2019 ASHS Annual Conference

River Birch (Betula nigra) is one of the more popular shade trees planted in landscapes across USDA Zones 4-9 the United States. River birch is marketed as the most borer resistant birch, tolerant of both wet and dry summers. Yet, landscape managers in western states regularly report drought-stress symptoms including tip die-back, leaf-scorch and ultimately plant death. Stressed trees are often more susceptible to insect attack. Trees in residential landscapes are often experience sudden and drastic changes to growing conditions when lawns are removed, irrigation is altered, hardscapes are installed, or the landscape topography is changed. Biostimulants are a group of organic substances marketed to provide “benefits” including increasing plant tolerance to and recovery from abiotic stresses. However, at this time there is not a legal definition for biostimulants in the United States, which engenders a skepticism about the validity of the benefit from these products. To investigate the effect of these emerging products we established an experiment to learn if selected biostimulants would improve the drought tolerance of river birch. Our test was conducted in a wind-screened can-yard, at Oregon State University, North Willamette Research and Extension Center, in Aurora Oregon. We transplanted 72 bareroot whips into #5 black plastic nursery containers ( 19 L volume) filled with 100% Douglas-fir amended bark media. The 72 potted trees were dispersed in a completely randomized one-way factorial design. In total, we had six treatment groups (n =12). Treatment groups included one group that was not subjected to drought-stress. All other trees were subjected to drought conditions: four groups treated with different biostimulants, and one untreated drought group. Plants were evaluated for growth, aesthetics, and leaf-chlorophyll fluorescence. The results showed significant differences ( p < 0.05) between the treatment groups, suggesting that certain biostimulants may mitigate the harmful effects of drought stress more than others.