Johnson grass (Sorghum halepense) is recognized globally as one of the most invasive and challenging weeds to manage due to its aggressive growth habit and deep-seated root structure. This perennial grass quickly dominates agricultural fields, lawns, and non-crop areas, causing significant losses. Because it regenerates rapidly from underground structures, simple mechanical removal is often ineffective, making chemical control necessary. The specific choice of herbicide depends entirely on the location of the weed and whether desirable plants are growing nearby.
The Biological Challenge of Johnson Grass
The difficulty in controlling Johnson grass stems directly from its biology as a warm-season perennial grass. Unlike annual weeds that rely solely on seed, this plant thrives through an extensive network of underground stems called rhizomes. These thick, fleshy rhizomes store carbohydrates, providing the energy reserves necessary for the plant to survive winter and rapidly sprout new shoots in the spring.
A single plant can produce extensive rhizome growth, sometimes extending hundreds of feet per year, and can also generate over 80,000 seeds in a single growing season. This dual reproductive strategy—vegetative spread by rhizomes and sexual reproduction by seed—ensures its persistence and rapid colonization. Seedlings begin producing new rhizomes in as little as three weeks, establishing the plant’s perennial nature quickly. Because the rhizomes can be found up to five feet deep, they are protected from shallow tilling or surface treatments, demanding a systemic approach to control.
Systemic Herbicides for Complete Eradication
For areas where Johnson grass is growing without desirable vegetation, such as fence rows, driveways, or fallow ground, a non-selective, systemic herbicide is the most effective solution. The most common active ingredient is glyphosate, which is absorbed through the leaves and moves throughout the entire plant, including the deep rhizome system. This systemic action allows the chemical to reach the underground storage organs that fuel regrowth.
Glyphosate works by inhibiting a specific enzyme, 5-enolpyruvylshikimate-3-phosphate synthase (EPSP synthase), which is necessary for the plant to produce aromatic amino acids required for protein synthesis. Since this enzyme is not present in humans or animals, the chemical targets plant physiology directly. Applying the herbicide to mature, actively growing Johnson grass—ideally when it is 10 to 25 inches tall and before it forms seed heads—optimizes chemical translocation down to the rhizomes.
The slow action of systemic herbicides allows for maximum movement within the plant before it dies, which is why symptoms like yellowing and wilting may take up to a few weeks to appear. Always adhere to the product label instructions regarding the appropriate concentration for perennial weed control and the use of personal protective equipment (PPE). Following these guidelines ensures both treatment effectiveness and applicator safety.
Selective Herbicides for Targeted Control
When Johnson grass needs to be removed from an area containing desirable plants, such as a lawn, pasture, or specific crops, selective herbicides must be employed. These chemicals are designed to kill one type of plant while leaving another unharmed. For instance, in grass-only areas like bermudagrass or bahiagrass pastures, herbicides containing active ingredients like sulfosulfuron or nicosulfuron can be used, as they target Johnson grass specifically.
Another class of selective herbicides are the ACCase inhibitors, which include chemicals like sethoxydim and clethodim. These are effective in controlling Johnson grass and other grasses while leaving broadleaf plants, such as soybeans, alfalfa, or garden vegetables, mostly unaffected. ACCase inhibitors disrupt the enzyme acetyl-CoA carboxylase, which is necessary for grass plants to build fatty acids and cell membranes.
In turfgrass settings, specialized options like Monosodium Methyl Arsonate (MSMA) may be used to suppress Johnson grass in certain warm-season grasses like bermudagrass. However, its use is often restricted by regulations and proximity to water bodies. The effectiveness of all selective treatments depends upon the weed’s growth stage and requires careful selection to ensure desirable plants are tolerant to the chosen chemical.
Practical Application Strategies
Effective Johnson grass control requires more than just applying a chemical; it demands a strategic approach to timing and follow-up. The ideal period for a systemic application is in late summer or early fall, when the plant naturally begins to move carbohydrates from its leaves down to the rhizomes for winter storage. This downward flow, known as source-to-sink translocation, pulls the herbicide directly into the underground storage system, maximizing impact on the perennial roots.
Regardless of the herbicide used, repeat treatments are necessary to achieve long-term control because of the plant’s massive rhizome and seed reserves. A common strategy involves an initial application followed by a second spot treatment on any regrowth that appears a few weeks later or the following season. To allow the herbicide sufficient time to translocate fully to the rhizomes, avoid mowing, tilling, or disturbing the treated plants for at least seven to ten days after application.
Environmental factors must also be considered during application to ensure safety and effectiveness. Avoid spraying on windy days to prevent drift onto non-target plants, and ensure there is no rain in the immediate forecast to allow the chemical time to be absorbed by the foliage. Long-term management should include the use of pre-emergent herbicides the following spring to control new Johnson grass seedlings.