Johnsongrass grows up to eight feet tall and can produce thousands of seeds. Introduced in 1830 as a forage crop and for stabilizing ditches, johnsongrass crowds out desirable plants, destroys food and shelter for wildlife and causes trouble for farmers and ranchers. Here's an overview of your most effective methods of getting rid of it.
As with many weeds, pulling out Johnsongrass by hand can break the roots. Those broken roots can simply grow back. The same is true with hoeing or tilling.
Attack Before Summer
Before the weed produces a seed head in July, apply a broad-spectrum product such as Roundup® Ready-To-Use Weed & Grass Killer III. It goes down to the root for effective control. Just be careful not to spray it on the plants you like, since it's a non-selective herbicide and will harm the plants in comes in contact with.