Fraser fir (Abies fraseri) is economically important as a premium Christmas tree in the United States. Christmas tree producers in the Midwest incur substantial monetary losses because of early and heavy cone production. Ironically, cone production is often sporadic in natural stands and consistent cone production is difficult to achieve in conifer seed orchards. The ability to influence cone formation in Abies fraseri is important both for Christmas tree growers who want to reduce costs associated with undesired cone production and for seed orchard managers who require consistent seed production. Cultural practices or chemical treatments may influence the formation of reproductive buds, but only when applied concurrent with cone bud initiation and differentiation. We surveyed shoot phenology at nine Christmas tree plantations in Michigan and used the data obtained to develop a phenology model that predicts the timing of cone bud differentiation in growing degree days. We used that model to time treatments and interpret results in subsequent experiments. Heat and drought are important environmental regulators of reproduction in conifers. Therefore, we investigated the effects of irrigation and mulch on cone production in two Fraser fir plantations in Michigan. We also tested the effects of overhead misting for evaporative cooling and polyethylene tenting for solar heating of tree crowns. Irrigation did not affect cone production even under drought conditions, compared to non-irrigated control plots. Mulch during a hot summer reduced cone production by 51%, but mulch had no effect during a cooler summer. Misting decreased average daily maximum temperatures of lateral shoots by 5.0°C, and tenting increased maximum temperatures by 3.8°C, but neither affected cone production. Gibberellins (GAs) regulate reproductive development in conifers and are used to enhance cone production in conifer seed orchards. We identified several plant growth regulators (PGRs) that inhibit GA biosynthesis and evaluated their effects on cone production and vegetative growth at four locations over a three-year period. Paclobutrazol was the most effective PGR for cone control. When applied by soil injection, it reduced average cone density (cones/tree) by 39% two years after application, compared to control. Cone production is under complex regulation, and environmental factors interact with internal controls through multiple pathways. Therefore, it is difficult to influence cone formation by modifying the environment to alter tree stress. Cultural practices that reduce heat stress to the roots may reduce cone formation during particularly hot summers, and paclobutrazol or other GA inhibitors may limit unwanted cone formation. Future research should examine the interaction of environmental and chemical controls, as they may have greater influence on cone production when used in combination.
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