Smooth Crabgrass

Smooth Crabgrass  

Biology: Smooth crabgrass (Digitaria ischaemum), is a summer annual grassy weed that is found throughout the Midwestern United states. Closely related to large crabgrass (Digitaria sanguinalis) and southern crabgrass (Digitaria ciliaris), these three species are often considered to be the most problematic weeds in lawns due to their ability to survive extremely low mowing heights, poor quality soils, and dry/hot climates.

Identification: Smooth crabgrass is a summer annual grassy weed that is commonly found in home lawns throughout the US. Smooth crabgrass can tolerate very dry, poor quality soils and is found in multiple cropping systems ranging from agronomics, to landscape, to horticulture, to vegetable gardens, along with both cool and warm season lawns in Indiana. Crabgrass plants are very opportunistic, so look for it to germinate in areas of bare soil like walkways, gardens, newly seeded turf areas, as well as a very open, thin turfgrass canopy.

Germination typically occurs from mid- to late-spring (late March-early May) in Indiana. If germination occurs while the temperatures are still slightly cooler, the plant will remain small for an extended amount of time. As the temperatures warm, more leaves will start to develop and the plant will begin tillering by early- to mid-summer (June). The young leaves of smooth crabgrass will have very little hairs, while large and southern crabgrasses have a dense covering of very fine hairs. All three crabgrass varieties will have a membranous ligule that has a torn or ‘shredded’ appearance.

Smooth crabgrass leaf emerging from coleoptile.
1-leaf size smooth crabgrass.
2-leaf size smooth crabgrass.
3-leaf size smooth crabgrass.
Usually some hairs are present on smooth crabgrass but many fewer than on large crabgrass.
Photo shows membranous ligule and few hairs on leaf sheath or leaf blade.

The development of multiple tillers can contribute to the clumpy appearance of a crabgrass plant, though smooth crabgrass is generally smaller in size than large crabgrass. Once mature, smooth crabgrass produces seedheads in the form of 3-5 spikes that are clustered at the top of long, erect stems. Seedheads mature at the end of summer prior to fall/autumn. As crabgrass dies with the first killing frost, the plant can appear as unsightly brown patches surrounded by green cool-season turf in lawns.

Prostrate growth of smooth crabgrass in short cut turfgrass.
Dense turf keeps out crabgrass. Here a patch of creeping bentgrass in a lawn is keeping out the crabgrass.
Chilling injury to crabgrass in cool fall months prior to the first frost.
Brown, dead crabgrass plants in a lawn following the first hard frost.
Cultural control: Maintaining a high quality lawn through higher mowing heights, proper fertilization (some fertilization is always better than none), and supplemental irrigation during drought will help the desired turf to outcompete crabgrass.
Biological control: None known specifically for control of smooth crabgrass in home lawns.
Chemical control: For more information about crabgrass control, see our recent post, Common Questions About Crabgrass Germination and Preemergence Herbicides Answered.

Both smooth and large crabgrass can be controlled using preemergence and postemergence herbicides.

There are multiple options for preemergence herbicides that are available for purchase by homeowners, including dithiopyr (Dimension), pendimethalin (Pendulum), and prodiamine (Barricade). These herbicides prevent the crabgrass from successfully germinating. For adequate control, these products must be applied before the crabgrass starts to germinate. One exception is dithiopyr, which has the ability to control crabgrass after it germinates until it reaches 1- to 2-tillers in size.

Once the crabgrass has already emerged, another option for control is the use of postemergence herbicides. Products that include active ingredients like dithiopyr (smaller than 1- to 2-tiller in size), quinclorac (Drive), mesotrione (Tenacity), fenoxaprop (Acclaim Extra) can be used to adequately control crabgrass that has already emerged. Each of these individual fertilizers provide a slightly different spectrum of control and works well on specific sizes (based on number of tillers) of crabgrass. Homeowners can choose a product based on the size of their crabgrass as well as its ability to control other weeds that you might have present.

For more information on weed control, search this blog and check out our Turfgrass Weed Control for Professionals Publication.

For archives of past weed of the month postings, visit our Weed of the Month Archive.

Aaron Patton, Turfgrass Extension Specialist, Purdue University
Leslie Beck, Weed Extension Specialist, New Mexico State University
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