Johnson grass (Sorghum halepense) is native to the Mediterranean region and was introduced into the United States in the 1800s for livestock forage. It spreads by underground rhizomes and has the ability to produce thousands of seeds each season that remain viable in the soil for several years. Its aggressive nature has allowed it to spread over the southern 2/3 of the United States and become a problem for gardeners and landscapers. It grows taller and faster than lawn grasses, ornamental plants and row crops, creating competition for water and nutrients and an unsightly appearance.
Identify the Johnson grass and the extent of the area it has invaded. The rhizomes can spread 20 feet or more in each direction in one season. Johnson grass has blades like thin corn leaves and grows rapidly up to 10 feet tall. It produces a loose pyramid-shaped seed head. For best control, you must treat the Johnson grass before the seed head opens and matures.
Mix the herbicide containing glyphosate according to the container label and apply to the Johnson grass with a garden sprayer designated specifically for herbicides. The grass should begin to decline within two weeks.
Reapply if the Johnson grass sprouts again from the ground or fails to respond to the herbicide application. Reapplication of the herbicide will eventually kill the Johnson grass. If you cannot spray the area, you can use a mop, cloth, brush or other device to soak in the solution and brush onto the leaves. Wear protective rubber gloves if using this method.