Johnson grass (Sorghum halepense) is globally recognized as one of the most invasive and difficult weeds to control. Its aggressive nature and rapid spread require a comprehensive, integrated, multi-year strategy for successful eradication. Eliminating this perennial grass demands that any control method address both its underground storage system (rhizomes) and its prolific seed production. This guide details proven, targeted methods—from mechanical removal and smothering to precise chemical application—necessary to overcome this persistent species.
Understanding Johnson Grass: The Biology of Persistence
The difficulty in killing Johnson grass is linked to its two-pronged survival strategy: an extensive rhizome network and a persistent seed bank. Rhizomes are thick, horizontal underground stems that store carbohydrates, allowing the plant to rapidly sprout new shoots even after the above-ground portion is destroyed. These rhizomes can produce new shoots when the soil temperature reaches approximately 60°F, often emerging earlier than seedlings.
A single plant can generate over 200 feet of new rhizomes in one season, quickly forming dense, interconnected colonies. This underground energy reserve allows rhizome sprouts to grow faster than new seedlings, making them highly competitive. Successful eradication must focus on depleting these reserves, as they sustain the plant through multiple defoliation events.
The seed bank ensures long-term survival in the soil. A mature plant can produce up to 80,000 seeds in a single season, and these seeds can remain viable for up to six years. New seeds germinate when soil temperatures warm to about 70°F, creating a continuous, season-long emergence pattern. Control programs must prevent seed production and manage the existing seed bank to avoid recurring infestations.
Non-Chemical Eradication Strategies
Non-chemical methods focus on exhausting the plant’s stored energy reserves and preventing seed formation. Repeated, shallow tillage is one of the most effective mechanical approaches. Tillage must be timed to coincide with the lowest point of the plant’s carbohydrate reserves, typically 10 to 30 days after new shoots emerge, when the plant is 6 to 12 inches tall. Tillage at this stage cuts the rhizomes into fragments, forcing the plant to use remaining energy to produce new sprouts.
Tillage must be repeated every three to four weeks throughout the growing season to prevent shoots from replenishing rhizome reserves. Although disking fragments can spread the weed, frequent repetition starves the small pieces of energy. In late fall, deep plowing can bring rhizomes to the soil surface, exposing them to lethal freezing temperatures over winter.
Smothering and solarization are effective for smaller patches by depriving the plant of light or killing underground structures with heat. For solarization, remove surface debris, lightly till the soil 6 to 12 inches deep, and moisten the area. Cover the ground with clear, UV-resistant plastic sheeting, sealing the edges tightly by burying them in a trench.
The clear plastic creates a greenhouse effect, raising soil temperatures to 110°F to 140°F in the top few inches, which is lethal to rhizomes and seeds. This process must be maintained for four to six weeks during the hottest, sunniest part of the summer. Opaque black tarps can be used for smothering (occultation), which kills plants by light deprivation but does not generate the high temperatures needed to destroy deep rhizomes.
Mowing and hand-pulling are insufficient as stand-alone solutions for established infestations, but they weaken the plant and prevent seed production. Mowing must be done frequently—ideally every two to four weeks—to keep the plant from reaching the boot stage. This repeated defoliation depletes the rhizomes and is useful for managing pastures or non-crop areas. Hand-pulling is only practical for very young seedlings that have not yet formed an extensive rhizome system.
Targeted Herbicide Application Protocols
Chemical control often requires systemic herbicides that translocate down into the rhizomes. The most widely used non-selective option is glyphosate, which is absorbed by the foliage and moves throughout the plant, killing the entire root and rhizome system. For maximum efficacy, glyphosate must be applied when the plant is actively growing and is at least 18 inches tall, up to the boot or early flowering stage.
Application timing is crucial; the plant must be actively moving nutrients to the roots for the herbicide to be pulled into the rhizomes. A fall application is often the most effective single treatment, as the plant naturally translocates carbohydrates to underground storage organs before winter dormancy. Severe infestations require multiple sequential applications, often involving an initial treatment followed by a second treatment of regrowth three to four weeks later.
Where desirable broadleaf plants are present, selective herbicides must be used. Acetyl-CoA carboxylase (ACCase) inhibitors, such as clethodim, fluazifop-P-butyl, and sethoxydim, are effective selective options that kill grasses but spare broadleaf plants. These products are applied post-emergence when the Johnson grass is small (8 to 18 inches tall) and usually require two or more applications for control of established rhizome plants.
Strict adherence to the product label is required for safety. Necessary personal protective equipment (PPE) includes long-sleeved shirts, long pants, chemical-resistant gloves, and eye protection. To prevent non-target injury, especially with non-selective products, use spot-treatment and apply in calm weather to minimize drift.
Post-Eradication Management and Soil Restoration
The initial eradication effort targets established rhizomes, but the dormant seed bank remains a source of re-infestation. Continuous monitoring and competitive ground cover are necessary to manage the resulting flush of seedlings. Scouting for new seedlings should be frequent, and any new growth must be controlled before it establishes new rhizomes, which can occur as early as three weeks after emergence.
Planting competitive cover crops suppresses new Johnson grass seedlings by outcompeting them for resources. Alfalfa is highly competitive and can significantly reduce populations when subjected to frequent mowing over a two- to three-year rotation. Small grain cover crops, such as cereal rye, are also beneficial because they produce biomass that suppresses seedling growth and may contain allelopathic compounds that inhibit rhizome survival.
Improving soil health is part of a long-term strategy for preventing re-establishment. Practices like rotating crops or planting dense covers disrupt the environment that allows Johnson grass to thrive. While aggressive tillage is used for initial control, subsequent management should transition toward less soil disturbance to avoid bringing deeply buried seeds to the surface. Integrating these cultural controls into a multi-year plan is necessary to establish a healthy, weed-resistant landscape.