Weeds must be well managed during the establishment phase (1-3 years) of Christmas tree production as weed competition directly relates to the rate of Christmas tree growth during this time. The objectives were to evaluate the weed control efficacy of organic mulch and herbicide combinations and to determine their phytotoxic effects on four different species of Christmas trees during the establishment stage and also to determine alternative postemergence control options for clopyralid resistant common ragweed (Ambrosia artemisiifolia L.), which has recently been discovered in Michigan Christmas tree farms. A field and greenhouse experiment were conducted. Four species of Christmas trees in their establishment stage were used for the field experiment: Fraser fir [Abies fraseri (Pursh) Poir], blue spruce (Picea pungens Engelm.), white pine (Pinus strobus L.), and Scotch pine (Pinus sylvestris L.). Twelve weed control treatments were applied for the field and greenhouse experiment with four replications each in complete randomized block design. The treatments were cypress bark organic mulch and the herbicides clopyralid, oxyfluorfen, and glyphosate, which were either applied alone or in combinations with each other. It was found that herbicides in combination with mulch resulted in better longer lasting weed control, but treatments involving clopyralid + glyphosate, even if mulch was also included proved to be the most phytotoxic treatments to the Christmas trees. Growth indices would likely only be affected later in the tree's life and foliar nitrogen percent levels were found not to be affected by the use of mulch or any of the treatment combinations. Overall, the best treatments for controlling clopyralid resistant common ragweed were mulch + clopyralid + glyphosate and mulch + clopyralid + oxyfluorfen.
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