Dallisgrass (Paspalum dilatatum) is a highly invasive perennial weed that commonly infests warm-season turfgrasses, including St. Augustine. It thrives in the same hot, humid conditions, allowing it to compete aggressively for resources. Controlling this weed is challenging due to its biology, requiring a targeted, multi-season strategy rather than a single solution.
How to Identify Dallisgrass and Why It Resists Removal
Accurate identification is the first step in successful removal. Dallisgrass grows in distinct, coarse clumps that are lighter green and more upright than the surrounding St. Augustine turf. Its leaves are wide, flat, and often have a slight hairiness near the base, featuring a prominent central midrib. The most distinct feature is the seed head, which grows on tall stalks that quickly shoot up above the lawn, even soon after mowing. These seed heads consist of two to ten short, finger-like spikes (racemes) that alternate along the stem.
The weed’s tenacity comes from its perennial nature and dual reproduction strategy. Unlike annual weeds, Dallisgrass returns each year from the same established root system, spreading through prolific seeding and tough underground stems called rhizomes. The fibrous root system and rhizomes store carbohydrates, making the plant exceptionally resilient and allowing it to regrow even after top growth is damaged. This deep-rooted structure is why simply pulling the weed or applying a general surface herbicide is ineffective.
Manual and Cultural Removal Strategies
For small, isolated clumps of Dallisgrass, mechanical removal is the most effective way to ensure complete eradication. This process involves digging out the entire plant, including the fibrous root crown and all associated rhizomes. Use a sharp spade or trowel to dig deep and wide enough to remove the entire structure, as any remaining root material can lead to regrowth. Afterward, the resulting bare spot should be immediately filled with new topsoil and repaired with St. Augustine plugs or sod to prevent new weeds from germinating.
Targeted spot treatment can be used if avoiding selective herbicides. Spot-treating individual clumps with a non-selective product like glyphosate will kill the Dallisgrass, but it will also kill the surrounding St. Augustine grass. To protect the turf, contain the spray by cutting the bottom out of a plastic jug, placing it over the weed, and spraying the herbicide inside. For organic solutions, concentrated horticultural vinegar or iron-based herbicides can be used, though these typically require multiple applications and may only suppress the weed temporarily.
Choosing Selective Herbicides Safe for St. Augustine
Chemical control of Dallisgrass requires highly specific selective post-emergent herbicides because St. Augustine grass is sensitive. Many effective Dallisgrass killers, such as MSMA, are not labeled for residential use or injure St. Augustine turf. The most recommended selective herbicide for suppression is an active ingredient combination found in products like Celsius WG. While Celsius alone can suppress Dallisgrass, it often requires a tank mix with a companion herbicide for better control, though users must be cautious as some combinations may be too harsh for St. Augustine.
Application timing is important; post-emergent herbicides are most effective when Dallisgrass is actively growing, usually during the late spring and summer months. The weed should be healthy and not under drought stress when treated so the herbicide can translocate effectively to the roots. Because Dallisgrass is a perennial with a robust root system, a single application is rarely sufficient for complete control. Sequential applications, typically spaced two to four weeks apart, are necessary to deplete the root reserves and achieve a higher level of control.
Always follow the product label instructions precisely regarding mixing ratios and application rates to avoid damaging the St. Augustine turf, which is susceptible to herbicide burn, especially during high heat or stress. While pre-emergent herbicides will not kill established perennial Dallisgrass, they are valuable for preventing new seedlings from germinating in the spring when soil temperatures reach 60 to 65 degrees Fahrenheit. This preventative measure helps reduce the overall population pressure by limiting the spread from prolific seed production.
Long-Term Prevention of Dallisgrass Return
The most effective long-term defense against Dallisgrass is maintaining a dense, healthy St. Augustine lawn. Promoting a thick turf canopy naturally shades the soil, which helps suppress the germination of Dallisgrass seeds. St. Augustine should be mowed at its maximum recommended height, typically between two to four inches, to ensure fewer weed seeds receive the light necessary to sprout.
Proper irrigation and fertilization schedules also play a significant role in creating a competitive environment. Watering deeply and infrequently encourages a deep-rooted, resilient turfgrass. A consistent fertilization program that meets the St. Augustine variety’s nutritional needs will promote vigorous growth, allowing the desired turf to outcompete any emerging weed seedlings. Focusing on these cultural practices makes the lawn itself the primary barrier against the return of Dallisgrass.