Regional response of zoysiagrass turf to glufosinate and glyphosate applied during post-dormancy transition based on accumulated heat units

Abstract

Turfgrass managers apply nonselective herbicides to control winter annual weeds during dormancy of warm-season turfgrass. Zoysiagrass subcanopies, however, retain green leaves and stems during winter dormancy, especially in warmer climatic regions. The partially green zoysiagrass often deters the use of nonselective herbicides due to variable injury concerns in transition and southern climatic zones. This study evaluated zoysiagrass response to glyphosate and glufosinate applied at four different growing-degree-day (GDD) based application timings during post-dormancy transition under different geographic regions, including Blacksburg, VA; Starkville, MS; and Virginia Beach, VA, in 2018 and 2019. GDD was calculated using a 5C base temperature with accumulation beginning January 1 each yr, and targeted application timings were 125, 200, 275, and 350 GDD5C. Zoysiagrass injury response to glyphosate and glufosinate was consistent across a broad growing region from northern Mississippi to coastal Virginia but varied by application timing. Glyphosate application at 125 and 200 GDD5C can be utilized safely for weed control during post-dormancy period of zoysiagrass, while glufosinate caused unacceptable turf injury regardless of application timings. Glyphosate and glufosinate exhibited a stepwise increase to maximum injury with increasing targeted GDD5C application timings. Glyphosate applied at 125 or 200 GDD5C did not injure zoysiagrass above a threshold of 30%, while glufosinate caused greater than 30% injury for 28 and 29 d when applied at 125 and 200 GDD5C, respectively. Likewise, glyphosate application at 125 or 200 GDD5C did not affect the zoysiagrass green cover area under progress curve d-1, while later application timings reduced it. Glyphosate and glufosinate caused higher injury to zoysiagrass when applied at greater cumulative heat units and were attributed to increasing turfgrass green leaf density, as heat unit accumulation is positively correlated with green leaf density. Accumulated heat unit-based application timing will allow practitioners to apply nonselective herbicides with reduced injury concerns.