Evaluating Dormant Application Timing for Winter Weed Control On Zoysiagrass (Zoysia spp.).

Treating winter-dormant zoysiagrass (Zoysia spp.) with non-selective herbicides is common practice in the transition zone to eradicate winter annual weeds prior to spring green-up.  Timing for this application is important to effectively remove weeds without causing phytotoxicity or delaying zoysiagrass spring green-up.  Repeated applications of glyphosate over multiple seasons have resulted in selection of glyphosate-resistant weed populations, for example annual bluegrass (Poa annua L.).  Therefore, this study was conducted to determine the ideal timing for dormant applications on zoysiagrass turf by using glyphosate (0.561 kg a.e. ha^-1) and glufosinate, another non-selective herbicide, at two rates (0.842 and 1.684 kg a.e. ha^-1).  Field plots were established in both Columbia, MO and Carbondale, IL.  Timing of application and herbicides were arranged as a factorial design with four replications, repeated over two years.  Application timings are described as early dormant (soil temperature ≤ 42^oF), late dormant (soil temperature ≤ 48 ^oF), stolon green-up (soil temperature ≤ 60 ^oF), and 50% leaf green-up (soil temperature ≤ 66 ^oF).  Results indicate that once zoysiagrass stolons began to green-up and soil temperatures reached 60 ^oF, applications caused substantial injury and delayed green-up up to four weeks.  Zoysia injury was minimal when treatments were applied at soil temperatures less than 50 ^oF.  Glufosinate caused less damage to zoysia than glyphosate approaching 50 ^oF.  Weed control was less effective at the early dormant stage when soil temperatures were below 45 ^oF, since only 58% of weed populations existed at that time.  The best timing for both herbicides was at late dormant stage, when the four inch soil temperatures were at ~48 ^oF.  No significant differences were found in weed control by using glyphosate or glufosinate at the 1.684 kg a.e. ha^-1 rate.