Impact of Overhead Irrigation Timing on Ornamental Plant Phytotoxicity Following Preemergence Herbicide Applications

The use of preemergence herbicides is the primary method of controlling weeds in container-grown ornamental plants, but it may cause injury to common popular ornamentals. The objective of this research was to evaluate the use of overhead irrigation to reduce phytotoxicity in ornamental plants. Dimethenamid-P and flumioxazin were applied at standard label rates to container-grown coneflower (Echinacea purpurea), lady fern (Anthyrium filix-femina), and blue plumbago (Plumbago auriculata). Plants were subjected to one of four irrigation regimes at the time of herbicide treatment, including receiving 1.3 cm of overhead irrigation before treatment, immediately after treatment, both immediately before and after treatment, and no irrigation until the next irrigation cycle resumed at 4 h after treatment. For all three species, irrigation timing had minimal effect on visual injury ratings following treatment with dimethenamid-P, as injury was minimal overall. Severe injury was observed following treatment with flumioxazin, but significant recovery was noted in both lady ferns and echinacea when irrigation was applied immediately after treatment. The results indicate that irrigating plants immediately after treatment could improve crop tolerance to preemergence herbicide applications and should be further investigated as an injury management strategy for container-grown ornamental plants.