Lawn and Turfgrass Weeds: Smooth Crabgrass and Large Crabgrass

Both species are warm-season summer annual grasses that infest lawns, golf course turf, athletic fields, parks, and institutional grounds. Crabgrass species are found throughout Pennsylvania and are most prevalent in sites with ample sunlight and thinning turf. Smooth and large crabgrass can grow in many different soil types and persist in both droughty and wet conditions. Because of their low-growth habit, crabgrass species can tolerate mowing heights of ½ inch or less.

Crabgrass species reproduce by seed, which is disseminated by foot traffic, maintenance equipment, and soil introduced from crabgrass-contaminated sites. Homeowners and professional turf managers find their coarse texture and spreading growth habit objectionable in lawns, golf courses, and sports turf. Over time, crabgrass species can dominate turf stands and lead to severe thinning of turf. Fortunately, smooth and large crabgrass can be controlled by employing good cultural practices as well as selective preemergence and postemergence herbicides applied at the correct time of year and growth stage.

Life Cycle

Crabgrass species belong to the Poaceae family and have a summer annual life cycle. Seeds of these grasses begin to germinate and emerge in spring, and continue into summer. Following germination and emergence, crabgrass plants undergo vegetative growth during the summer. Seedheads are produced from July through September and can generate thousands of seeds per plant. Crabgrass plants turn reddish-brown, then die following frost events in the fall. Seeds produced in late summer and early fall give rise to new plants the following spring and summer, given proper conditions for germination and emergence.

Identification

Leaf blades of smooth and large crabgrass are light to medium green, approximately ½ to ¾ inch wide, and 2 to 10 inches in length, with a thin, conspicuous midvein running lengthwise in the center. On smooth crabgrass, few long, sparse hairs are present at the base of the leaf blade on the upper surface, and plants are generally smaller than those of large crabgrass. Large crabgrass (sometimes referred to as hairy crabgrass) has long, sparse hairs on both the upper and lower leaf blade surfaces and on the leaf sheaths. Both crabgrass species have a short (1 to 1.5 mm) membranous ligule. The ligule of large crabgrass is somewhat jagged at the tip, whereas the ligule of smooth crabgrass is not jagged. Both species lack auricles in the collar region (located at the junction of the leaf blade and sheath). Stems, also referred to as tillers, grow outward from a single growing point in a rosette pattern, lying mostly flat against the soil surface under low mowing heights and more upright under higher heights of cut. As plants mature in summer, tiller numbers increase to over a dozen per plant. Seedheads consist of 3 to 5 spikes that radiate in a "V" formation from a central stem. Each spike has two rows of spikelets arranged in an alternating pattern along the spike. Seeds are 1 to 2 millimeters long, elliptical, and have pointed or rounded tips.

Cultural Management

Crabgrass seeds are usually introduced into turf stands through transfer on shoes, maintenance equipment, and soil used for renovation projects. Although there is no practical way to detect seeds in the soil, using soils from sites with no previous history of crabgrass infestation can lessen the chances of contamination of turfgrass sites. If only a few crabgrass plants are infesting a stand, they can be removed by hand-pulling or using a trowel or other hand-held weed removal tool. Improving turf density through fertilization, irrigation during dry conditions, and using turfgrasses well-adapted to site conditions will help reduce crabgrass infestations. Keeping lawn mowing heights at about 3 inches and mowing on a regular basis will help deprive young emerging crabgrass plants from obtaining light, thus reducing populations in lawns.

Control with Preemergence Herbicides in Established Turf

Crabgrass can be controlled with certain preemergence herbicides if applications are made in spring before crabgrass seeds begin germinating. Preemergence herbicides are typically the most effective chemical control measure for crabgrass. Active ingredients that have shown long-term preemergence control of crabgrass in Pennsylvania include prodiamine (Barricade and other prodiamine-containing products), pendimethalin (Pendulum, Pre-M, and other pendimethalin-containing products), and dithiopyr (Dimension and other dithiopyr-containing products). For maximum effectiveness, preemergence herbicides should be applied uniformly over the entire turf stand at the label-recommended rate and watered into the soil within 2 to 3 days of application.

The timing of preemergence herbicide applications is the most critical component of an effective crabgrass control program. The best time to apply a preemergence herbicide is approximately 10 to 14 days before spring's earliest expected crabgrass germination period. Provided that adequate moisture is present in the soil, crabgrass seeds begin to germinate when temperatures in the upper inch of soil reach approximately 55 to 58°F at daybreak for 4 to 5 days. Plant development indicators such as full bloom of forsythia flowers are sometimes used to time preemergence herbicide applications for crabgrass control but are not always reliable in years with abnormal weather patterns. Normally, treatment with preemergence herbicides for crabgrass control in Pennsylvania should take place according to the following dates:

• Southeastern Pennsylvania: March 15 to April 15
• Northern tier and high-altitude counties: April 20 to May 10
• Other Pennsylvania areas: April 1 to May 1

Keep in mind that not all crabgrass seeds germinate at the same time. Peak germination occurs a few weeks after the beginning of the initial germination period. Crabgrass seed germination and seedling emergence usually begin to taper off in mid-June, but some seeds can germinate in July. Because of the extended germination period of crabgrass, preemergence herbicides should remain viable for 6 to 8 weeks or longer.

In some areas of Pennsylvania where crabgrass populations are high, two spring applications of preemergence herbicides may be necessary for commercially acceptable control. For late germinating crabgrass and/or when goosegrass is present in the stand, a second preemergence herbicide application is often needed 6 to 8 weeks after the initial application for acceptable control. If using more than one preemergence herbicide application in a single season, be sure to follow label directions on rates, as most product labels recommend using rates lower than the maximum annual application rate for each of the two applications. Dithiopyr-containing products are often used by professional turfgrass managers in Pennsylvania as the second preemergence herbicide application in a two-application program because dithiopyr controls early emerging crabgrass (up to the 1-tiller growth stage).

Control with Preemergence Herbicides in Newly Seeded Turf

Most preemergence herbicides have long residual activity in soil and may inhibit the germination of newly seeded turfgrasses. Thus, the seeding of turfgrasses should be postponed for the duration of time specified on the herbicide manufacturer's label. This is especially important for spring turfgrass seedings, where preemergence herbicides may have been applied before the optimum spring seeding period. Depending on the preemergence herbicide and application rate, the seeding restriction period may be up to 6 months following application.

When seeding of turfgrasses must take place in spring or summer, be sure that no non-selective preemergence herbicide has been applied earlier in the spring. Alternatively, if crabgrass seed is suspected or known to be present in soil and seeding of turfgrass is planned in spring or summer, consider using a selective preemergence herbicide that inhibits crabgrass germination and emergence but has little or no influence on turfgrass seed germination. Such products include siduron (Tupersan) or mesotrione (Tenacity or Trione). Tupersan can be safely used on new seedings of all cool-season turfgrass species. Tenacity is labeled for use on new seedings of all cool-season turfgrasses except fine fescues. Both these herbicides can be applied before, during, or shortly after seeding turfgrasses. Selective preemergence herbicides do not typically provide the same degree of crabgrass control as non-selective preemergence herbicide products containing prodiamine, dithiopyr, pendimethalin, benefin, bensulide, and others. However, they can significantly reduce the number of emerging crabgrass plants, which results in less competition with new turfgrass seedlings. Tenacity can also be used as a postemergence herbicide for crabgrass control after young turfgrass plants have been mowed at least twice, or four weeks after seedling emergence.

Control with Postemergence Herbicides

Poor control of crabgrass can occur when preemergence herbicides are applied after crabgrass seedlings have already emerged. In these situations, postemergence herbicides may be required for commercially acceptable control. Postemergence control of crabgrass involves using products that kill plants after they appear in the turf. For postemergence herbicides to be effective, crabgrass plants must be uniformly covered and at the stage of growth specified on product labels. Environmental conditions also play an important role in the postemergence control of crabgrass, with adequate soil moisture and moderate temperatures generally providing the best results.

Selective postemergence herbicides labeled for control of crabgrass include products containing fenoxaprop-p-ethyl (Acclaim Extra and Last Call); mesotrione (Tenacity and Trione); quinclorac (Drive XLR8, Quinclorac 75DF, and combination products containing quinclorac); and topramezone (Pylex). Be sure to follow label precautionary statements, restrictions, and directions regarding rates and timing of applications when using these herbicides.

Acclaim Extra and other Fenoxaprop-p-ethyl-containing Products

Acclaim Extra and other fenoxaprop-p-ethyl-containing products can provide postemergence control of crabgrass and are relatively safe to use on most cool-season turfgrasses. However, Acclaim Extra should only be used on bentgrass at 3.5 fl oz/acre. This herbicide may injure some Kentucky bluegrass and zoysiagrass cultivars at rates exceeding 28 fl oz/acre, especially during periods of high temperatures. Rates of Acclaim Extra range from 3.5 to 39 fl oz/acre, with suggested use rates increasing as crabgrass plants develop from the non-tillered leaf stage (3.5 to 13 fl oz/acre) up to 3 to 5 tillers (20 to 39 fl oz/acre). This herbicide should not be applied if cool-season turfgrasses show signs of drought stress. Acclaim Extra is less effective when tank mixed with phenoxy-type herbicides such as 2,4-D and MCPP, but can be mixed with pendimethalin or other preemergence herbicides.

Drive XLR8 and Other Quinclorac-containing Products

Drive XLR8 and other quinclorac-containing products can provide postemergence control of crabgrass, foxtail, barnyardgrass, and a number of broadleaf weed species in established turfgrass. Drive XLR8 and other quinclorac-containing products control newly emerged crabgrass and mature plants (plants with more than four tillers). However, under certain conditions, this herbicide may not provide complete control of crabgrass with 2, 3, or 4 tillers. Drive XLR8 can also be used as a preemergence and postemergence herbicide in new turfgrass seedings, provided label restrictions are followed (the Drive XLR8 label suggests different application intervals following the emergence of specific turfgrass species). The label rate of Drive XLR8 for postemergence control of crabgrass is 64 fl oz/acre and should be combined with a methylated seed oil for best performance. Do not exceed 128 fl oz/acre over the course of one season. Drive XLR8 can be mixed with pendimethalin products for preemergence control of late-germinating crabgrass.

Pylex

Pylex is labeled for postemergence control of crabgrass in newly established or established turf. This herbicide provides postemergence control of crabgrass up to the 4-tiller stage when applied at 1.0 to 1.5 fl oz/acre under favorable conditions. Pylex can be applied before seeding or 28 days after seeding tolerant turfgrass species. Tolerant species include Kentucky bluegrass, perennial ryegrass, tall fescue, and fine fescues. Creeping bentgrass is only marginally tolerant of Pylex, and rates exceeding 0.25 fl oz/acre can result in unacceptable injury. Pylex should not be used on managed stands of bermudagrass or zoysiagrass, as severe injury will result. Applications up to 2.0 fl oz/acre can only be made on pure Kentucky bluegrass stands. The total seasonal amount of Pylex is restricted to 4.0 fl oz/acre. This herbicide can be mixed with pendimethalin products for preemergence control when used on established turf in early summer. For enhanced postemergence crabgrass control, Pylex can be mixed with Drive XLR8 at the label-specified rate. Pylex is most effective in controlling crabgrass when mixed with a methylated seed oil or crop oil concentrate. Applicators and turf users should be aware that treated crabgrass and other susceptible weeds turn white for several days following the application of Pylex. Also, some turfgrass species and cultivars may show a white tinge on foliage when Pylex is used at high rates. The whitening effect can be reduced somewhat when triclopyr is mixed with Pylex.

Tenacity and Other Mesotrione Products

Tenacity and other mesotrione products work best for newly emerged crabgrass in new seedings of Kentucky bluegrass, perennial ryegrass, and tall fescue. This herbicide should not be used in new stands of fine fescues. The best postemergence control of crabgrass is obtained when applied to plants with fewer than four tillers. Rates of Tenacity range from 5 to 8 fl oz/acre and vary with turfgrass species. Repeat applications may be necessary for complete control of crabgrass, but the total seasonal application rate should not exceed 16 fl oz/acre. Label instructions suggest spraying Tenacity only after newly germinated turfgrasses have been mowed twice or four weeks after emergence. Tenacity can be mixed with prodiamine, dicamba, triclopyr, fluroxypyr, or carfentrazone when used on established turf. This herbicide is most effective in controlling crabgrass and other summer annual grass weeds when mixed with a nonionic surfactant. Susceptible weeds turn white following Tenacity applications, creating a highly visible treatment effect for 1 to 3 weeks following application.

*Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

**Not for use on residential lawns.

*Follow label precautionary statements, restrictions, and directions regarding tolerant turfgrass species, rates, and timing of applications.

References

Patton, A.J. 2020. Turfgrass weed control for professionals. Purdue University Extension. 128 pp.

Uva, R.H., J.C. Neal, and J.M. DiThomaso. 1997. Weeds of the Northeast. Cornell Univ. Press. 397 pp.