Mid-February – Early March is the time to apply pre-emergent herbicides to lawns in southeastern North Carolina. These products are usually applied as a granular and then watered in to the lawn. If your summer lawn is typically weedy, applying a pre-emergent now could help reduce summer weeds, depending on the type of weeds you have and the health of your lawn.
Pre-emergent herbicide products, often sold as crabgrass preventers, are effective for controlling summer annual grassy weeds such as crabgrass, sandbur, and goosegrass, but do not control perennial weeds such as dallisgrass, Florida betony, dollarweed, or bahiagrass. They will also have no effect on winter weeds such as chickweed, annual bluegrass, burrweed, or henbit, which are already growing. So, manage your expectations by learning which weed is which. Warm season lawns should not be fertilized until late April. Applying fertilizers too early to warm season lawns can increase the risk of cold injury and disease problems. So for now, skip the so-called ‘weed and feed’ products on the shelves that contain fertilizer in addition to herbicide.
To be effective, pre-emergent products must be applied before seeds of summer weeds sprout, which usually happens around the time dogwoods begin to bloom. Applying a pre-emergent now will ensure the product is in place before weed seeds start to sprout. The downside of using pre-emergent herbicides is that they stunt root growth on turfgrasses. In a healthy, vigorous lawn the impact is usually not serious, but on a lawn that is already struggling, the effect can be severe.
Because of the unusually cold weather experienced this winter, turf specialists with N.C. Cooperative Extension are recommending NC residents only apply half the amount of pre-emergent herbicide recommended on the package now, and wait to apply the other half in early May. According to NCCE turf specialists it is probably a safe bet that warm-season grasses (particularly centipedegrass and shorter cut bermudagrasses) have been weakened by the cold weather. The bottom line is the weather from now until the end of March will dictate the extent of winter injury. If there is some green-up in March followed by temperatures in the low 20s, there is a good chance we will have winter-kill.
Anytime there is a heightened concern of winter injury, it is wise to consider herbicide selection and use patterns on warm-season grasses. Because winter injury results in turf needing to be grown back, full rates of some herbicides can slow that process down. To further complicate the process, PRE herbicides need to be applied in February and early March. This is before we know the extent of winter injury. Pre-emergent herbicides, including dinitroaniline herbicides (prodiamine, pendimethalin, oryzalin, etc.) and dithiopyr, can inhibit root growth on stolons as turf recovers and grows into thin areas. Inhibiting stolon rooting may cause stolons to be cut off during mowing, significantly reducing lateral spread and recovery. Consider reducing application rates. For instance, instead of using 3 lbs active ingredient/acre (ai/a) of pendimethalin in late February, use 1.5 lbs ai/a in late February and follow-up with the remaining 1.5 lbs 8 to 10 weeks later.
By late April or early May, it will be much easier to quantify if, or how much winter injury has occurred. If significant injury has occurred, you can then choose not to apply the remaining 1.5 lbs. If no injury has occurred, you can go ahead with the application.”
Out competing the weeds with healthy turf is the most effective long term solution. This means following correct cultural practices, such as mowing at the correct height, having any insect or disease problems correctly diagnosed before treating, sending soil samples to the NC Department of Agriculture to determine your nutrient or lime needs, and following turf care recommendations for your lawn type available. Give your extension office a call about maintenance calendars for the type of lawn you have.
For more information about how to have great gardens and landscapes, join us for this spring and summer’s Garden Life classes. Learn how to create, care for and enjoy a beautiful, bountiful and life-enriching garden. The Garden Life series offers 10 classes through North Carolina Cooperative Extension in Lee County. To sign up for classes go to https://lee.ces.ncsu.edu/garden-life-series/
Minda Daughtry is Horticulture Agent for North Carolina Cooperative Extension in Lee County.