Smooth Crabgrass
Biology of Smooth Crabgrass
Smooth crabgrass (Digitaria ischaemum) is a summer annual grassy weed commonly found throughout the Midwestern United States. It is closely related to large crabgrass (Digitaria sanguinalis) and southern crabgrass (Digitaria ciliaris). These three species often rank among the most problematic weeds in lawns because they thrive at extremely low mowing heights, in poor quality soils, and under dry/hot conditions.
Identifying Smooth Crabgrass
You can commonly find it in home lawns across the US. It stands out for its ability to tolerate very dry, poor quality soils. This weed appears in various cropping systems, including agronomics, landscape, horticulture, and vegetable gardens. It also invades both cool and warm season lawns in Indiana. Crabgrass plants are opportunistic; they typically germinate in areas of bare soil such as walkways, gardens, newly seeded turf areas, and places where the turfgrass canopy is open and thin.
Germination Patterns of Smooth Crabgrass
In Indiana, smooth crabgrass typically starts germinating from mid- to late-spring (late March to early May). If germination happens during cooler temperatures, the plant stays small for a longer period. As temperatures rise, the plant develops more leaves and begins tillering from early to mid-summer. Unlike large and southern crabgrasses, which have a dense covering of fine hairs, the young leaves of smooth crabgrass have very few hairs.
Cultural Control Strategies
To outcompete crabgrass, maintain a high-quality lawn. Practices such as higher mowing heights, proper fertilization (any amount is better than none), and supplemental irrigation during droughts are effective.
Biological Control Options
Currently, there are no known biological control methods specifically for smooth crabgrass in home lawns.
Chemical Control Measures
For detailed information about crabgrass control, refer to our recent post, “Common Questions About Crabgrass Germination and Preemergence Herbicides Answered.”
You can control both smooth and large crabgrass with preemergence and postemergence herbicides. Homeowners have several preemergence herbicide options, including dithiopyr (Dimension), pendimethalin (Pendulum), and prodiamine (Barricade). These herbicides prevent successful crabgrass germination and must be applied before germination begins. Dithiopyr is an exception, offering control for germinated crabgrass up to 1- to 2-tillers in size.
For already emerged crabgrass, postemergence herbicides are effective. Products containing dithiopyr (for smaller than 1- to 2-tiller size), quinclorac (Drive), mesotrione (Tenacity), or fenoxaprop (Acclaim Extra) can control emerged crabgrass. Each herbicide offers a slightly different control spectrum and is effective on specific crabgrass sizes (based on the number of tillers). Homeowners should select a product based on their crabgrass size and the need to control other present weeds.