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Winter annual weeds begin to germinate as zoysiagrass enters winter dormancy in autumn. These weeds can suppress zoysiagrass shoot development the following spring through competition for sunlight, moisture, and nutrients. Previous research involving winter annual weed control in dormant turfgrass has been conducted primarily on bermudagrass, but less is known about how various herbicides used for this purpose will influence zoysiagrass post-dormancy transition. Two field studies were conducted at 7 site yr between 2016 and 2020 to evaluate 17 herbicide treatments that are typically marketed for broadleaf weed control in spring and 18 herbicide treatments that are typically marketed for annual bluegrass control during winter for effects on a variety of weeds and semi-dormant ‘Meyer’ and dormant ‘Zeon’ zoysiagrass, respectively. Glufosinate, glyphosate + simazine, and indaziflam + simazine controlled Persian speedwell over 90% and significantly higher than auxin-type and other herbicide combinations evaluated. The combination of simazine and glyphosate had better dandelion and Persian speedwell control than glyphosate applied alone. Glufosinate controlled dandelion, hairy bittercress, and Persian speedwell more effectively than glyphosate. In ‘Meyer’ zoysiagrass, glyphosate and glufosinate controlled annual bluegrass equivalently while in ‘Zeon’ zoysiagrass, glyphosate controlled annual bluegrass more than glufosinate. Foramsulfuron or simazine containing treatments controlled annual bluegrass >90%. Flumioxazin admixture with diquat, glufosinate, or glyphosate improved annual bluegrass control. Herbicide treatments containing diquat, glufosinate, glyphosate, and metsulfuron alone or in a tank-mix should not be applied to ‘Meyer’ zoysiagrass with 5% visual green turf cover due to high injury potential. In both studies, glufosinate was more injurious to ‘Meyer’ and ‘Zeon’ zoysiagrass than glyphosate. Overall, several herbicides that control annual bluegrass or broadleaf weeds can be safely applied to ‘Zeon’ zoysiagrass during dormancy or ‘Meyer’ zoysiagrass during post-dormancy transition.

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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.