Not all herbicides, or weed killers, are the same. To get rid of weeds efficiently, it is important to apply the right weed killer at the correct time. One option is to prevent the germinated weeds from growing into established weeds; the chemicals to treat weeds at this early stage are called pre-emergent herbicides.

Here’s what you need to know about pre-emergent herbicides, how they are used, and when to apply them.

What Is Pre-Emergent Herbicide?

Pre-emergent herbicides act as growth inhibitors of weed seedlings. They prevent germinated weed seeds from developing roots, shoots, or both by depriving them of nutrients and erecting a chemical barrier in the soil surface so that the weeds cannot break through.

Although pre-emergent herbicides do not prevent weed seeds from germinating, they need to be present in the soil when the weeds germinate.

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How to Use Pre-Emergent Herbicide

Measure the area you want to treat and mark it with flags or landscape marking paint. Use a broom or a rake to clear the area of any leaves and other plant debris.

Liquid Pre-Emergent Herbicides

Wait for a day without wind or rain. Follow the instructions on the product label to mix the herbicide with the required amount of water to cover the square footage. Use a sprayer to apply the herbicide on the target area. Overlap passes slightly to avoid gaps.

To minimize drift, keep the sprayer nozzle close to the soil surface.

Granular Pre-Emergent Herbicides

Follow the instructions on the product label to measure the amount of granules required to cover the square footage.

Fill your spreader with the granules and calibrate the spreader according to the label instructions. Test the spreader on a small area and make adjustments if necessary. Cover the entire area you want to treat, overlapping each pass slightly to avoid gaps.

If there is no substantial rainfall in the immediate forecast, you’ll need to water the area with about half an inch of water to activate the herbicide.

Best Time to Use Pre-Emergent Herbicide

The timing of pre-emergent herbicides depends on the type of weeds, specifically at what temperature the seeds germinate. But applying pre-emergence herbicides too early, say in January or February, is not a problem.

“Pre-emergence herbicides are primarily degraded by soil microorganisms, however, when soil temperatures are in the 40s and 50s, microorganism activity in soil is low so very little degradation of the herbicide happens at that stage,” says Dr. Grady Miller, professor of sustainability and extension turfgrass specialist at North Carolina State University.

Once the soil warms, most pre-emergence herbicides will last between eight and 12 weeks. Dr. Grady points out, therefore a second application may be necessary.

Calvin Norman, assistant teaching professor of forestry at Penn State University, recommends applying pre-emergent herbicide against highly invasive grasses in a continental climate from late February to late March.

“To be effective, the herbicide needs to be in contact with seeds, so do not use them when there is snow on the ground,” Norman says.

If an area is severely invaded, he suggests following up the first pre-emergent herbicide with an application of a post-emergence herbicide in the growing season.