Johnson grass (Sorghum halepense) is a perennial weed belonging to the grass family (Poaceae) that is recognized as a serious agricultural pest. Originally introduced to the United States as a forage crop, this nonnative, warm-season grass quickly spread and is now listed as a noxious or prohibited weed in many states. Its ability to thrive in disturbed soils makes it one of the world’s most problematic weeds.
Identifying Johnson Grass
Johnson grass is an upright perennial that can grow quite tall, frequently reaching heights of six to eight feet. The leaf blades are typically broad, up to an inch wide and two feet long, and feature a noticeable whitish midrib running down the center.
The seed heads, or panicles, are large, open, and spreading, often displaying a distinctive yellow to purplish color as they mature in the fall. The most distinguishing feature is the extensive, underground rhizome system, which is the primary means of survival and spread. These thick, cream-colored, creeping rhizomes form a tangled sod capable of regenerating new shoots.
Why Johnson Grass is a Nuisance
Johnson grass is a nuisance because of its aggressive growth and the specific dangers it poses to livestock. Its extensive rhizome system and prolific seed production allow it to spread rapidly, giving it a competitive edge over desirable plants. This aggressive habit quickly depletes nutrients and water from the soil, crowding out seedlings and significantly reducing crop yields.
The rhizomes are easily broken into fragments by tillage equipment, which encourages vegetative reproduction and spreads the infestation further. This fragmentation makes eradication extremely difficult and requires long-term management. Furthermore, the foliage can produce toxic amounts of cyanide (prussic acid) when the plant is under stress, such as from frost or drought. This cyanide, along with high nitrate levels in the stressed plant, can be poisonous to grazing livestock.
Effective Control Strategies
Controlling Johnson grass requires an approach that targets both the seeds and the established rhizome system. For small patches, hand-pulling is possible, but it requires removing all plant parts, including every piece of the fragile rhizome, which is difficult because the rhizomes break easily. Repeated close mowing or intensive grazing can prevent seed production and inhibit rhizome regrowth, helping to weaken the established plants over time.
Mechanical control often involves repeated tillage, which severs the rhizomes and brings the fragments to the soil surface where they can be killed by desiccation or freezing winter temperatures. This shallow plowing should be done repeatedly, such as six times at two-week intervals during the growing season, to prevent new rhizome development. Conversely, poorly timed tillage can encourage the weed’s proliferation by spreading the rhizome pieces throughout the area.
Chemical control typically employs systemic post-emergent herbicides, like glyphosate, which are translocated down to the rhizomes to kill the entire plant. For best results, these herbicides should be applied when the grass is actively growing and before it reaches the seedhead stage, often when the grass is 18 inches tall. Pre-emergent options can also provide partial control by stopping new plants from emerging from seed. Since some variants have developed resistance to glyphosate, rotating between different chemical classes is a recommended strategy to maintain efficacy.