Ethephon Substrate Drenches Suppress Growth, but Not Tiller Number of ‘Jester’ Ornamental Millet

Ethephon (2-chloroethylphosphonic acid), traditionally used as a foliar spray to control growth, is now available for drench applications with the commercial product Collate ® 2L (Fine Americas Inc., Walnut Creek, CA, USA). This study evaluated the efficacy of ethephon drenches on the growth of ‘Jester’ ornamental millet ( Pennisetum glaucum L.). Each container was drenched with 59.1 mL aliquots of solution containing deionized water (0 mg·L −1 ; control) or 25, 50, 75, 100, or 200 mg·L −1 ethephon. After 28 days, height, diameter, and shoot dry weight (SDW) decreased as the ethephon concentration increased. Height was suppressed with drenches of 50 to 200 mg·L −1 by 3% to 36%, while diameter and SDW of plants drenched with 25 to 200 mg·L −1 ethephon were 31% to 52% smaller and 32% to 52% lower, respectively, when compared with controls. The ethephon drench concentration did not impact tiller number. These results demonstrate that substrate drenches of 25 to 200 mg·L −1 ethephon can effectively control growth in ‘Jester’ ornamental millet, thereby supporting its suitability as a growth management tool for commercial ornamental millet production.