During the last few week, I have visited some hay meadows in Hopkins County infested with Dallisgrass. For some, Dallisgrass was not recognized because either the ranchers never seen it before or was never seen in their pastures. According to UC Davis IPM website, Dallisgrass, Paspalum dilatatum, is a tufted perennial grass that was introduced into the United States from Uruguay and Argentina. It is now naturalized in much of the southern United States. It has been used as a pasture grass in wet areas or irrigated sites. The seed heads are susceptible to an ergot fungus that is toxic to livestock when ingested. Dallisgrass is primarily a weed in turfgrass, wet roadside areas, irrigation ditchbanks, and in some orchards and vineyards. It is closely related and similar in appearance to knotgrass, Paspalum distichum, which is a mat-forming perennial grass with good forage qualities and more desirable attributes for natural areas. Bahiagrass, Paspalum notatum, is also a warm-season tufted perennial with short rhizomes that could be confused with dallisgrass or with Florida paspalum, a more robust relative of Dallisgrass. Dallisgrass is a coarse-textured grass that grows in a clump and slowly increases in diameter as its shallow, underground stems (short 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 such as crabgrass. As the clump matures, the center may die and a different grass or weed may be growing in its center. Where large numbers of dallisgrass plants grow together they can form almost a solid planting with uneven texture and poor turfgrass qualities.The leaf blades of dallisgrass are fairly wide (1/4–1/2 inch) compared to desirable thinner grasses. If left unmowed, blades will grow 4 to 10 inches long. At the base of each leaf blade is a collar with a membranous ligule about 1/4 inch long and no auricles or projections. At the base of the collar is the leaf sheath, which is slightly flattened. Frequently there is purplish coloration at the base of the grass stems (technically called culms). The flowering stalk (raceme) grows 14 to 65 inches tall and the flower head (inflorescence) consists of 2 to 10, often drooping, spikelets (delicate branches) that arise from different points at the top of the flower stalk. Each spikelet has two rows of flat, egg-shaped seeds along its entire length and is pale green to purplish in color. Dallisgrass produces abundant amounts of seed, which are its primary means of dispersal. Water, mowers, and humans or pets spread the seed to new places. Seeds usually germinate in spring and summer when soil temperatures are in the 60° to 65°F range and grow to form new clumps. The optimum air temperature range for growth is 80° to 90°F and when temperatures are in this range, plants grow very rapidly. This weed is often found growing in wet areas such as drain ditches, low places, and in heavily irrigated turfgrass. It tolerates both sandy and heavy clay soils and, once established, is drought-resistant and frost-tolerant. Dallisgrass does not become off-color in winter like many warm-season grasses. It responds to nitrogen fertilizer and competes well against grasses in fertilized sites. The importance of dallisgrass infestations will depend on the ultimate use of the land and the crop: for homesites is a lawn weed were creates an unsightly clump in turfgrass that can be a problem in golf courses, sports playing fields, and home landscapes. The stiff clumps are much coarser in texture than other grasses common in developed recreational areas such as lawns, golf courses, or parks and can present a hazard in sports fields and play areas, causing people to fall. It has a faster growth rate than turfgrasses. The flower stalks (racemes) often escape mowing and spring back up above the rest of the turfgrass, causing problems in golf courses and sports fields as well as lending a rough, uneven appearance to lawns. In pastures, dallisgrass can cause toxicity problems in cattle if flower stocks are present or can reduce hay quality in haymeadows. Basically, cattle will graze well on dallisgrass but care must be done not to overexpose cattle to head stocks without cattle getting them used to it because the potential intoxication caused by the fungus that typically grows in the seed stock.
A major component of dallisgrass management is preventing establishment of new plants. In home landscapes, removing young plants by digging them out before they form rhizomes or set seed is the best strategy for control. Mature plants can also be dug out, but they sometimes grow back if rhizomes are left behind. In professionally managed turfgrass areas, prevention is an important component in managing this weed. When dallisgrass is abundant or the plants are located over a large area, it may be necessary to supplement cultural practices with herbicides. For infested pastures, the only alternative is glyphosate treatment of areas with high numbers of Dallisgrass.
Dallisgrass can be introduced into lawn areas with new turfgrass seed or sod, but often 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.
Because dallisgrass is a perennial plant, persistence is required to kill it with cultural practices. In lawn areas the clumps can be removed by digging. Mowing will not remove dallisgrass, but when grass is mowed at its optimum height, it is better able to resist an invasion of this weed.
When dallisgrass has been established for some time in the grass, seed will be abundantly present in the soil. In well-established grass areas, seedlings may not be able to establish, but if there are open areas in the turf, seed will germinate in these areas. If bare areas are present, overseed them with desirable grass species to reestablish the grass.
Where digging out clumps of dallisgrass in turfgrass is not practical, herbicides may be used. Herbicides to control established plants are referred to as postemergent herbicides. These herbicides are either selective and kill only specific weeds, or they are nonselective and kill any plant they come in contact with. To control germinating seed, preemergent herbicides are used. In order to obtain complete control of this perennial grass weed, it is necessary to control both the established dallisgrass plant and the germinating seed.
Preemergent herbicides can be used in established turfgrass to control germinating dallisgrass seed. Apply preemergent herbicides in late winter or early spring before dallisgrass seed germinates. Herbicides that control crabgrass such as benefin + oryzalin, bensulide, DCPA, dithiopyr, oryzalin, oxadiazon pendimethalin or prodiamine, are also effective on dallisgrass. (Bensulide, DCPA, and oxadiazon are for professional use only and may have some restrictions for use on residential lawns.) Preemergent herbicides used on lawns need to be irrigated into the soil with about 1/2 inch of water relatively soon after application in order to become effective. Consult the label for application details. Thee preemergent herbicides napropamide, oryzalin, pendimethalin, or combinations of benefin plus oryzalin are effective to prevent dallisgrass seed from germinating. Benefin plus trifluralin is also available, but only for use by commercial pest controllers. Once seedlings appear, then postemergent herbicides may be necessary to control them chemically. Follow label recommendation and product availability since those change quite often. For more information on this, please contact the Hopkins County Extension Office at 903-885-3443 or email me at [email protected].tamu.edu
Mario Villarino DVM, Ph.D.
Hopkins County Extension Agent for Ag and NR
1200B Houston Street
Sulphur Springs, Texas 75482