Gibberellic acid inhibitors control height growth and cone production in Abies fraseri

Precocious cone production in Fraser fir (Abies fraseri) results in substantial economic losses for Christmas tree growers in the United States. Cone removal often represents the second highest labor expense, after shearing. Gibberellins (GAs) regulate both reproductive development and shoot elongation, and are used to enhance cone production in conifer seed orchards. We identified plant growth regulators (PGRs) that inhibit GA biosynthesis – paclobutrazol, applied by foliar spray (PBZ-foliar) or soil injection (PBZ-soil), and chlormequat. We evaluated the effects of PGR treatments on cone production and vegetative growth in three tree size classes (small, medium, and large) at four locations over a three-year period. PGRs were applied in spring 2013, but did not affect growth until 2014, nor did they affect cone production until 2015. In 2014, PBZ reduced leader length by 21–29% across size classes and sites, while also increasing bud density by 11–20%. In 2015, PBZ-soil reduced average cone density (cones/tree) in small, medium, and large trees by 33%, 54%, and 40%, respectively. PBZ-soil also reduced leader length by 20–29% and increased bud density by 16–25%, making it the most effective PGR at controlling height growth and cone production in Fraser fir.