Dallisgrass and crabgrass are two common weed grasses in lawns throughout the United States. When immature, they can be easily confused. For healthy turf, it's important to control these species, as they can compete with your lawn for the same nutrients and water, reducing the density of the surrounding turf. Control can be obtained by both pre-emergent weed control as well as post-emergent control.
Dallisgrass (Paspalum dilatatum) is a perennial grassy weed. Dallisgrass can appear as clumps much like crabgrass, but in fact is comprised of loose bunches. When allowed to grow, dallisgrass can reach heights of up to 5 feet. The individual blades of dallisgrass are stiffer than those of crabgrass, which may be the only distinguishing feature in lawns that are mowed. The flowers of dallisgrass, if allowed to bloom, are distinguishable from crabgrass. Each stem can produce three to six long, loose flower spikes.
Crabgrass (Digitaria spp.) is an annual weed grass. It spreads by seeds from the previous year. It can be easily distinguished from lawn grass by its clump-like form. Blades of crabgrass are between 1/4 of an inch to 1/3 of an inch wide, with a prominent mid-vein. Crabgrass blades can grow up to 5 inches. If unmowed, it will bloom in the fall. The flowers of crabgrass occur in one or more whorls at the tip of the stem.
Pre-emergent herbicides can be used to prevent the seeds of both crabgrass and dallisgrass from germinating. They must be applied in early to mid-spring to be effective. Crabgrass emerges almost entirely from seed and so is more effectively controlled by pre-emergent herbicides than dallisgrass. Pre-emergent herbicides will help to prevent new dallisgrass from emerging, but will not control established dallisgrass. Herbicides containing the active ingredients pendimethalin, dithiopyr, benefin plus trifluralin or benefin plus oryzalin have all been proven as effective pre-emergent control for many lawn weed species.
If the window of opportunity for pre-emergent control is missed, post-emergent control for dallisgrass and crabgrass can be applied when the weeds are actively growing. Post-emergent herbicides must not be applied when the weeds are dormant, as they will not be effective. Take caution to select a lawn herbicide that will kill the weeds and not the lawn. Herbicides containing the active ingredients MSMA, quinclorac and fenoxaprop are commonly used to control grassy weeds, while not harming turf.