°C to 20°C than
from 20°C to 29°C (Table 2). For all hostas except H. plantaginea, ‘Royal Standard’,
and ‘Undulata’ increasing the temperature from 20°C to 29°C resulted in an increase in
days to VB and in days to FLW (Table 2, 6). Increasing the temperature from 20°C to
26°C also increased the time to VB and time to FLW for H. montana plants (Table 2, 6).

Flowering uniformity varied with temperature and genotype (Figures 1-11A-C).
Variability in time to VB and FLW increased for ‘Francee’ and H. montana plants as the
temperature increased from 14°C to 26°C (Figures 1A, C; 3A, C). Days from VB to
F LW were generally uniform for all hosta clones except ‘Tokudama’ green (Figures 1-
113). However, the variability in time to VB and F LW, and from VB to FLW of
‘Tokudama’ green plants can in part be explained by a smaller sample size of 3 to 5
plants per treatment (Figure 2A-C).

The parameters of linear regression equations relating forcing temperature to plant

development are presented in Table 7. There was a signiﬁcant linear relationship

128

 

between temperature and the rate of progress toward VB, from VB to F LW, and toward
FLW for most hostas (Table 7). However, the rates of progress toward VB and F LW for
‘Royal Standard’, toward VB for ‘Undulata’, and toward FLW for ‘Tokudama’ green
were not signiﬁcant (Table 7).

The reciprocal of the linear regression line was plotted against original data

 

points. The regression line ﬁt well with the original data for all clones except ‘Royal
Standard’, ‘Tokudama’ green, and ‘Undulata’ (Figures 1-1 1A, B, C). The relationship
between forcing temperature and the rate of progress toward visible bud and ﬂowering

were weak for these cultivars. However, the original data did ﬁt the reciprocal of the

 

linear regression line for the rate of progress from visible bud to ﬂowering well.

The base temperature and the cumulative thermal time required for each
developmental process can be used to predict ﬂowering in greenhouse environments
where temperatures ﬂuctuate by using the average forcing temperature (Tf). The number
of days required to complete a developmental event can be calculated as °days/(TrTb).
The calculated base temperatures of signiﬁcant lines for ﬂowering of hosta clones ranged
from -28.8°C to 8.7°C (Table 7). The cumulative thermal time to ﬂower ranged from 723

units above a base temperature of 8.7°C for H. montana to 3663 thermal units above a

 

base temperature of -28.8°C for ‘Undulata’. The calculated base temperatures and
thermal times required for ﬂowering for most hosta clones were physiologically
unrealistic, and the relationship between time to ﬂower and temperature should be
examined over a wider range of cool temperatures.

Temperature has been shown to impact ﬂoral characteristics of many herbaceous

plant species (Whitman et al., 1997; Yuan et al., 1998). The average number of ﬂowers

129

per inﬂorescence of most clones was not affected by temperature (Table 4). However,
twice as many ﬂowers were formed on ‘Hyacinthina’ and ‘Lancifolia’ plants grown at
14°C than on plants grown at 29°C (Table 4). The lavender color of ‘Golden Scepter’,
‘Golden Tiara’, and ‘Lancifolia’ ﬂowers intensiﬁed at 14°C and faded under temperatures
_>_26°C (personal observation). The inﬂorescence height of all clones at ﬁrst open ﬂower
was greatest at 14°C or 17°C and decreased with increasing temperature (Table 4).

Plant height generally decreased in 1998-1999 with increasing temperature for all
hostas except ‘Tokudama’ gold and ‘Undulata’ (Table 3). ‘Francee’, ‘Hyacinthina’, H.
plantaginea, and ‘Royal Standard’ plants grown at 14°C were 9 to 13 cm taller than
plants grown at 29°C. Differences in plant height of other clones were less. Height of
‘Undulata’ plants increased to a maximum at 26°C then sharply decreased at 29°C.
Leaves of ‘Tokudama’ gold plants exhibited an uncharacteristic vertical orientation when
grown at 29°C which elevated plant height at this temperature.

Gardeners typically plant hostas in shady areas for optimal performance as
temperature and light intensity impact the physical characteristics of Hosta. Hostas
grown in full sunlight are reportedly shorter, have a larger number of leaves per division,
and suffer from marginal leaf burn (Solberg, 1988). Cultivars grown in full sun may also
lose characteristics which make them distinctive, such as a decrease in leaf size, a
narrowing of the leaf shape, or a change in coloration. The alteration in plant habit and
physical characteristics of hostas grown in sunlight may be attributed to an elevated plant
temperature and not necessarily light intensity as leaves exposed to sunlight may reach a
higher temperature than the surrounding air (Serrano et al., 1995). Evidence to support

this hypothesis can be found with the altered leaf color and uncharacteristically small

130

_m _‘ﬂ
‘
I

 

 

 

leaves of hostas grown in deep shade (Solberg, 1988).

Hostas grown at high temperature exhibited several characteristics commonly
observed for plants grown in full sunlight. Temperature affected the average leaf size in
this experiment but did not impact the number of leaves per shoot of most hosta clones
(Table 5). Average leaf size decreased with increasing temperature for all hostas except
‘Golden Scepter’ (Table 5). Average leaf size of H. montana and H. plantaginea
increased as temperature increased from 16°C, then declined when exposed to warmer
temperatures (Table 5).

Temperatures 2 26°C greatly altered plant habit and the physical appearance of all
clones. Plants grown under high temperatures exhibited a rosette growth habit with long,
narrow leaves attached to short petioles. Foliage pigmentation was also adversely
affected by high average daily temperatures. ‘Tokudama’ gold and ‘Golden Scepter’
plants lost their characteristic gold color when grown at 29°C, and all leaves of the white-
variegated ‘Undulata’ and the yellow-variegated ‘Golden Tiara’ became green. The white
marginal variegation of ‘F rancee’ turned yellow and narrowed to the extreme edge of the
leaves when grown at 29°C. The wax bloom which gives ‘Tokudama’ green and
‘Hyacinthina’ their blue or grey hue was lost or did not form under 26°C and 29°C.

Marginal leaf hum of Hosta may be a direct response to water stress of plants with
poorly developed root systems. Shade cloth was raised in the spring of 1998 when
marginal burn was observed on newly planted divisions grown at Z 23°C. Leaf burn was
exacerbated on plants that appeared to be underwatered. Plants grown in 1998-1999 had
well-established root systems at the start of forcing, and fewer incidents of marginal burn

were observed.

131

 

 

 

Hostas are almost exclusively grown for their ornamental foliage. Temperatures
5 23°C produced the highest quality plants with large leaves and vibrantly colored
foliage. Plants of a salable size can be forced in four to six weeks at 20 and 23°C, and in
six to eight weeks at 14 and 17°C. Although plants of some clones ﬂowered faster under
_>_ 26°C temperatures, they exhibited undesirable qualities that would prevent their sale.
The fastest ﬂowering of plants without a reduction in plant quality will occur in 8 to 15

weeks at 20 or 23°C, depending upon the genotype grown.

132

 

 

 

REFERENCES

F ausey, B. A. 1999. The inﬂuence of cold treatment, photoperiod, and forcing
temperature on the dormancy, growth, and ﬂowering physiology of Hosta. MS
Thesis, Michigan State University, East Lansing.

Finical, L. A., A. Franc, P. Koreman, A. Cameron, R. D. Heins, and W. Carlson. 1998a.
Forcing Perennials: Campanula ‘Birch Hybrid’. Greenhouse Grower. August:
121 -124.

Finical, L. A., E. S. Runkle, R. D. Heins, A.C. Cameron, and W. Carlson. 1998b.
Forcing Perennials: Echinacea purpurea ‘Bravado’. Greenhouse Grower.
Septemberz49-52.

Graves, W. 1992. National Geographic Atlas of the World, Revised, 6‘h edition.
National Geographic Society, Washington, DC.

Heins, R. D., A. C. Cameron, W. H. Carlson, E. Runkle, C. Whitman, M. Yuan, C.
Hamaker, B. Engle, and P. Koreman. 1997. Controlled ﬂowering of herbaceous

perennial plants. In: Plant production in closed ecosystems. Kluwer Academic
Publishers, Netherland.

Nau, J. 1996. Ball Perennial Manual: Propagation and production. Ball Publishing,
Batavia, IL, USA.

Roberts, E. and R. Summerﬁeld. 1987. Measurement and prediction of ﬂowering in
annual crops. In: J. G. Atherton (ed.). Manipulation of Flowering.

Butterworth’s, London.

Schmid, W. G. 1991. The Genus Hosta -- Giboshi Zoku. Timber Press, Portland, OR.

Serrano, L., J. A. Pardos, F. I. Pugnaire, and F. Domingo. 1995. Absorption of radiation,
photosynthesis, and biomass production in plants. In: M. Pessarakli (ed.)
Handbook of plant and crop physiology. Marcel Dekker, New York.

Solberg, R. M. 1988. Hostas are shade plants! The Hosta Journal 19(2):54-56.

Solberg, R. M. 1997. Hosta Physiology. The Hosta Journal. 28(2):]11-113.

Whitman, C. M., R. D. Heins, A. C. Cameron, and W. H. Carlson. 1997. Cold treatment
and forcing temperature inﬂuence ﬂowering of Campanula carpatica ‘Blue Clips’.

HortScience 32(5):861-865.

Yuan, M., W. H. Carlson, R. D. Heins, and A. C. Cameron. 1998. Effect of forcing
temperature on time to ﬂower of Coreopsis grandiﬂora, GaiIIardia xgrandiﬂora,

133

 

 

Leucanthemum xsuperbum, and Rudbeckiafulgida. HortScience 33(4):663-667.

Zhang, D., A. M. Arrnitage, J. M. Affolter, and M. A. Dirr. 1996. Environmental control
of ﬂowering and growth of Achillea millefolium L. ‘Summer Pastels’.
HortScience 31(3):364-365.

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APPENDIX A

Development of Hosta ‘Halcyon’ Lateral Buds

154

 

 

Objective: To determine the inﬂuence photoperiod, benzyladenine, and plant orientation have on

development of Hosta ‘Halcyon’ lateral buds.

Plant Material and Culture. F ield-grown Hosta ‘Hacyon’ plants were received from
Twixwood Nursery (Berrien Springs, Michigan) on August 18, 1997. The foliage and ﬂower
stalks had been cut 10 cm above each crown, and all visible lateral buds had been removed.
Eighty uniform plants were potted upright into 13-cm (1.1 L) square containers ﬁlled with a
commercial soilless medium composed of composted pine bark, vermiculite, Canadian sphagnum
peat, and coarse perlite with a wetting agent, lime, and starter fertilizer charge (High Porosity
Mix, Strong-Lite Products, Pine Bluff, AR). Plants were fertilized at every irrigation with a
nutrient solution made from well water (EC of 0.70 mScm" and 105, 35, and 23 mgL‘I Ca, Mg
and S respectively) acidiﬁed with H280, to a titratable alkalinity of approximately 130 mgL'l
CaCO3 and water soluble fertilizer providing 125-12-125-13 N-P-K-Ca mgL'I (30% ammonical
N) plus 1.0-0.5-0.5-0.5-O.1-O.l (Fe, Mn, Zn, Cu, B, Mo) mgL’l (MSU Special, Greencare

Fertilizers, Chicago, IL).

Treatments. Zero or five ml of a SO-ppm benzyladenine (BA) (Accell, Abbott Laboratories,
Chicago, IL) solution were poured onto each hosta crown planted in an upright position.
Twenty-four hours aﬁer BA application, plants were tilted horizontally or remained upright.
Plants were placed under long days (LD) or 9-hour short days (SD) on August 19, 1997, and
remained under these conditions for five weeks. Photoperiods were established by covering
plants with opaque black cloth from 1700 to 0800 to provide 9-hr of natural light. Sixty-watt
incandescent lights delivered 3 to 5 pmol m'zs‘l from 2200 to 0200 to simulate long days. Four-
hundred-watt high-pressure sodium (HPS) lamps provided a photosynthetic photon ﬂux (PPF) of

100 nmol-m'2 s’l starting at 0800 and continuing until the outside PPF exceeded 400 umol-m'2 s".

155

 

If the outside PPF then dropped below 200 pmolm'z's" lamps were again turned on until 1700.

Data Collection and Analysis. Lateral shoot number, lateral bud number, and bud length were
evaluated ﬁve weeks after treatment. A lateral bud was recorded when the bud measured at least
2 mm in length. Buds were measured and placed in categories to less than 1 cm, equal to 1 cm,
or greater than 1 cm in length. A lateral shoot was recorded when at least one leaf had unfolded
following emergence from a lateral bud. The total number of available buds per plant was
calculated by summing the number of shoots and the number of visible buds present ﬁve weeks
after treatment. The percentage of available buds developing into a shoot and those measuring
greater than 1 cm, equal to 1 cm, and less than 1 cm was determined for each treatment. Data
were arcsin transformed then analyzed using analysis of variance (ANOVA). Results are

presented in Table 1.

Results and Discussion. Benzyladenine is a cytokinin that promotes lateral bud development in
many plants, including Hosta (Garner et al, 1997; Garner et al, 1998; Keever, 1994). Generally,
short days (SD) induce dormancy of Hosta lateral buds and promote lateral bud development,
while long days (LD) promote vegetative grth (Finical et al, 1997).

Benzyladenine had no effect on average bud number or on the percentage of buds that
formed shoots or measured less than, equal to, or greater than 1 cm in length. A low application
rate and reduced volume may account for Hosta ‘Halcyon’s lack of response to BA. However,
BA has been shown to markedly affect the number of lateral shoots produced on a single plant
(Garner et al, 1997); therefore, higher rates of BA at greater volumes might increase the number
of Hosta ‘Halcyon’ lateral shoots produced.

Photoperiod did not affect total bud number for each treatment; yet, photoperiod did

affect the percentage of buds that developed into shoots and the length of remaining buds. Short

156

 

photoperiods promoted lateral bud enlargement but not shoot development. Long photoperiods
promoted shoot development from lateral buds.

Plant orientation affected the percentage of total buds that developed into a lateral shoot
with horizontally-oriented plants producing more shoots than vertically-oriented plants.
Photoperiod, benzyladenine, and plant orientation interacted to affect the percentage of buds that
developed into a shoot. A greater percentage of buds on horizontally-oriented plants with or
without BA application and vertically-oriented plants with a BA application developed into
shoots than other treatments.

Average lateral bud number was unaffected by benzyladenine, plant orientation, or
photoperiod. It is assumed that a predetermined number of latent buds were present on plants
prior to experimentation, therefore total bud number would not be affected by treatment.
However, commercial hosta producers can expect a greater number of shoots to emerge from
plants grown under LD conditions as few shoots developed from the lateral buds of plants grown
under SD conditions despite BA application or plant orientation. A horizontal orientation also
encouraged vegetative growth from lateral buds, and plants placed on their side are expected to
develop more shoots than those planted upright. BA application has been shown to induce
growth from lateral buds, yet the volume and concentration used in this experiment were

insufﬁcient to signiﬁcantly affect lateral bud development and shoot growth.

157

 

 

Table 1. Effect of benzyladenine, plant orientation, and photoperiod on development of Hosta
‘Halcyon’ lateral buds. Plants used in this experiment were dug in mid-August, 1997, foliage
removed to 10 cm above the crown, and all lateral buds 3 5 mm in length removed.

 

 

 

 

 

Average Bud Developmental Stage
number
Photoperioda Orientation BA of buds
< 1 em = 1 cm >1 cm Shoots
(ppm) ( % of total buds)

SD Horizontal O 1 .7 29 12 41 18
50 1.4 14 0 S7 29

Vertical O 2.0 25 0 65 10

50 1.7 47 18 35 0

LD Horizontal 0 l .6 0 O 6 94
50 1.7 12 O 35 53

Vertical 0 1.2 33 8 8 50

50 1.4 7 7 14 71

ANOVAb

BA NS NS NS NS NS

Photoperiod NS NS NS * *

BA*Photo NS NS NS NS NS

Orientation NS NS NS NS *

BA*Orien NS NS NS NS NS
Photo*Oricn NS NS NS NS NS
BA*Photo*Orien NS NS NS NS *

 

' SD: 9-hr photoperiod; LD: 9—hr photoperiod plus 4-hr night interruption.
b NS, Not sigiﬁcant; *, signiﬁcant at P<0.05.

158

 

References

Finical, L., A. Cameron, R. Heins, W. Carlson, and C. Whitman. 1997. Inﬂuence of cold
treatment and photoperiod on development and ﬂowering of Hosta. The Hosta Journal
28(2):88-89.

Garner, J. M., G. J. Keever, D. J. Eakes, and J. R. Kessler. 1997. Benzyladenine-induced offset
formation in Hosta dependent on cultivar. HortScience 32(1):91-93.

Garner, J. M., G. J. Keever, D. J. Eakes, and J. R. Kessler. 1998. Sequential BA applications
enhance offset formation in Hosta. HortScience 33(4):707-709.

Keever, G. J. 1994. BA-induced offset formation in Hosta. J. Environ. Hort. 12(1):36-39.

159

 

APPENDIX B

Fall-Induced Dormancy of Hosta ‘Royal Standard’

160

 

Objective: To determine temperature, photoperiod, and foliage removal effects on dormancy of

Hosta ‘Royal Standard’.

Plant Material and Culture. Container-grown Hosta ‘Royal Standard’ plants were received from
Twixwood Nursery (Berrien Springs, Michigan) on August 18, 1997. Plants were fertilized at
every irrigation with a nutrient solution made from well water (EC of 0.70 mScm‘l and 105, 35,
and 23 mgL’l Ca, Mg and S respectively) acidiﬁed with H280, to a titratable alkalinity of
approximately 130 mgL'l CaCO3 and water soluble fertilizer providing 125-12-125-13 N-P-K—Ca
mgL‘l (30% ammonical N) plus 1.0-0.5-0.5-O.5-0.l-O.1 (Fe, Mn, Zn, Cu, B, Mo) mgL‘1 (MSU

Special, Greencare Fertilizers, Chicago, IL).

Treatments. Plants were initially placed in a 20°C greenhouse or moved outside and exposed to
natural temperatures (Figure 1). Plants received natural photoperiods (ND), long photoperiods
(LD), or short photoperiods (SD) for six weeks beginning August 22, 1997. SD and LD
photoperiods were established by covering plants with opaque black cloth from 1700 to 0800 so
plants received 9-hr of natural light. Long days were simulated with 60-W incandescent bulbs
delivering 3 to 5 umol m'2 s'1 from 2200 to 0200. Plants receiving natural photoperiods were
exposed to 13 hours 36 minutes of light in August declining to 11 hours 55 minutes of light in
October as determined by sunrise and sunset. F our-hundred-watt high-pressure sodium (HPS)
lamps provided supplemental lighting with a photosynthetic photon ﬂux (PPF) of 100 pmolm'2 s"
starting at 0800 and continuing until the outside PPF exceeded 400 umol'm'2 s". If the outside
PPF then dropped below 200 pmol-m'z-s'l lamps were again turned on until 1700. Foliage was
removed 10 cm above the crown 2, 4, or 6 weeks after the start of photoperiod treatments, and
plants were transferred to a 20°C greenhouse with LD night-interruption lighting to ensure

continued growth of nondorrnant plants. Foliage was not removed from control plants.

161

 

Data Collection and Analysis. Shoot emergence, shoot number, lateral bud number, and bud
weight were recorded ﬁve weeks after foliage removal for half the plants. Data on the remaining
plants were recorded eleven weeks after foliage removal. Preliminary analyses revealed no
signiﬁcant differences between collection timings, and data were combined. Data were analyzed
using analysis of covariance (ANCOVA) and treatment means are presented in Table 1. Shoot
number, bud number, and bud weight treatment means were adjusted to a covariate, whether the
plant ﬂowered prior to photoperiod treatments (ﬂowering shoot). Treatment means were adjusted
using the linear model y”. = y,- +ﬂ(x,j.- x”) + ea. where ,u, is the treatment mean, ,8 is the slope for the
linear regression, x”. is the covariate for the jth plant in the ith treatment, x__ is the average covariate

value, and e”. is the experimental error. Statistical analysis is presented in Table 2.

Results and Discussion. Hosta ‘Royal Standard’ shoot number, lateral bud number, and lateral
bud- weight were dependent on whether the plant was reproductive before photoperiod treatments.
Reproductive plants produced 1.3 more shoots, 1.3 more lateral buds, and buds that weighed 0.7
more grams than those of nonreproductive plants. In Hosta, ﬂowering releases apical dominance
of lateral buds that have the potential to develop into new shoots under favorable environmental
conditions.

The hosta leaf appears to be an additional source of growth inhibition with foliage removal
releasing the correlative inhibition of lateral buds by the leaves. The time of foliage removal after
photoperiod treatment affected Hosta ‘Royal Standard’ shoot number, lateral bud number, and
lateral bud weight. Lateral bud outgrowth was suppressed in control plants with intact leaves.
compared to plants where leaves were removed. Consequently, the average bud number and bud
weight of control plants and plants with foliage removal after six weeks was greater than two or
four week treatments since buds did not develop into shoots. The average bud weight was also

greater for control plants than for those cut back after six weeks. The inhibition of lateral bud

162

 

outgrowth resulted in control plants developing larger buds over the duration of the experiment
with a smaller proportion of those buds developing into lateral shoots.

Clearly, the presence of leaves greatly inhibits the outgrth of lateral buds into shoots. It
is unclear from this experiment at what point Hosta species and cultivars enter dormancy in the
fall, and if the dormant state can be reversed by photoperiod or temperature. Hosta ‘Royal
Standard’ plants in this experiment did not show signs of dormancy after six weeks of controlled
photoperiods and declining temeperatures. Hosta ‘Royal Standard’ is a hybrid cross of Hosta
plantaginea and Hosta sieboldiana (Schmid, 1991). Hosta plantaginea and ‘Royal Standard’ do
not require cold temperatures to break dormancy (Fausey, 1999). If the plants were entering a
dormant state from experimental treatments, the subsequent long day photoperiod or greenhouse
temperatures were sufﬁcient stimuli to promote growth. However, cultivars that require cold to
overcome dormancy would be expected to show true signs of dormancy, namely few lateral shoots

emerging from buds upon transfer from natural to greenhouse conditions.

References
F ausey, B. A. 1999. The inﬂuence of cold treatment, photoperiod, and forcing
temperature on the dormancy, growth, and ﬂowering physiology of selected Hosta

species and cultivars. MS Thesis, Michigan State University, East Lansing.

Schmid, W. G. 1991. The Genus Hosta -- Giboshi Zoku. Timber Press, Portland, OR.

163

 

Table I. Fall-induced dormancy of Hosta ‘Royal Standard’. Plants were grown outdoors and in a
20°C greenhouse and were exposed to SD, ND, or LD photoperiods beginning August 22, 1997 for
six weeks. Foliage was removed from plants at two-week intervals from the start of the
experiment, and plants were transferred to a 20°C greenhouse with LD lighting for six weeks. The
number of shoots and buds, and the weight of the buds were determined for each plant.

 

 

Time of foliage Photoperiod' Temperature” Shoots“ Budsc Bud
removal weightc

(wk) (g)
2 SD Natural 1.5 3 .0 0.5

20 1.7 1.8 0.2

ND Natural l .8 2.9 0.4

20 1.8 l .9 0.3

LD Natural 3.0 2.5 0.4

20 2.0 2.3 0.3

4 SD Natural 2.1 3 .7 0.6

20 1.7 2.8 0.5

ND Natural 2.2 3 .1 0.4

20 2.4 2.2 0.3

LD Natural 1 .5 3 .2 0.8

20 l .7 2.2 0.5

6 SD Natural 2.4 3 .4 0.8

20 1 .7 3 .7 l .1

ND Natural 2.5 5.7 1.2

20 l .7 3 .5 0.9

LD Natural 3. l 3 .2 0.9

20 1.9 3 .3 0.7

Control SD Natural l .7 4.4 l .9
20 2.0 4.3 1.6

ND Natural 1.8 5.0 1.8

20 0.6 5.4 3 .0

LD Natural 1.6 3 .1 1.2

20 0.6 4.1 2.3

 

' SD: 9-hr photoperiod; ND: natural photoperiod from 08/97 to 10/97; LD: 9-h plus 4-h night
interruption from 2200 to 0200.

" Natural: outdoor temperature from 08/97 to 10/97; 20°C: greenhouse temperature.

° Least squares means adjusted for a covariate, ﬂowering shoot.

d NS, Not signiﬁcant; *, signiﬁcant at P<0.05.

164

I”:

 

Table 1 (cont’d).
Signiﬁcanced

Temperature * * NS
Photoperiod NS * NS
Temp*Photo NS * NS
Time * * *
Temp*Time NS * *
Photo*Time * NS NS
Temp*Photo* NS NS *
Time
Flower shoot * * "‘
Contrasts Natural vs * * NS
20°C
LD vs SD NS NS NS
LD vs ND NS * NS
ND vs SD NS NS NS
2 week vs 4 week NS * NS
2 week vs 6 week NS * *
2 week vs Control * * *
4 week vs 6 week NS * *
4 week vs Control * * *
6 week vs Control * NS *

 

 

’ SD: 9-hr photoperiod; ND: natural photoperiod from 08/97 to 10/97; LD: 9-h plus 4-h night
interruption from 2200 to 0200.

b Natural: outdoor temperature from 08/97 to 10/97; 20°C: greenhouse temperature.

‘ Least squares means adjusted for a covariate, ﬂowering shoot.

d NS, Not signiﬁcant; *, signiﬁcant at P<0.05.

165

. Fit-Zu'

 

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W

 

 

—l
o

 

Temperature Degrees Celsius
a

 

OI

 

 

 

 

 

o
08/31 09105 09110 09115 09120 09125 09130
1997 Natural Temperatures

 

Figure 1. Natural air temperatures from 08/31/97 to 09/30/97, East Lansing, MI.

166

         

    

     

         

   

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