Johnson grass is an aggressive plant that spreads through large seeds and rhizomes. The rhizomes, or roots, move quickly underground to establish thick areas of the tall perennial. Eradication of this invasive grass proves to be difficult. Burning the plant back or excessive grazing may cause more vigor to the overall establishment of the grass. Perhaps the most problematic areas for the spread of Johnson grass are that in plowed fields. Breaking up the rhizomes will only spread the plant to more areas within the field.
Keep the Johnson grass areas mowed to the lowest height possible. Constant mowing will disrupt seed head production. Over time the rhizomes will become weakened and will produce less aboveground portions of the grass. The mowing method may not kill all of the Johnson grass, but it will weaken it substantially to apply a post-emergent herbicide.
Mix a glyphosate herbicide with one part chemical to two parts water.
Apply the herbicide in the heavy concentration using a rope wick applicator. The rope wick will allow for spot treatment of the individual blades of Johnson grass.
Cultivate the Johnson grass areas 10 days after chemical application. The herbicide-weakened plant maybe easier to kill by exposing the dying rhizomes.