How to Manage Pests
Pests in Gardens and Landscapes
Dallisgrass (Paspalum dilatatum) is a warm-season perennial bunch grass that was introduced into the United States from South America in the 1800s for use as a forage plant. It is now a troublesome weed that has naturalized in much of the southern United States. In California, dallisgrass is found throughout the state except at high elevations, such as the Great Basin and Sonoran Deserts. It has been used as a pasture grass in wet areas or irrigated sites but is now rarely used as forage in California because of its extreme weediness. In addition, the seedheads are susceptible to a fungus that is toxic to livestock when ingested.
Dallisgrass is a weed in turfgrass, wet roadside areas, irrigation ditch banks, orchards, vineyards, and ornamental landscaped areas. It is closely related and similar in appearance to knotgrass (Paspalum distichum), a mat-forming perennial grass with good forage qualities and more desirable attributes for natural areas. Bahiagrass (P. notatum) is another related warm-season perennial bunch grass with short rhizomes, which is sometimes grown as a turfgrass in southern California and could also be confused with dallisgrass. Additionally, seedlings and growing sprouts of dallisgrass can be mistaken for hairy and smooth crabgrass (Digitaria sanguinalis and D. ischaemum), two very common warm-season annual lawn weeds in California.
Dallisgrass grows in a clump that slowly increases in diameter as its shallow, short, underground stems (rhizomes) grow outward. The rhizomes have short internodes (the length of stem between the joints) that look like concentric rings on its surface. The presence of these distinctive rhizomes is a good way to distinguish dallisgrass from other common clumping grasses in lawns, such as crabgrass. As the clump matures, the center may die and a different grass or weed may be growing in its place. Where large numbers of dallisgrass plants grow together, they form almost a solid planting with uneven texture and poor turfgrass qualities. New dallisgrass plants can emerge from rhizomes that were established in previous years as well as from seeds that will develop into plants with overwintering rhizomes.
The coarse-textured gray-green leaf blades are wide (1/4–1/2 inch) compared to most desirable turfgrasses. If left unmowed, blades will grow 4 to 10 inches long with a prominent midrib. At the base of each leaf blade is a collar with a membranous tissue (ligule) about 1/4 inch long and no projections (auricles) except for a few hairs. At the base of the collar is the leaf sheath, which is slightly flattened and sometimes hairy. Frequently there is purple coloration at the base of the grass stems.
Dallisgrass seedheads are easy to identify. The inflorescence consists of a flowering stalk growing 1 to 5 feet tall with 2 to 10, often drooping, delicate branches alternately arranged toward the top of the stalk. Each branch has 2 to 4 rows of flat, egg-shaped seeds with a fringe of hairs along its entire length. The flowering stalk is pale green to purple in color. Dallisgrass produces abundant amounts of seed, which are its primary means of dispersal. Water, lawn mowers, and humans or pets spread the seed to new places. Seeds usually germinate in spring and summer when soil temperatures reach 60° to 65°F and the seedlings grow to form new clumps. The optimum air temperature for growth is 80° to 90°F and when temperatures are in this range, plants grow rapidly.
This weed is often found growing in wet areas such as drainage ditches, low spots, and in heavily irrigated turfgrass. It tolerates both sandy and heavy clay soils and, once established, is drought-resistant and frost-tolerant. Dallisgrass tolerates lower temperatures than bermudagrass before going dormant and may not become off-color in winter like many warm-season grasses. It responds well to nitrogen fertilizer and is highly competitive against turfgrasses in fertilized sites.
Dallisgrass creates unsightly and uneven clumps in turfgrass, causing problems in golf courses, sports playing fields, recreational parks, and home landscapes where a uniform texture and surface is necessary for balls to roll and people to walk, run, and play. It has a faster growth rate than most turfgrasses and adapts to low mowing heights. In mowed sites the seedheads can grow low and almost parallel to surface which allows them to escape mowing and spring back up above the mown turfgrass. The stiff clumps are much coarser in texture than other turfgrasses and present a tripping hazard. In other sites such as ornamental landscapes, dallisgrass is a competitor for nutrients and water.
A major component of dallisgrass management in home landscapes and professionally managed turfgrass areas is preventing establishment of new plants. The best strategy is to remove young plants by digging them out before they form rhizomes or set seed. Mature plants can also be dug out, but they sometimes grow back if rhizomes are left behind. When dallisgrass is abundant or the plants are located over a large area, it may be necessary to supplement cultural practices with herbicides or consider partial or complete renovation of the lawn. Dallisgrass is a very difficult weed to control or eradicate in lawns, even when herbicides are used.
Dallisgrass can be introduced into lawn areas with new turfgrass seed or sod. Commonly, the seed is introduced on mowers that have been used in contaminated sites and then moved to weed-free sites. Cleaning a mower after mowing a contaminated site should reduce the chance of invasion into new areas. Inspect sod before taking delivery to make sure dallisgrass is not present. Don’t use soil from dallisgrass-contaminated areas to repair low or bare spots in lawns. In dallisgrass-infested areas, delay or minimize the amount of aeration performed on the turfgrass in spring when new seedlings germinate to avoid small open areas where dallisgrass plants might become established.
Cultural and Physical Control
Cultural controls are practices that change the conditions surrounding the desirable plant to be unfavorable for the pest and more favorable for the plant This could include adjusting irrigation or fertilization practices, planting competitive plants, changing mowing height or frequency, or digging out the unwanted plant with physical means.
Lawns: Because dallisgrass is a perennial plant, persistence is required to kill and remove it with cultural practices. In lawn areas the clumps can be removed by digging. Mowing the turfgrass will not remove dallisgrass. Turfgrass is better able to resist an invasion of this weed when mowed at its optimum height and fertilized and irrigated consistently to increase vigor and density. Dallisgrass is often found in low-maintenance turf where irrigation is not well managed (i.e., application of a large amount of water with long intervals between applications). This situation favors dallisgrass over the desirable turf species. When dallisgrass has been established for some time in the turfgrass, the soil is likely to contain large amounts of seeds. In well-established dense turfgrass, dallisgrass seedlings may not be able to establish, but if there are open areas in the turf, seed will germinate in these spaces. If bare areas are present, overseed or sod them with the desirable turfgrass species to reestablish the turf.
Landscapes: Dallisgrass is not normally a problem in ornamental beds, but if it does occur, the weeds can be carefully dug out, removing all rhizomes. Apply a thick layer of mulch over the area to prevent or minimize new germination and seedling establishment. If the dallisgrass population was abundant, consider using a preemergent herbicide to control germinating seeds before applying mulch. Mulching with organic materials is not very effective for the control of mature dallisgrass. However, if the tops of the plants are removed down to the soil line, laying black plastic or landscape fabric over the area will control the rest of the plant as well as any new seedlings.
Other sites: Along roadsides and fences, the plants can be dug out during summer and left in place for the clumps of rhizomes to desiccate. If all the rhizomes are dug up and dried, the plant will not regrow. Buried seed will continue to germinate and establish so those seedlings will need to be removed or the seeds prevented from germinating.
Summer solarization with clear plastic significantly helps control dallisgrass seed and reduces rhizome regrowth. For information on solarization, see the publication Pest Notes: Soil Solarization for Gardens & Landscapes.
Herbicides may be used, when digging out clumps of dallisgrass in turfgrass is not a practical solution. Herbicides to control established plants that have already germinated are referred to as postemergence herbicides. These herbicides are either selective and kill only specific weeds, or they are nonselective and kill most plants they contact. To control germinating seeds, preemergence herbicides are used. To obtain complete control of this perennial grass weed, it is necessary to control both the established dallisgrass plant and the germinating seed.
Most herbicides mentioned in this publication are only available to licensed professional applicators. In home lawns with significant dallisgrass populations, it is often more time-saving and less costly to completely renovate the lawn. Turf managers of sports fields and golf courses should consult with professional weed control specialists.
Preemergence Herbicides to Prevent Dallisgrass Seed Emergence
*This is a partial listing of some common herbicide products. Always read the label to be sure the product is safe to use on your specific site.
Postemergence Herbicides for Use in Warm Season Turfgrasses
The herbicides listed in Table 2 have specific directions for dallisgrass suppression or control on their label. They can only be purchased and applied by a licensed professional applicator. Revolver, Celsius, Monument, and Certainty belong in the same herbicide class (sulfonylurea) and weed resistance may develop, if these herbicides are used repeatedly and exclusively. In general, they selectively remove cool season grasses, sedges, and a few other tough weeds (such as dallisgrass) from warm season turfgrasses.
When using any of these products at least 3 applications are necessary for control of established dallisgrass plants. Make the first application in the fall when air temperatures drop to about 70°F as this is when dallisgrass growth is starting to slow and the herbicide will be more effective. Make the second application 4 to 6 weeks later. The third application should be in the mid to late spring when dallisgrass starts growing. Properly timing the applications will lead to better control. Starting the applications in late spring and summer will be less effective and more applications will be needed to suppress the dallisgrass. Follow label directions closely and use the recommended surfactants and adjuvants.
Spot sprays rather than broadcast sprays bring better results, but avoid the tendency to overdose the weeds which can result in turfgrass injury. Revolver and Celsius are often combined to control dallisgrass since 2 active ingredients are more effective for control of this weed. Celsius contains 3 active ingredients, but only thiencarbazone-methyl acts on dallisgrass. The other 2 ingredients will control many broadleaf weeds.
Dallisgrass suppression and control is enhanced when MSMA (mono sodium methane arsenate) is used in tandem or combination with the sulfonylurea herbicides (Revolver, Celsius, Certainty, and Monument). MSMA is a postemergence herbicide that was widely used for many years in home lawns, parks, and sports fields. It is very effective on hard to control grass weeds, such as dallisgrass and sedges growing in warm season turfgrass. However, MSMA has been severely restricted to specific sites because arsenic has the potential to become a contaminant in ground water. MSMA can only be used on partial acreage of golf courses and sod farms, and there are additional restrictions that must be followed. MSMA is no longer registered for use on home lawns in California. Although the product Certainty is registered for use on home lawns for weed control, dallisgrass suppression depends on combining it with MSMA, so Certainty is not an effective dallisgrass treatment in home lawns.
Fluazifop (sold as Fusilade II) may be used in zoysiagrass, but turf tolerance varies by cultivar. Most California zoysiagrass lawns are Zoysia japonica cultivars such as Meyer, El Toro, DeAnza, and Victoria, which are more tolerant than Zoysia matrella cultivars. More information about fluazifop herbicide follows in the next section about cool season turfgrasses.
*WARNING: Do not use these products on cool season grasses such as tall fescue, fine fescue, Kentucky bluegrass, perennial ryegrass, or creeping bentgrass as turfgrass injury will occur.
Postemergence Herbicides for Use in Cool Season Turfgrasses
Only two herbicides can be used in cool season turfgrass to control dallisgrass plants. Fusilade II herbicide kills many grasses but not broadleaf plants. This herbicide is most commonly used by licensed professional applicators to kill grasses growing in broadleaf crops and ornamental plants or in noncrop situations, such as roadsides and fence rows. In cool season lawns Fusilade II may only be used on fine fescue and tall fescue turfgrasses to selectively remove several annual and perennial grass weeds. Do not apply this herbicide if the lawn is under stress because the turfgrass could be injured. For best results make applications in spring and fall and avoid treatments in hot summer months, especially July and August. Several monthly applications will be needed to adequately control established dallisgrass.
MSMA can be used on Kentucky bluegrass turfgrass, but as said earlier its use is restricted to golf courses and sod farms. There are no selective herbicides for dallisgrass control in bentgrass, ryegrass, or creeping fescue.
Nonselective Postemergence Herbicides for Dallisgrass Control in Turf, Landscapes, and Other Sites
Spot Treatments with Glyphosate: The herbicide glyphosate will kill any desirable turfgrass and most broadleaf plants it contacts in addition to dallisgrass plants. Yet in many situations, spot treatments with glyphosate may be the fastest, easiest, and cheapest solution to dallisgrass control. Be aware that even when glyphosate is carefully applied, spot treatments will damage the desirable turf. Bare areas in lawns, especially non-creeping turfgrasses such as fescues, will need to be replanted to improve the aesthetic appearance and to prevent new weed invasions. There are many formulations and concentrations of glyphosate, and it is sold under various trade names, including RoundUp, KleenUp, Remuda, and Weed & Grass Killer. Each label lists how much to use for specific perennial weeds, including dallisgrass.
In summary, managing dallisgrass infestations in turfgrass, especially home lawns, is a difficult, tedious process that usually takes several years. Unfortunately, there is no simple and quick option. In heavily infested dallisgrass sites a complete lawn renovation might be the easiest and least costly solution.
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AUTHORS: Michelle LeStrange, UC Cooperative Extension, Tulare County (emeritus) and John A. Roncoroni, UC Cooperative Extension, Napa County (emeritus).
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