Johnsongrass, or Sorghum halepense, is one of the world’s most aggressive and difficult perennial weeds to control. Its rapid growth allows it to quickly dominate landscapes, making a single application of any control method ineffective. Eradicating this plant requires a sustained, multi-year strategy that targets its reproductive biology above and below the soil line. Successful removal depends on persistence and a precise understanding of the weed’s life cycle.
Understanding Johnsongrass Persistence
The difficulty in eliminating Johnsongrass stems from its dual reproductive strategy, relying on prolific seed production and an extensive rhizome system. Rhizomes are the thick, scaly, underground stems that function as the plant’s primary energy storage. Most rhizomes are concentrated in the top eight inches of the soil, but they can grow 200 to 300 feet in a single growing season.
These underground structures allow the plant to resprout rapidly after the visible portion has been destroyed, drawing on stored carbohydrates for quick regrowth. Plants emerging from these reserves are more vigorous than new seedlings.
A single Johnsongrass plant can produce up to 80,000 seeds, which remain viable in the soil for five to twenty years, creating a massive, long-term seed bank. Seedlings can begin forming new rhizomes in as little as nineteen days after emergence. This combination of a long-lived seed bank and a regenerative rhizome network is why casual weeding or a single treatment will fail.
Manual and Cultural Removal Techniques
Manual and cultural controls focus on physically exhausting the plant’s carbohydrate reserves and preventing seed set.
Mowing
Mowing must be timely and frequent to be effective. Start mowing when the plant reaches 8 to 12 inches in height, before the boot stage when the seed head is forming. This process must be repeated every three weeks throughout the growing season to prevent seed production and continually deplete the energy stored in the rhizomes.
Tillage
Tillage can also be used to exhaust the rhizome system, but it requires precision. The goal is to interrupt regrowth when the plant’s carbohydrate reserves are lowest, typically 10 to 30 days after shoots emerge (when plants are six to twelve inches tall). Cultivations should be performed every fourteen to eighteen days at a depth of three to five inches, ensuring the entire plant is severed. Shallow or infrequent tilling is counterproductive, as it cuts rhizomes into smaller pieces that sprout into new plants.
Soil Solarization
Soil solarization uses heat to kill the plant and its seeds. This involves covering the infested area with clear plastic sheeting for a minimum of thirty days, or ideally eight weeks, during the hottest months. The clear plastic traps solar radiation, raising the soil temperature in the top six inches to as high as 140°F, which is lethal to the rhizomes. Before applying the plastic, remove all existing top growth and ensure the soil is moist to enhance heat transfer.
Competitive Planting
Competitive planting relies on aggressive, desirable plants to suppress Johnsongrass growth. Planting a dense perennial forage crop like alfalfa for three to four years can significantly reduce the weed population. Annual cover crops, such as cereal rye, are also effective, as their rapid, dense growth can outcompete and suppress the emergence of Johnsongrass rhizomes.
Chemical Control Strategies
For established infestations, chemical control using systemic herbicides is required for long-term eradication. The effectiveness of these herbicides, which are absorbed by the leaves and transported throughout the plant, depends entirely on timing. Application must coincide with the plant actively moving sugars down to its rhizomes, a process known as source-to-sink translocation.
The most effective period for post-emergent application is when the plant is between the boot stage and the head stage (twelve to twenty-four inches tall), or in the fall. During the fall, the plant naturally moves carbohydrates to the rhizomes to prepare for winter dormancy, maximizing the amount of herbicide translocated below ground. Systemic chemicals like glyphosate or grass-selective ACCase-inhibitors (e.g., clethodim and sethoxydim) are required to reach the underground rhizomes.
A single application is often insufficient to kill the entire rhizome network, so sequential applications are necessary. Follow-up treatments are typically applied two to three weeks after the initial application, targeting new regrowth. Pre-emergent herbicides, such as imazapic, can prevent new seeds from germinating, but they do not affect established plants.
When using any chemical, read the product label carefully for specific application rates and safety precautions. Non-selective herbicides, like glyphosate, will kill any desirable plant they contact. If treating a pasture, hay should not be harvested for two to three weeks after application to ensure the herbicide moves into the rhizomes for maximum control.
Long-Term Management and Prevention
Eradication is a multi-year process, and long-term success hinges on constant monitoring and strict prevention protocols. Because Johnsongrass seeds remain viable for decades, vigilance is necessary for several years after initial control efforts. New sprouts and seedlings must be removed immediately, as a young plant can begin forming new rhizomes in less than three weeks.
Preventing the introduction of new seeds and rhizomes is paramount. All farm and garden equipment, especially tillage tools and harvesters, should be thoroughly cleaned using an air compressor or pressure washer before moving to a non-infested area. This practice removes seeds and small rhizome fragments that can easily establish a new infestation.
Avoid importing contaminated materials such as hay, straw, or soil from areas with a known Johnsongrass problem. Utilizing crop rotation strategies that include dense, competitive crops can help prevent the soil seed bank from re-establishing a dominant population.