Johnson grass (Sorghum halepense) is a highly invasive perennial weed that challenges homeowners seeking a uniform lawn. It is considered one of the world’s most aggressive weeds due to its ability to spread and persist across various environments. Its survival relies on a robust underground structure, making superficial removal or simple mowing ineffective. Successfully eliminating this weed requires a multi-season, integrated approach targeting both the plant above ground and its extensive root system below.
Identifying Johnson Grass and Its Root System
Identifying Johnson grass is the first step toward effective control, as it is often mistaken for other common lawn invaders. Mature plants are distinct, growing tall, sometimes reaching eight feet, and feature broad leaves with a prominent, thickened white midvein. The plant produces large, open, and often purplish seed heads, known as panicles, throughout the summer and early fall.
The weed is difficult to eradicate due to its aggressive network of rhizomes, which are horizontal underground stems. These fleshy structures can spread more than six feet laterally and store carbohydrates, allowing the weed to regenerate quickly even after the foliage is cut or killed. New shoots sprout from buds along these rhizomes, typically found in the top eight inches of soil. Simply pulling the top growth leaves behind these energy-storing rhizomes, guaranteeing the plant’s rapid return.
Manual and Cultural Control Methods
For small or isolated patches of Johnson grass, persistent physical removal can be effective. This manual control requires careful, deep hand-digging to ensure the complete removal of the entire rhizome system. Every piece of the fleshy, white to purple-spotted rhizome must be removed, as fragments left in the soil can sprout new plants. This method is practical only for minor infestations where the soil can be thoroughly sifted.
Cultural methods focus on exhausting the weed’s energy reserves and suppressing growth. Repeated, close mowing prevents seed production and forces the plant to continuously draw on stored carbohydrates in its rhizomes. Consistent mowing over one or multiple seasons significantly weakens the population, making it more susceptible to other treatments. Small, isolated areas can also be treated with solarization by covering the patch with clear plastic sheeting during the hottest months. This process traps solar heat, raising the soil temperature high enough to kill the rhizomes beneath the surface.
Selective Herbicide Strategies for Lawns
For widespread infestations, selective post-emergent herbicides are the most reliable method, though the choice depends on the type of lawn grass present. Homeowners with warm-season grasses like Bermuda, Zoysia, or Centipede grass have selective options containing active ingredients such as sulfosulfuron. These products control grassy weeds without harming the turf and often require two applications for satisfactory control. Another option for warm-season turf is an herbicide combining thiencarbazone-methyl, iodosulfuron-methyl-sodium, and dicamba.
Effective application timing is essential for systemic herbicides, which must be absorbed by the leaves and transported down to the rhizomes. Application should occur when the Johnson grass is actively growing, typically from late spring through late summer, and when the plant is at least 10 to 18 inches tall. Because Johnson grass leaves have a natural waxy coating, adding a non-ionic surfactant to the mixture is necessary to help the spray solution stick and penetrate the foliage. For cool-season lawns (Kentucky Bluegrass, Fescue, or Perennial Ryegrass), there are no selective post-emergent herbicides that will kill Johnson grass without damaging the turf. The only practical chemical control method is careful, non-selective spot-treatment using a product containing glyphosate, targeting only the Johnson grass foliage.
Managing Seedlings and Preventing Recurrence
Long-term management is required because a single Johnson grass plant can produce thousands of seeds that remain viable in the soil for several years. The seeds germinate when soil temperatures reach 60 to 70 degrees Fahrenheit, making spring a crucial time for preventive action. Pre-emergent herbicides form a chemical barrier in the soil to stop seeds from sprouting and are highly effective against new Johnson grass seedlings.
Active ingredients like prodiamine or pendimethalin can be applied in the spring to prevent new weed growth from seed, but these products will not affect established plants emerging from rhizomes. Beyond chemical control, improving the density and health of the turf is a powerful cultural prevention method. Maintaining the lawn at the highest recommended mowing height for your grass species creates a dense canopy that shades the soil, discouraging the germination of Johnson grass seeds. Providing appropriate fertilization and ensuring the lawn receives about one inch of water per week encourages the turf to outcompete any new weed seedlings.