Johnson grass propagates through horizontal root rhizomes and seed dispersal. According to the University of Missouri, once Johnson grass is established, it will take several tries to completely kill the plant from any area. In most cases, eradicating well-established stands of Johnson grass is a two-fold process: mowing and applying herbicides.
Mow the stands of Johnson grass as short as possible with a mower, and keep them mowed short. This will place stress on the rooting rhizomes.
Mix the herbicide glyphosate, following the label directions pertaining to the specific plant species. Various herbicides contain different strengths of the chemical.
Spray on all sides of the Johnson grass plant approximately one week after mowing. New grass blades will emerge after mowing the plant in approximately one week. Apply the herbicide to the Johnson plant only, using short spurts of chemical from the sprayer. Over spray from the chemical will damage and kill other green plants.
Wait two weeks. Inspect the plants for visible signs of browning on the leaves.
Apply another herbicidal treatment to any plants that show signs of growth. According to the University of Illinois, it may take up to two years to fully eradicate the plant, as seed from previous Johnson grass plants will germinate.