Johnson Grass is a grassy weed that requires particular attention because it is a spreading perennial that can ruin lawns and landscapes if not dealt with. While this grass is generally not a pest in well-managed areas, in lawns that are infrequently mowed, and in overgrown flowerbeds, it can be a serious issue. Unmowed johnson grass produces large blades up to 2 feet long by an inch wide. The entire plant often grows to 7 feet tall. It has thick rhizomes or underground roots that allow the clump to spread. From May until fall, unmowed Johnson grass produces rusty-red seedheads.
This weed can be very hard to control for different reasons in lawn settings, vegetable gardens, and shrub beds. It is best to deal with it before it becomes established. Small clumps of the grass can be hand-dug with a shovel in all settings as long as care is taken to remove all of the rhizomes. If it is established in the lawn, it may require up to 3 applications of a general-purpose killer such as Hi-Yield’s “Killsall” or Bonide’s “KleenUp.” This will also kill all the lawn grass around it so it is only practical if the lawn will be overseeded in the fall or sod will be applied to the dead patches. For best effect, spray in the spring after the Johnson grass has started growing with applications about 2 weeks apart.
Make sure the grass is well watered and let it grow slightly taller to give more surface area for the chemical to penetrate. Heavy stands in vegetable areas and shrub beds are even more resistant to being sprayed, although the same application practices apply. If 3 sprays of a glysophate product do not control the Johnson grass, it will have to be hand-dug or tilled repeatedly for 1 to 2 years to eliminate it.