Trifluralin (Preen) Dissipation from the Surface Layer of a Soilless Plant Growth Substrate1

Southeastern U.S. container nursery crop production relies upon frequent applications of preemergence herbicides for broad-spectrum weed control. An experiment was conducted to determine the length of residual control and the sensitivity of two grass species to Preen (trifluralin) applied to the surface of a pine bark:sand (7:1 by vol) potting substrate. The experiment was conducted twice at each of two locations during 2001. Samples of the potting substrate were taken 0, 1, 3, 7, 14, 28, 42 and 56 days after treatment (DAT) for laboratory analysis. Trifluralin was extracted from the potting substrate and quantified using gas chromatographic techniques. Large crabgrass and perennial ryegrass were seeded 0, 7, 14, 28, 42 and 56 DAT to bioassay for herbicide residues. Shoot and root length were measured two weeks after each seeding date. Preen residues in the surface of the potting substrate decreased rapidly after application and leveled off approximately 21 DAT. Based on the bioassay, March applications of Preen controlled large crabgrass and perennial ryegrass for 37 days or greater while May and June applications of Preen controlled the two species for only 3 to 21 days. These results suggest that trifluralin reapplication should be more frequent than the common interval of 56 to 70 days in the southeastern United States. Index words: herbicide dissipation, container nursery production, dinitroaniline, preemergence herbicides. Species used in this study: large crabgrass (Digitaria sanguinalis (L.) Scop.); perennial ryegrass (Lolium perenne L.). Herbicides used in this study: Preen (trifluralin) 2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl)benzenamine.