Concerned About Weed Control?

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Weed control in the lawn begins with proper management practices to encourage a dense, healthy turf.  A healthy turf shades the soil so that less sunlight reaches the ready to germinate weed seeds.  A thick turf minimizes the space available for weeds to become established.

A healthy, vigorous lawn is encouraged by following the best management practices. They include proper: mowing height and frequency, watering rate and frequency, fertilizer analysis, rate, and timing, liming as recommended by a soil test, core aeration to reduce soil compaction, and dethatching as needed. To determine the recommendations for your turf grass, look for Lawn Care Calendars available from Cooperative Extension. Good management also includes consideration of the changes in the environmental conditions.

If your summer lawn is typically weedy, applying a pre-emergent now might help reduce summer weeds, depending on the type of weeds you have and the health of your lawn. 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

To be effective, pre-emergent products must be applied before seeds of summer weeds sprout, which usually happens around the time dogwoods begin to flower.  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 turf grasses.  In a healthy, vigorous lawn the impact is usually not serious, but if your lawn is already struggling, the effect can be severe.

If you choose to apply pre-emergent herbicides do not choose products that are mixed with fertilizer.  Warm season lawns should not be fertilized until mid May.  Applying fertilizers too early to warm season lawns can increase the risk of cold injury and disease problems.

Pre-emergent herbicide products, often sold as crabgrass preventers, are effective for controlling summer annual grassy weeds such as crabgrass, sandbur, and goosegrass. They 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.

Warm-season grasses like centipedegrass can been weakened by cold weather especially the weather from now until the end of March. Winter injury frequently occurs when there is some green-up of the grass in March followed by temperatures in the low 20s.

Anytime there is increased concern of winter injury, it is wise to carefully consider herbicide selection and use on warm-season grasses. Winter injury requires that the turf must grow back and full rates of some herbicides can slow the process down.

Pre-emergent herbicides need to be applied in early March which is before the known 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. Apply only half the amount of pre-emergent herbicide recommended on the package now. 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, refrain from applying the remaining amount.  If no injury has occurred, you can proceed with the application.

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.

Written By

Photo of Peg GodwinPeg GodwinExtension Agent, Agriculture - Horticulture (252) 527-2191 peg_godwin@ncsu.eduLenoir County, North Carolina
Posted on Mar 1, 2016
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