Johnson grass (Sorghum halepense) is a highly invasive perennial weed known for its aggressive growth and difficulty in eradication. The plant spreads through two primary mechanisms: prolific seed production and an extensive network of underground stems called rhizomes. A single plant can produce tens of thousands of seeds in a season, and its rhizomes can grow up to 275 feet, forming dense colonies. These rhizomes store energy reserves, allowing the plant to survive harsh conditions and making control efforts that only target the above-ground foliage largely ineffective.
Non-Chemical Eradication Strategies
Controlling Johnson grass without chemicals requires intensive, repeated efforts focused on depleting the rhizome’s stored energy reserves. Tillage can be an effective mechanical control, but it must be deep and frequent to be successful. Shallow or infrequent tilling can worsen the infestation by cutting the rhizomes into small pieces and spreading viable fragments throughout the soil.
Intensive cultivation during the growing season, perhaps six times at two-week intervals, prevents the plant from developing new rhizomes and starves the existing ones. Fall tillage is also useful; deep inversion plowing brings the rhizomes to the soil surface, exposing them to freezing temperatures. This exposure weakens or kills the underground structures.
Smothering and solarization offer alternative methods by blocking sunlight and raising soil temperatures to lethal levels. Covering the infested area with thick black plastic sheeting deprives the plant of light, heat-killing the rhizomes over time. Heavy mulching can also suppress growth and prevent the plant from emerging.
Frequent mowing and grazing serve as suppression tactics, preventing seed formation and gradually exhausting the plant’s energy stores. Mowing must be done often enough to prevent the plant from reaching the seed-head stage, typically requiring cutting every three weeks during active growth. While this will not eliminate an established stand, it weakens the plants and is an effective part of a multi-year management plan.
Targeted Herbicide Application Strategies
Effective chemical control depends on using systemic herbicides that can be transported throughout the plant, reaching and killing the underground rhizome network. Herbicides that only burn down the foliage result in rapid regrowth from the protected rhizomes. Therefore, application timing is crucial to ensure the chemical moves actively to the energy-storing root structures.
The optimal time to apply a systemic herbicide is when the grass is actively growing and has sufficient leaf area to absorb the chemical. This occurs when plants are 8 to 12 inches tall, but before they produce seed heads. Applying the herbicide during this stage ensures the plant translocates energy, and the herbicide, downwards to the rhizomes.
Glyphosate is a widely used non-selective systemic herbicide highly effective against Johnson grass, especially when applied in the fall. During the fall, the plant naturally moves carbohydrates from the leaves down to the rhizomes to prepare for dormancy, enhancing herbicide movement. For smaller infestations, spot treatment with concentrated glyphosate can target individual plants without affecting surrounding desirable vegetation.
For areas where non-selective treatment is not feasible, such as within crops, selective grass killers can be used. These include ACCase inhibitors like fluazifop-P-butyl, quizalofop-p-ethyl, sethoxydim, or clethodim, which control grasses while sparing broadleaf plants. Even with optimal timing, multiple sequential applications are usually necessary for complete eradication, as regrowth from surviving rhizomes is common.
Long-Term Management and Monitoring
Eradicating Johnson grass is a multi-year effort because the seeds can remain viable in the soil for an extended period. Seeds possess a hard coat and can survive for five to ten years, meaning new seedlings can emerge long after the parent plants are gone. This persistence necessitates a long-term vigilance plan focused on preventing re-establishment.
Regular monitoring is essential to catch any new growth from surviving rhizomes or germinating seeds. Seedlings must be treated immediately, ideally before they develop their own rhizome network, which can happen quickly. Immediate spot treatment with a selective herbicide or physical removal of the entire plant stops a new infestation before it takes hold.
Planting competitive cover crops or maintaining dense turf helps suppress new Johnson grass growth. These species utilize soil resources and block sunlight, making it difficult for weed seedlings to establish. Crop rotation strategies can also disrupt the weed’s life cycle, weakening its stand over time.
A final component of management is preventing mechanical spread. Rhizome fragments and seeds can easily be carried on equipment like mowers, tillers, and vehicles. Thoroughly cleaning all equipment after working in an infested area, using compressed air or high-pressure water, is necessary to avoid inadvertently spreading the weed.