Assessing herbicides for Japanese stiltgrass (<i>Microstegium vimineum</i>) control in cool-season turfgrass

Abstract Japanese stiltgrass is one of the most troublesome invasive weed species in the eastern United States. Strategies for controlling the weed in managed lawns are limited because most previous research was conducted in forest understories or on golf course natural areas. Eight field experiments were conducted in Virginia from 2014 to 2019 to evaluate the response of Kentucky bluegrass, tall fescue, and Japanese stiltgrass to selective herbicides traditionally marketed for use on cool-season turfgrass. Only treatments that contained mesotrione caused transient injury to tall fescue of 15% to 25% at 2 wk after treatment (WAT). When fenoxaprop was applied at a rate of 35 g ha −1 or higher, at 8 WAT Japanese stiltgrass was controlled by ≥90%, relative cover was reduced to &lt;15% compared with a nontreated control, and shoot density was reduced to ≤6 shoots m −2 . Sequential applications of topramezone at 27 g ha −1 at 3-wk intervals, or a single application of topramezone at 54 g ha −1 alone or with triclopyr, resulted in ≥80% control of Japanese stiltgrass and a reduction in relative weed cover and shoot density to ≤22% and &lt;35 shoots m −2 , respectively. Fenoxaprop applied at 0.25× of the labeled rate and herbicide combinations that contained topramezone selectively controlled Japanese stiltgrass without injuring tall fescue. Fluazifop applied at 53 g ha − 1 resulted in 25% injury to Kentucky bluegrass and digitally assessed turf cover was reduced by 20% at 4 WAT, but turfgrass recovered by 6 WAT. Reduced rates of fluazifop provided 85% control of Japanese stiltgrass, and a reduction in weed shoot density to &lt;20 shoots m −2 , and relative cover to &lt;20% at 8 WAT. Kentucky bluegrass did not appear to be injured with premixed applications of dicamba, fenoxaprop, and fluroxypyr, but Japanese stiltgrass was controlled by ≥92%, the relative weed cover was reduced to ≤7%, and shoot density was reduced to ≤5 shoots m −2 at 8 WAT. Our research provides herbicide options for turf managers for controlling Japanese stiltgrass in lawns of Kentucky bluegrass and tall fescue.