Crabgrass is a warm-season, annual grassy weed. There are two species common throughout Louisiana: smooth crabgrass (Digitaria ischaemum [Schreb.] Schreb. ex Muhl.) and large crabgrass (Digitaria sanguinalis [L.). Crabgrass spreads through seed and usually germinates beginning in spring. It is typically found growing in areas with thin turfgrass and is tolerant of low mowing. Though their appearances are similar, smooth crabgrass is sparsely hairy compared to large crabgrass.
Crabgrass leaves are wide and tapered at the tip with smooth edges. Leaf edges can be crinkled near the base of the leaf. Smooth crabgrass is sparsely hairy with hairs occurring at the leaf nodes. Large crabgrass is densely hairy with hairs present on the leaf nodes, stems and leaves. Smooth crabgrass leaf nodes and stems can have a reddish tinge, and rooting can occur at nodes closer to the base of the plant. Both smooth and large crabgrass produce seed heads with two to six linear spikes (clusters of flowers), which are white to light green in color. Crabgrass generally has a low, prostrate growth habit. For more information on crabgrass identification and characterization, visit the USDA Plants Database at https://plants.usda.gov.
The best way to prevent or reduce weed encroachment is to maintain a healthy lawn through proper fertilization and soil pH and regular mowing. Properly maintaining a lawn through these cultural practices promotes dense and vigorous turfgrass, allowing it to better compete with weeds. Below are the recommended mowing heights and nitrogen fertility rates recommended per turfgrass species. In addition to these lawn care practices, manual removal of weeds may also be necessary.
In addition to cultural practices, herbicide applications may be required to achieve effective weed control. Pre-emergence herbicides are heavily relied upon for crabgrass control in turf. These types of herbicides prevent the emergence of crabgrass plants from the soil, but timing of these applications is highly important. It is critical that these types of herbicides are applied prior to crabgrass germination. Crabgrass germinates when soil temperatures warm to approximately 60 degrees Fahrenheit. Depending on the severity of the winter, pre-emergence herbicides should be applied as early as late January in the New Orleans area to late February in the Shreveport area.
Post-emergence herbicides are available, but these herbicides are more effective when applied to young (fewer than two tillers) crabgrass.
When applying any type of herbicide, you must follow the manufacturer's labeled directions. For more information regarding pesticides for turfgrass, please reference the Louisiana Suggested Chemical Weed Control Guide at the LSU AgCenter website www.lsuagcenter.com.