These weeds are mostly mistaken for crabgrass. They look like crabgrass on steroids. They are in fact quite different than crabgrass! They both are warm season perennials and crabgrass is a summer annual. This means crabgrass dies every fall after the first frost. Johnsongrass and Dallisgrass do not die after the first frost, however, the top foliage disappears after the first frost. When April arrives, the plants put out new top growth from the rhizome underground. The top left photo, shows a johnsongrass rhizome. Therefore, crabgrass pre-emergent does nothing to deter these perennial grasses. Pre-emergent only keeps annual plants that germinate from seed(crabgrass) from emerging. There is NO selective herbicide that will kill these grasses without killing your desirable grasses. Therefore, the best time to control these grasses is at the end of the summer, right before overseeding time. Roundup or Fusilade should be applied twice, ten days apart for best results. The BEST control method is a shovel!! The rhizome is very difficult to kill, even with Roundup or Fusilade. Therefore, digging them out is the best method. If you have several of these plants in close proximity, they are most likely connected by the rhizome underground. If you spray 10 of these plants with either Roundup or Fusilade, 7 will not return in the same spot the following April, but up to three may remain. If you choose to spray these grasses instead of digging them up, be prepared to take 3 summers for 100% control. What a pain!!
Why do I get these??? Surface soil moisture is the key. A poorly drained soil or light, frequent irrigation makes these grasses go crazy. See irr 101 to correct bad irr habits. To improve drainage, aerate every fall. We typically treat these grasses during 4th app in late July or August. If you do not want the dead brown spots that accompany the killing of these grasses, please let us know prior to 4th app!!!