Johnson grass (Sorghum halepense) is recognized globally as one of the most problematic invasive weeds, known for its ability to aggressively colonize diverse environments. Its persistence stems from an elaborate underground network, making it difficult to eradicate using conventional methods like mowing or simple cultivation. The challenge for property owners is finding a solution that selectively targets this grass without harming desirable plants, such as turfgrass, garden vegetables, or ornamental beds. Successfully controlling this weed requires understanding its unique biology and utilizing specialized herbicides designed for grass-on-grass control.
Identification and Growth Cycle
Johnson grass is a perennial warm-season grass characterized by its height, often reaching six to eight feet, and a prominent white midvein running down the center of its leaf blades. As a persistent perennial, it spreads through both seed and extensive rhizomes, which are robust, horizontal underground stems. These rhizomes are stout, scaly, often possess a purplish spotting, and typically reside in the top six to eight inches of the soil.
The rhizome system is the primary reason for the weed’s survival, acting as a massive energy storage unit that allows the plant to regenerate year after year. A single plant can produce up to 60 feet of rhizomes in a growing season, and new shoots emerge from buds along these segments. Even if the above-ground foliage is cut or killed, the plant rapidly regrows from the stored carbohydrates, requiring a systemic approach for long-term control.
Selective Herbicides for Grass-on-Grass Control
Targeting a grass weed like Johnson grass within a stand of desirable grass requires a highly specific chemical mechanism of action. The most effective selective post-emergent herbicides for this task belong to Acetyl-CoA carboxylase (ACCase) inhibitors, sometimes called “graminicides.” These chemicals specifically target the ACCase enzyme, which is necessary for fatty acid synthesis in grasses, disrupting the plant’s growth.
Common active ingredients in the ACCase inhibitor family include clethodim, sethoxydim, and fluazifop-P-butyl, often utilized in broadleaf crops, vegetable gardens, and flower beds. Because the ACCase enzyme in broadleaf plants is structured differently, these herbicides kill Johnson grass without harming plants like tomatoes, cotton, or ornamental shrubs. However, these chemicals will also kill desirable turfgrasses, making their use non-selective in a lawn.
For situations where Johnson grass infests certain established turfgrasses, like Bermuda grass, specific sulfonylurea herbicides offer a limited selective option. Active ingredients such as sulfosulfuron are sometimes labeled for use in specific warm-season grasses because the turf has a greater tolerance to the chemical than the Johnson grass. Monosodium methyl arsonate (MSMA), an organic arsenic-based herbicide, was historically used but is now heavily restricted or declining due to environmental concerns. The success of any selective treatment depends entirely on the tolerance of the desirable plant, so consulting the product label to ensure it is safe for the intended application site is necessary.
Application Timing and Strategy for Eradication
The effectiveness of selective herbicides against Johnson grass depends heavily on the timing and method of application, which must be strategic to overcome the rhizome defense system. Systemic herbicides must be applied when the plant is actively growing and translocating nutrients downward to its rhizomes, usually during late spring and summer. The optimal time for the initial treatment is when the grass is between 15 and 25 inches tall, but before the plant develops seed heads.
Applying the herbicide at this stage ensures the chemical is absorbed by the leaf surface and efficiently moved down into the rhizomes, where it depletes stored energy reserves. To maximize absorption, mowing should be avoided for several days both before and after the application. This allows the plant to develop enough leaf area to absorb a lethal dose and gives the systemic chemical time to move throughout the plant before the foliage is removed.
Because rhizomes often sprout unevenly throughout the season, a single application is rarely sufficient for eradication, especially in established infestations. True control requires a sequential treatment approach; a second application must be made to any regrowth that appears, typically three to four weeks after the first treatment. Repeat applications over one or two growing seasons are necessary to exhaust the entire rhizome network and prevent the weed from returning.