Description. Two species of crabgrass are found throughout the semi-tropical and temperate zones of the U.S. - smooth crabgrass (Digitaria ischamum) and hairy crabgrass (Digitaria sanguinalis). Both are annual grasses that emerge in early to mid-spring and are killed by the first frost in fall. Crabgrass flowers throughout the summer and is a prolific seed producer.
Crabgrass has one significant weakness in that the seed requires light to germinate. Consequently, a dense turf cover effectively resists invasion by crabgrass. But, anything that weakens the turf during the spring and
summer such as disease, insect damage, traffic or winterkill increases the likelihood of a crabgrass invasion. Often, cultural practices such as aeration and dethatching increase the crabgrass problem by exposing the seed to favorable conditions-sunlight, moisture and high temperatures.
Once crabgrass germinates it rapidly dominates a turf. Crabgrass is a vigorous plant that grows faster than the most desirable turfgrasses. It grows under stress conditions such as drought, heat and low fertility where turfgrasses suffer. Crabgrass thrives under low mowing heights because of its prostrate growth habit.
Control. Crabgrass control requires a sound turf maintenance program together with a planned herbicide program. A dense turf is the best protection against invasion by crabgrass. Cultural practices that promote a dense, healthy turf include regular mowing and watering together with timely fertilization, aeration and dethatching. Pest management including insect and disease control is also essential to preventing crabgrass invasions.
For the past 25 years, preemergence control of crabgrass has been the target of considerable research. A number of very good herbicides have been developed for turf as a result of that research. DCPA (Dacthal), simazine (Princep), besulide (Betasan, Pre-san), benefin (Balan), dithiopyr (Dimension), oxadiazon (Ronstar), oryzalin (Surflan), prodiamine (Barricade), pendimethalin (Pre-M) and napropamide (Devrinol) are some of the materials available for preemergence crabgrass control.
Since crabgrass germinates from April through September in most areas of the country (slightly shorter periods in other areas), few of these herbicides provide season-long control. All of these products should be applied about 2 weeks prior to the expected date of emergence of crabgrass. Since this date varies from North to South and from year to year, a specific date must be developed by turf managers from past experiences, climatic conditions and, perhaps, biological indicators.
The presence of thatch is another factor that influences the effectiveness of preemergence herbicides. Some products such as benefin and oxadiazon are more injurious to turf where a thatch layer is present. Other products including DCPA and benefin break down more rapidly in thatch than in soil. Thus lighter and more frequent applications of preemergence herbicides may be required for effective season-long control of crabgrass in thatchy turf.
Occasionally, turfgrass injury may appear during stress periods after the application of preemergence materials. Injury usually resembles drought stress and may be attributed to root injury by the herbicide. The symptoms should be treated as drought stress by increasing the frequency of irrigation and applying fungicides to reduce disease occurrence on the weakened grass.
Selective postemergence control of crabgrass with chemicals is effective in many situations. The organic arsenicals including MSMA, DSMA, AMA and CAMA effectively control crabgrass with little injury to bermudagrass or zoysia when used properly. These materials should not be applied to fescue, St. Augustine, centipede or bahia turf. A single application of one of these materials at 2 pounds active per acre in 80 to 100 gallons of water per acre will control seedling crabgrass. Repeat applications at 14-day intervals may be required to control more mature crabgrass plants. For best crabgrass control, turf should not be mowed for 2 to 3 days prior to application or 2 to 3 days after application and temperatures should be 80 to 90°F at the time of application.
In some states, Asulox is labeled for postemergence control in St. Augustine grass turf. A single application at 5 pints per acre is recommended for crabgrass control in St. Augustine grass.
Dimension Turf Herbicide (dithiopyr) is a preemerge product that provides effective postemerge control of crabgrass when plants are in the 3 to 4 leaf stage. Dimension will control more mature crabgrass plants, but may require 4 to 6 weeks to kill the plants.
Where crabgrass and other weeds are dominant and renovation is required, nonselective herbicides such as glyphosate (Roundup) can be used. These products will kill existing weeds and grasses prior to planting. Again, label recommendations with respect to turf renovation should be followed. Keep in mind that crabgrass and other weeds will germinate from seed after treatment with these products. Thus, selective postemergence materials may be required after planting.