Controlling Leader Growth on Noble and Turkish Fir with S-ABA

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.