Any type of grass that's growing where it's not wanted becomes a weed. Field grass like Johnsongrass, which is considered an invasive species throughout much of North America, can be tough to kill once it's established. Homeowners battling invasive field grass will find their efforts most effective by combining a natural weakening process with chemical control. As with any invasive plant, killing the field grass can be challenging, lengthy and demanding process, but its reward is a weed-free landscape.
Cut down tall field grass to a height of a few inches using a scythe. Discard clippings in the garbage bin or compost.
Till over your lawn to a depth of 3 to 5 inches. This breaks apart the soil and disturbs the roots of the invasive field grass. Alternately, hoe over the lawn instead of renting a tiller. The Kansas Department of Agriculture advises this as an effective strategy for homeowners suffering from invasive field grass.
Wait 10 to 14 days—the grass will start to grow again—then till or hoe the ground again. Continue this pattern of tilling or hoeing over the grass, then waiting for it to grow again. The plant expends extensive energy trying to grow, so continuous cutting will exhaust its resources.
Select a herbicide to fully kill the field grass once you've weakened it following the process above. Contact your local county extension office for a list of currently approved herbicides. For example, the Kansas Department of Agriculture recommends several brands of herbicide on their website for citizens of that state (see Resources).
Mix the herbicide with water following the manufacturer's ratio and directions. Apply herbicide over remaining field grass using a garden sprayer.