Crabgrass is a warm-season annual weed that germinates from seed each spring and thrives throughout the summer if not controlled. Two species of crabgrass are commonly found in Iowa lawns: smooth crabgrass (Digitaria ischaemum) and large crabgrass (Digitaria sanguinalis). Both plants have light green foliage color and prostrate growth habit. Seedheads, which look like finger-like projections, begin appearing in late summer and continue until frost. Individual plants have the potential to produce thousands of seeds each.
Control in Lawns | Control in Garden Beds | More Information
Controlling Crabgrass in Lawns
The best way to prevent crabgrass infestations in lawns is to maintain a thick, healthy lawn through proper mowing, irrigation, and fertilization. Crabgrass will have a difficult time germinating and surviving in thick turf. Fertilize your existing turf in the fall. A healthy turf stand will have fewer weeds due to a more uniform canopy with few openings for crabgrass to move into. Seed thin spots help thicken the turfgrass to reduce any openings in the turfgrass, which allows the light needed for crabgrass seed germination. Maintain turf at 3-4” height of cut. Taller turfgrass stands tend to have less weed pressure since the leaves shade out the germinating crabgrass.
If crabgrass is an issue in the lawn, you have several options for control.
Preemergence Herbicides
Control of crabgrass with preemergence herbicides in the spring is the most effective way to gain control over this weed. Preemergence herbicides work by disrupting the germination process. They must be applied before the seeds germinate and are not effective on emerged weeds. Several preemergence herbicides will control crabgrass in established lawns. These include benefin, bensulide, pendimethalin, and dithiopyr. Often, these herbicides are combined with a turf-type fertilizer (i.e., weed and Feed). This allows the gardener to apply a preemergence herbicide and fertilizer with one application.
Most preemergence herbicides also prevent the germination of all turfgrass seeds. If sowing grass seed in spring, the only preemergence herbicide that can be used is siduron (Tupersan). Siduron effectively controls crabgrass without affecting the germination of turfgrass seeds. Grass seed can be successfully sown in late summer (mid-August to mid-September) after a spring application of any preemergence herbicide. By late summer, the preemergence herbicide will have broken down and not interfere with grass seed germination.
Crabgrass seeds germinate from spring to mid-summer. Germination begins when soil temperatures reach 55°F in the spring. Preemergence herbicides must be applied before the crabgrass seeds germinate. If the material is applied too early, crabgrass seeds that germinate late in the season will not be controlled. If applied too late, some crabgrass seeds will have already germinated.
Preemergence herbicides should normally be applied in early to mid-April in southern Iowa, mid-April to May 1 in central Iowa, and late April to early May in northern areas of the state. Try to time the application to receive a ½ inch of rainfall or irrigation within 3-5 days of applying the preemergence herbicide.
The weather in late winter and early spring often varies considerably from year to year in Iowa. Accordingly, gardeners should make minor adjustments in the timing of the preemergence herbicide application. If the weather in March and April is consistently warmer than normal, apply the preemergence herbicide early within the typical time period. Apply the herbicide late within the recommended time period if Iowa is experiencing a cold early spring.
If you’re still uncertain when to apply the preemergence herbicide, Mother Nature provides some helpful (colorful) clues. Preemergence herbicides should be applied when the forsythia blossoms start dropping or when redbud trees begin to bloom (color). Crabgrass seed germination typically (although not always) starts after these events.
To ensure the herbicide is applied properly, carefully read and follow the label directions on the package.
Postemergence Herbicides
If preventative strategies fail, crabgrass can be controlled with postemergence herbicides. Postemergence herbicides provide temporary control of crabgrass. These herbicides will kill actively growing plants but will not kill any seed. Since crabgrass is an annual that germinates new from seed every spring, the weed will return next year unless a preemergence herbicide is applied.
Postemergence herbicides are most effective when applied to small, actively growing plants within the 3 to 5-leaf stage of development. Non-selective herbicides, such as glyphosate, can be used as a spot treatment in areas with abundant crabgrass. Non-selective herbicides will kill all plants, including desirable turfgrasses, so they must be applied carefully. After the treated areas have been completely destroyed, reestablish the lawn by seeding or sodding.
Several selective herbicides can also kill crabgrass, but not affect most of the desirable cool-season grasses in the lawn, such as Kentucky bluegrass. Products containing the active ingredients mesotrione, fenoxaprop p-ethyl, quinclorac, or topramezone are all possibilities. Which herbicide to use must be considered carefully as some can impact certain desirable cool-season grasses differently. For example, mesotrione can be used in new seedings of Kentucky bluegrass, tall fescue, or perennial ryegrass but should not be used in new stands of fine fescues. Fenoxaprop p-ethyl can cause damage to Kentucky bluegrass if misapplied, such as when temperatures are high. These selective postemergence herbicides are often only available from professional lawn care services and must be applied by licensed applicators.
In general, postemergence herbicides are ineffective against large, well-established crabgrass plants. Home gardeners with a lawn badly infested with crabgrass should consider total lawn renovation in the fall.
Corn Gluten Meal
Products containing corn gluten meal can be applied to lawns in spring to prevent the germination of crabgrass seeds. Corn gluten meal is a corn milling byproduct. When applied in spring, it interferes with the germination of crabgrass seeds. Corn gluten meal is also an organic fertilizer. It contains approximately 10 percent nitrogen. As a preemergence herbicide, corn gluten meal becomes more effective after two or three years of repeated application. Corn gluten meal products should be applied at the same time as chemical preemergence herbicides. Corn gluten meal products are not as widely sold as chemical pre-emergents but can be found at garden centers and other retailers.
Hand Pulling or Digging
Hand pulling or digging can remove established crabgrass plants but does not prevent new seed from germinating. Removing plants before they set seed can help reduce the size of the seed bank in the soil, hopefully reducing the number of seeds that can sprout next spring. Still, even just one plant allowed to go to seed can produce thousands of seeds, so persistence and diligence are essential to use hand pulling as a management option. While this non-chemical means of weed control is an option, hand pulling has limited success because infestations tend to be large and widespread in the lawn, new seed readily germinates, and the process is very labor intensive.
Control of Crabgrass in Garden Beds and Landscape Plantings
In flower and vegetable gardens, hoeing and hand pulling are the best control options. When cultivating the garden, avoid deep tillage. The roots of many vegetables and flowers grow near the soil surface. Deep cultivation will cut off some of these roots. Also, deep cultivation will bring deeply buried weed seeds to the soil surface, where they can germinate. Hand-pulling is best done after a good rain.