Submitted By: Chris Hicks – UT Extension
One of the most troubling weed species in home lawns in TN is crabgrass (Digitaria spp.). Crabgrass is a summer annual that germinates primarily in the spring, grows through the summer, produces seeds in the fall and dies following the first killing frost. Left uncontrolled, it can greatly impact the usability and beauty of your lawn.
There are two primary crabgrass species in Tennessee – large crabgrass and smooth crabgrass. Large and smooth crabgrass can be differentiated by examining the leaves and stems of each species. Large crabgrass has hairs on its leaves and stems, while the leaves and stems of smooth crabgrass have very little hair except on the collar region (the intersection of the leaf and stem of the plant). Smooth crabgrass is the most prevalent crabgrass species infesting turf in Tennessee.
Like most weeds, a healthy lawn is the best defense against crabgrass. Making sure your soil is properly fertilized and mowing at the correct cutting height are two easy ways to keep your grass healthy and avoid weeds such as crabgrass. However, if you do have an infestation of crabgrass, herbicides might be needed to control it.
If you have had trouble with crabgrass in the past, consider using a preemergence herbicide. These herbicides prevent germinating seedlings from developing into mature plants. For preemergence herbicides to work properly, they must be applied before seed germination and need approximately 0.5 inch of rainfall or overhead irrigation within 24 to 48 hours after application to be activated.
Large and smooth crabgrass seed germinates in the spring when soil temperatures exceed 55 degrees F for four consecutive days and nights. The blooming of the forsythia plant, also known as golden bells, is a visual indicator that soil temperatures are increasing. Preemergence herbicides for crabgrass control should be applied before crabgrass seed begins to germinate.
Since crabgrass is already up in Smith County, using a postemergent herbicide will be a better option this time of year. When choosing a postemergence herbicide, make sure it is labeled for the turf where it is to be applied. Unlike the preemergence options for crabgrass control, many of the postemergence options may only be labeled on one or two turf species.
Postemergence herbicides can be applied anytime after crabgrass has germinated. Consider that these herbicides often provide better control when applied to small, actively growing crabgrass plants. When applied to larger plants repeat applications will likely be necessary for complete control.
The University of Tennessee Extension publication, “Crabgrass Species Control in Turfgrass” has specific recommendations on herbicides which are available in TN. This free publication is available at the UT Extension office or online at smith.tennessee.edu. For more information, feel free to call the UT Extension office at 615-735-2900.
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Smooth Crabgrass: Photo Courtesy of Virginia Tech
