Selecting a Preemergent Herbicide for Your Lawn

Jackie Jordan
County Extension Agent
Clemson University Cooperative Extension Service

HTG 0217

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Q: When and how do I apply a preemergent herbicide to my lawn?

A: Preemergent herbicides are applied to the lawn before weed seeds germinate. The seedlings absorb the herbicide as they grow in the treated zone. Preemergent herbicides do not control perennial weeds or all annual weeds. The first step in successful weed control is to correctly identify the weed. This can be done by exploring the HGIC section on weeds or by taking a sample to your local Extension office to be submitted to the Plant Problem Clinic for identification.

Push spreader
Preemergent herbicides are best applied with a push spreader.

Crabgrass

Crabgrass is a very common summer annual grass that is controlled by pre-emergent herbicides. Crabgrass begins to germinate when the upper inch of soil reaches a temperature of 55 ᵒF for at least three days. Germination rates increase as the soil temperature goes up. Crabgrass can continue to germinate throughout the summer. If you are not monitoring your soil temperatures, then a good rule of thumb is to apply a preemergent around March 1 from the Coastal Plain to the Sandhills regions and on or around March 15 to 30 in the Piedmont and Mountain areas. Goosegrass and other weeds germinate later, so properly timing your application for crabgrass emergence will help manage these weeds.

Broadleaf Weeds

Some broadleaf weeds can be controlled by preemergent herbicides so check the label to see if the weed you need to control is listed. Please refer to the following Home and Garden Information Center fact sheets for product names and a brief listing of weeds controlled: HGIC 2310 Managing Weeds in Warm Season Lawns, and HGIC 2309 Managing Weeds in Fescue Lawns.

Factors that Affect Herbicide Performance

For best results, apply preemergent herbicides according to the recommended dates for your area. Applying too early can waste the chemical and applying too late will allow weeds to mature beyond the point where the herbicide is effective.  Most preemergent herbicides are only effective for ten to twelve weeks so, typically a second application is required for season-long control.

Read the label fully, as the label is the law. Most granular herbicides need to be watered-in, with a half- inch of water in order to move the herbicide into the upper part of the soil. Do not rely on rainfall to water it in. Rainfall totals may not be enough or too much, and the herbicide could leach through the soil or run off the property. When preemergent herbicides are not watered-in within ten days, they can breakdown in sunlight and become ineffective.

Make sure that the product is applied uniformly across the lawn. Start by splitting the amount of herbicide needed in half. Apply half of it in vertical rows, and then apply the remaining half in horizontal rows over the turfgrass. This creates a more even distribution of the herbicide. Also, make sure to apply the herbicide at the correct rate. Calculate the square footage of your lawn and calibrate your spreader. This will ensure that you are not over-applying and wasting the product, or minimizing results by applying less than the effective amount. See fact sheet HGIC 1220, Measuring the Area of a Home Lawn for more information.

Preemergent herbicides will limit the growth of turf grass seedlings as well as weeds. Do not apply a preemergent herbicide if you are planning to reseed your lawn or lay out sod during the effective timeframe. The chemicals will limit the new sod’s ability to root into your soil.

Good Cultural Practices

It is important to employ good cultural practices when caring for your lawn. Herbicides are not the only option when it comes to winning the war on weeds. Good cultural practices can limit weeds by up to 70%. Thin or weak areas in the lawn are the first to be invaded by weeds. Make sure to grow the right turfgrass for your area and landscape conditions. It is critical that your lawn receives the minimum amount of direct sunlight required for adequate growth. Turfgrasses vary in their shade tolerance. See fact sheet HGIC 1214, Selecting A Lawn Grass for more information.

Some weeds can indicate nutritional problems. Have your soil sampled regularly to receive lime and fertilizer recommendations for your lawn. See fact sheet HGIC 1652 Soil Testing for more information.

Each turf grass has an ideal height range that encourages tillering or sideways growth. Weeds like chickweed and crabgrass can indicate that the grass is being cut too low. Keep the lawn at the recommended height and use good mowing practices to create a thick dense lawn that can shade out weed seeds and reduce their presence in your grass.  Do not water the lawn more than one time per week during the spring with one inch of irrigation water. This infrequent, but deep irrigation will reduce weed seed germination. Mowing, watering, and fertilizing the lawn correctly will establish a nice thick stand of turf that resists weeds.

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This information is supplied with the understanding that no discrimination is intended and no endorsement of brand names or registered trademarks by the Clemson University Cooperative Extension Service is implied, nor is any discrimination intended by the exclusion of products or manufacturers not named. All recommendations are for South Carolina conditions and may not apply to other areas. Use pesticides only according to the directions on the label. All recommendations for pesticide use are for South Carolina only and were legal at the time of publication, but the status of registration and use patterns are subject to change by action of state and federal regulatory agencies. Follow all directions, precautions and restrictions that are listed.