Johnson grass is a perennial weed that is extremely invasive. In fact, it is so invasive that several states now require its destruction whenever possible. There are two reasons Johnson grass is so hard to kill. One is that a single plant can produce up to 80,000 seeds in a single year and many of the seeds can lay dormant for up to 10 years before sprouting. The second reason is that Johnson grass sends rhizomes (roots that grow new plants) up to 275 feet in all directions. Killing Johnson grass takes several approaches used in tandem.
Mow Johnson grass frequently. Johnson grass can grow to 6 feet in height. Frequent mowing prevents the grass from producing seeds--one of its primary methods of propagation.
Spray with a broad-leaf weed killer. Follow all manufacturer's directions for mixing. Wet as many leaves of the Johnson grass as possible, as these weed killers soak in through the leaves and travel down to the roots through the plant itself, killing the roots. Do not spray on windy days or if rain is expected within 24 hours.
Spray a second time with a broad-leaf weed killer seven days after the first spraying (it takes up to seven days for these herbicides to do their work). Again, follow manufacturer's instructions for mixing strength, and soak the leaves of the grass you want killed. Do not spray on windy days or if rain is expected within 24 hours.
Spray a third time with the broad-leaf weed killer seven days following the second spraying. Follow manufacturer's instructions for mixing strength. Do not spray on windy days or when rain is expected.
Rototill the entire area seven days after the third broad-leaf spraying, exposing as many of the roots as possible. Rake the area with a hard-tinned rake to pull out as many roots as you can.
Spray the ground (and any remaining roots) with a broad-leaf weed killer one final time. Watch for new growth the following spring, and spray new growth immediately with a broad-leaf weed killer.