Abstract
Control of leader growth in Christmas tree production is a critical and expensive component in developing quality trees. Producers typically hand trim leaders to a desired length. With noble and Turkish fir this practice frequently results in poor top development in the subsequent year. This study investigated the use of S-abscisic acid (S-ABA) at various rates up to 6% active ingredient (a.i.) to determine if the plant growth regulator provides predictable leader control with minimal phytotoxicity. S-ABA was effective in controlling leader extension of both noble fir and Turkish fir. Shoot growth of Turkish fir was slightly more sensitive to S-ABA than noble fir, and effective growth control was achieved at 2% a.i., versus 3% a.i. for noble fir. With Turkish fir, we achieved an 80–90% reduction in growth across all sites, depending on rate. For noble fir, we achieved a 67–88% growth reduction. The addition of a surfactant did not increase the effectiveness of S-ABA for growth control and is not recommended. Repeating S-ABA applications 2 weeks after initial application resulted in a slight increase in growth control and phytotoxicity compared to a single application at the same rate. The results suggest that S-ABA can be an effective tool in Christmas tree production to control leader growth on fir species that often produce crooked or multiple leaders in response to conventional leader pruning.
Highlights
Noble fir (Abies procera Rehd.) is the most widely planted and important Christmas tree species in the Pacific Northwest
The results suggest that S-abscisic acid (S-ABA) can be an effective tool in Christmas tree production to control leader growth on fir species that often produce crooked or multiple leaders in response to conventional leader pruning
Results were conducted using SAS ver. 9.4 (SAS Institute, Inc., Cary, NC, USA). For both species and both treatment years, increasing rates of S-ABA resulted in in3
Summary
Noble fir (Abies procera Rehd.) is the most widely planted and important Christmas tree species in the Pacific Northwest. Turkish fir (A. bornmuelleriana Mattf.) and closely related Nordmann fir (A. nordmanniana (Steven) Spach) rank third in plantings in the Pacific Northwest with just over 400,000 trees planted yearly. Controlling the leader length on commercial true fir (Abies spp.) Christmas trees is one of the most expensive production practices associated with tree culturing. Leader control has been performed by hand trimming the leader to a preferred length. For certain species of firs such as noble fir, Turkish fir and Nordmann fir, hand trimming seldom produces straight tops in the subsequent growing season, presumably due to alterations in the balance of internal hormones resulting in a decrease in apical dominance and orthotropic growth [2]. Growers must frequently undertake costly measures to straighten leaders, such as tying up branches or leaders to wooden stakes or recutting tops, which further adds to production costs [3]
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