Applying Pre-Emergent Herbicides to Lawns
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Pre-emergent herbicides control summer annual grassy weeds such as crabgrass.
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. Avoid so-called ‘weed and feed’ products that contain fertilizer in addition to herbicide. 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.
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.
If temperatures in the winter are unusually cold, turf specialists with North Carolina Cooperative Extension recommend that 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. The following advice was shared by NCCE turf specialists on the NCSU TurfFiles website:
“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. I think it is safe to conclude that the winter so far, has set us up for a potential problem if we have even mildly bad luck with the weather from now until the end of March.
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, or in many cases, before we have winter injury because, as previously stated, significant winter injury can occur in late March.
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.
Research conducted in NC in the late 1990s showed these herbicides can be safely used on thin warm season grasses if they are used at reduced rates (usually half rates). Therefore, anytime winter-injury is a concern, it is important to split the applications of the above-mentioned herbicides. 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 refrain from applying the remaining 1.5 lbs. If no injury has occurred, you can proceed with the application.”
Corn gluten has been promoted as a natural pre-emergent herbicide but few studies have proven this product to be effective. In addition, corn gluten contains relatively high levels of nitrogen, which has the effect of fertilizing weeds.
For the long term, the most effective way to control weeds in turf is to nurture a healthy, dense lawn by following correct cultural practices. These include 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 (see lawn maintenance calendars below).
Learn More!
- Weed Control in Warm Season Grasses (Clemson Extension fact sheet): http://www.clemson.edu/extension/hgic/pests/weeds/hgic2310.html
- The NCSU TurfFiles Weed ID tool can help you identify problem weeds in your lawn: http://turfid.ncsu.edu/
- Lawn Maintenance Calendars from N.C. Cooperative Extension:
- What you can learn from a pesticide label: //pender.ces.ncsu.edu/2013/03/what-you-can-learn-from-a-pesticide-label/
Visit your local Cooperative Extension office to learn more about gardening and landscape care. Go to https://www.ces.ncsu.edu/local-
Contact your local Cooperative Extension office to get expert advice…
- If you live in Pender County, call 910-259-1235
- In New Hanover County, call 910-798-7660
- In Brunswick County, call 910-253-2610
- In Onslow County, call 910-455-5873
- In Duplin County, call 910-296-2143
Visit the Pender Extension Lawn and Garden webpage to stay up to date with all the latest gardening news.