Johnsongrass (Sorghum halepense or Sorghum halapense) is a large perennial grass that often appears in disturbed soil at the edges of flowerbeds or in newly planted lawns. Johnsongrass spreads quickly through underground rhizomes once it becomes established from seeds in an area. A seedling can begin developing rhizomes 19 days after emerging from the ground. Johnsongrass grows as a perennial in U.S. Department of Agriculture plant hardiness zones 8 through 10a and as an annual in cooler climates.
Remove individual johnsongrass plants by hand before they develop further and spread seeds. Wear heavy-duty gloves while handling the plants. Their leaves can cut skin. Check the sites from which you removed the plants after 19 days, and remove all johnsongrass seedlings from them.
Put on a face mask, waterproof gloves, long pants and a long-sleeved shirt, and spray the foliage of each johnsongrass plant with a premixed, 2-percent glyphosate herbicide spray. Spray until each entire johnsongrass plant is coated with the herbicide. Avoid spraying so much that the plants' leaves become dripping wet, and avoid spraying on a windy day to ensure the herbicide does not contact other plants.
Wait for the majority of each johnsongrass plant sprayed with herbicide appears withered and brown. Put on heavy-duty gloves, cut each of those plants at its base with pruning shears and dispose of the plants in a trash bag. Check the label of your glyphosate herbicide to determine how long it should take for the product to work, and spray the plants with the herbicide again after that time period if necessary.
Work the soil in an area several feet around the base of each johnsongrass plant, using a garden hoe or rotary tiller. Look for the thick, white rhizomes that johnsongrass produces, and remove as many of them as possible while chopping the remainder of them into fine pieces.
Wait until about 19 days after your sprayed johnsongrass with herbicide, and afterward check the sprayed areas weekly through the rest of the growing season. Cut down -- as they appear -- all johnsongrass seedlings that emerge from underground rhizomes.