Dallisgrass (Paspalum dilatatum) is a common, highly aggressive perennial grass weed that challenges homeowners maintaining a Bermuda grass lawn. This warm-season weed spreads relentlessly through seed production and short, thick underground rhizomes. Effectively managing this invader requires a targeted approach that combines specific post-emergent chemistry with precise timing to selectively kill the weed without damaging the desirable turf.
How to Positively Identify Dallisgrass
Accurate identification is the first step before applying herbicides. Dallisgrass is a clumping grass that grows in a coarse bunch habit, often appearing significantly taller than the surrounding Bermuda grass shortly after mowing. Its leaf blades are typically wider, coarser, and a lighter green color than Bermuda grass, sometimes tinged with purple near the base.
A defining characteristic is the ligule, a small, white, membranous structure at the junction of the leaf blade and the sheath, which is often tall and pointed. The plant produces short, thick rhizomes beneath the soil surface, allowing it to survive winter and regrow annually. The most visible identifier is the seed head, which emerges on a tall, slender stalk with three to five “finger-like” spikes, or racemes, that radiate from the stem. These spikes contain small, hairy spikelets that often develop distinctive black spots.
The Most Effective Selective Post-Emergent Treatments
The challenge of killing Dallisgrass selectively in Bermuda grass is that both are warm-season perennial grasses. Historically, the gold standard for control was Monosodium Methanearsonate (MSMA), which offered 60-80% control over the perennial weed. However, MSMA is now heavily restricted by the Environmental Protection Agency (EPA) for environmental reasons and is generally limited to commercial sites like golf courses and sod farms.
For homeowners, effective selective control relies on newer, specialized herbicides known as sulfonylureas. These systemic products are absorbed through the leaves and translocated down into the root system and rhizomes. Active ingredients such as foramsulfuron, trifloxysulfuron, and the combination product containing thiencarbazone-methyl are the primary alternatives. These products, often sold under trade names like Revolver, Monument, and Celsius, typically provide suppression rather than complete eradication.
The ACCase inhibitor pinoxaden, found in products like Manuscript, also shows activity against Dallisgrass. Successful treatment requires multiple, sequential applications because the weed’s robust rhizomes store carbohydrates for survival. Adding a non-ionic surfactant to the spray solution is often necessary to help the chemical penetrate the waxy leaf surface and improve its systemic action.
Essential Application Timing and Methods
Timing is the most important factor for maximizing the effectiveness of post-emergent herbicides. The ideal time for the initial application is in the early to late fall, while the weed is still actively growing but preparing for winter dormancy. During this period, the plant is actively moving nutrients and carbohydrates down to its root system for storage, which increases the translocation of the systemic herbicide.
Research suggests that the best control is achieved when the 24-hour average air temperature falls below 72°F in the fall. Multiple sequential applications are necessary for control, typically involving two to three sprays spaced 10 to 14 days apart. This repeat schedule is necessary to overwhelm the Dallisgrass’s perennial root system and prevent it from recovering.
Spot treatment is highly recommended to minimize the risk of injury to the surrounding Bermuda grass. Using a calibrated sprayer to ensure the correct mixing ratio is essential. Applying too much herbicide can injure the turf, while applying too little will not kill the weed. The treatment should focus on individual clumps, ensuring the leaves are thoroughly covered but avoiding excessive run-off.
Preventing Future Dallisgrass Emergence
Controlling established Dallisgrass must be paired with a proactive strategy to stop new weeds from emerging. Dallisgrass reproduces by seed, which germinates in the spring when soil temperatures consistently reach 60°F to 65°F. Therefore, the application of a pre-emergent herbicide in late winter or early spring is necessary to create a chemical barrier in the soil that prevents the seeds from sprouting.
Active ingredients like prodiamine (Barricade) or dithiopyr (Dimension) are effective pre-emergent options that prevent Dallisgrass seedlings from establishing. This preventative step is distinct from the post-emergent killing of existing perennial clumps, as pre-emergent products have no effect on established plants. Proper cultural practices that favor Bermuda grass growth are equally important for long-term control.
Maintaining a dense, healthy Bermuda grass lawn naturally suppresses weed growth by limiting available sunlight and space. Bermuda grass should be mowed at a low height, typically between 0.5 and 1.5 inches, to promote lateral spread and density. Additionally, watering deeply and infrequently encourages the Bermuda grass to develop deep roots while discouraging the shallow-rooted Dallisgrass.