Paclobutrazol (PBZ), a gibberrellin biosynthesis inhibiting plant growth regulator, was applied as a soil drench to potted American elm (Ulmus americana L.) seedlings in three greenhouse experiments. All plants were grown for one season in a #SP5 container and then transplanted to a larger #5 container before PBZ application in order to simulate planting in the landscape. In a test of the effects of PBZ on growth, a rate of 1.0 mg per plant reduced new shoot weight, shoot extension, and root weight (75, 38 and 63 percent, respectively) compared to controls, but new root elongation was unaffected. Root pruning, similar to that which occurs when transplanting fieldgrown nursery stock, resulted in a greater decrease in shoot growth from PBZ treatment at the moderate rate of 1.0 mg per plant, but for a shorter period of time, compared to non-root pruned, PBZ-treated plants. Shoot growth on all plants was unaffected by 0.5 mg PBZ, per plant. Shoot growth was greatly reduced on both root pruned and non-root pruned plants at 2.0 mg PBZ per plant. After 10 weeks of drought stress, stem water potential of elms treated with 1.0 mg PBZ per plant was the same as that of the well watered controls, whereas the stem water potential of drought-stressed elms lacking a PBZ treatment was significantly lower (more negative). These effects of PBZ may be able to aid in the establishment of newly planted trees.

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Author notes

The author would like to thank Patrick Kelsey of the Morton Arboretum for assistance with potting soil design and statistical analysis. This research was funded, in part, by grants from The Horticultural Research Institute, Inc., 1250 I Street, N.W., Suite 500, Washington, DC 20005 and the J. Frank Schmidt Family Charitable Foundation, P.O. Box 189, 9500 S.E. 327th Avenue, Boring, OR 97009.

2Senior Research Scientist.