Got Weeds? This is the Best Time to Apply Pre-Emergent Herbicide

Pre-emergent weed killers belong to a group of herbicides that don’t kill unwanted plants. Instead, they stop seeds from sprouting in the first place. Knowing when to apply pre-emergent herbicide can mean the difference between having a lush, weed-free lawn, a yard riddled with dandelions and other unwanted plants, or no lawn or flowers at all.

Applying pre-emergent weed killers too soon can keep beneficial seeds from sprouting, and applying them too late won’t do any good. “The most effective pre-emergence herbicides are nonselective,” says Alec Kowalewski, Ph.D., associate professor and turfgrass specialist at Oregon State University. “They will prevent the germination of any plant establishing from seed,” he says. Ahead, learn when to use pre-emergent herbicides—and when to wait.

Applying Pre-Emergent in Spring

Spring is the best time to apply a pre-emergent herbicide and prevent seeds from sprouting, because most weeds (like crabgrass, foxtail, and sandbur) germinate in late spring and emerge in summer. Kowalewski suggests using the herbicides in spring if there were heavy weed populations on the property the previous summer.

There are some situations in which you might want to defer or skip a spring pre-emergent treatment:

• If you are spreading grass seed in the area. It is best to avoid using pre-emergents altogether for 12 months after seeding or sodding of a lawn. “I would suggest [grass] seed or pre-emergence herbicides in the spring, not both,” says Kowalewski. He recommends that homeowners overseed (or interseed) and fertilize in those springs that follow low-weed summers.
• If you are planting a vegetable garden or flowerbed in the area. Pre-emergents can keep flower seeds, and seeds like corn, cucumbers, and squash from germinating, just as it does weeds.

Note: Sowing any seed while a pre-emergent herbicide is still effective will prevent them from sprouting. You should generally wait a minimum of 8 to 9 weeks after applying a pre-emergent to overseed grass or sow annuals.

Applying Pre-Emergent in Fall

A lawn might appear virtually weed-free going into fall, but then weeds suddenly appear. By this time, the pre-emergent applied in spring is no longer protecting the yard or beds, and a few invasive weed seed varieties, such as annual bluegrass, henbit, and chickweed, are lying in wait for cooler temps before sprouting. 

An additional pre-emergent application in fall can also help control next year’s summer weeds. After sprouting, hardy weeds will remain small, perhaps even unnoticeable, throughout the winter. But they’ll revive in early spring and grow with a vengeance. By then, it’s too late to use a pre-emergent. A post-emergent herbicide is necessary at that point. Break this cycle by controlling weeds before they have a chance to sprout. 

Note: With winter grass species like fescue, which need cool temperatures to sprout, avoid fall application of pre-emergents. Instead, overseed the lawn at that time, or alternate seeding and use of pre-emergents. 

Pre-Emergents, Temperature, and Climate

For spring application, “Some classic indicators for pre-emergence timing include atmospheric temperature that reaches 60 degrees Fahrenheit,” Kowalewski says. This will vary by region. For example, in southern USDA growing zones, such as Louisiana, appropriate temperatures typically hit around the first of March, while in North Dakota the temps will be more conducive to applying a pre-emergent in June. 

One telltale sign for when to apply pre-emergent herbicides is the blooming of forsythia shrubs, which Kowalewski says is “a classic indicator that temperatures are getting close to summer annual germination.” 

In the fall, apply pre-emergent herbicides when daytime temperatures drop to about 70 degrees Fahrenheit, since many winter/early spring weeds begin to germinate at temps below that. 

There are two types of pre-emergent herbicides, liquid and granular. Both work to keep weed seeds from sprouting, but one works a little better and more quickly. 

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Types of Pre-Emergents

Formulation

Formulation is a factor to consider when timing the application of pre-emergent herbicides.  

Liquid: Liquid herbicides, including pre-emergents, “are better than granular applications,” says Kowalewski. Think of them “as a blanket applied to the surface of the soil,” he says. Liquid herbicides can cover a wider surface area overall than granules can. Because they are in liquid form, they also act faster, whereas granules must first break down and bond with particles in the soil. Liquid products are easy to apply with a pump sprayer and come in either full strength or as concentrates that require diluting with water before use. 

Granular: Resembling tiny grains, granular pre-emergent products are applied with a drop spreader or a broadcast spreader, or by shaking the granules onto the soil with a cup. Using a spreader works best for even coverage on large areas. Still, gardeners might prefer sprinkling the granules around established plants in a vegetable or flower garden using a cup, and then working the granules into the soil’s surface.

“If granular applications are used, be sure to water these into the soil to improve activity,” says Kowalewski. It also helps to time application so that the granules on the ground before rainfall is expected.

Active Ingredients

The active ingredients in specific pre-emergent weed killers can vary. Identifying the unwanted plants in your hard can help you choose the best pre-emergent and determine the best time to apply, whether for summer or winter annual weeds. Examples of active ingredients are:

• Pendimethalin, the active ingredient in Scotts Halts Crabgrass and Grassy Weed Preventer, can stop crabgrass, plus several other weeds like purslane and henbit. It is safe for use on crops. It might not be as effective for fall application.
• Prodiamine is used often for turfgrass but is not safe for edible plants or crops. It is mildly toxic to earthworms and comes in liquid and granular options.
• Trifluralin, the active ingredient in Preen, is nonselective; it will stop a variety of unwanted weed and grass seeds but is more likely to also affect seeds of desired plants. It is not safe for pets, lawns, or edibles.
• Corn gluten meal is an organic approach to pre-emergent control. The natural method doesn’t work as well as chemicals, but it is better for the soil and environment.

Clemson University Extension posts a helpful list of the best active ingredients to tackle summer weeds and grasses. Herbicide products help consumers by listing the types of weeds they should kill on the packaging, in addition to the herbicide’s active ingredients and warnings.  

Safety Considerations

“Pre-emergent herbicides are generally safe on existing plants,” says Kowalewski. Check package labels to ensure that a particular formula is meant for the type of weeds you want to stop; identifying the types of weeds you’re trying to kill will also help you determine the best time to apply the pre-emergent. 

A pre-emergent’s label will also outline specific safety requirements you should follow while you’re applying it. With any pesticide, Kowalewski says it is best to wear “long pants, a long-sleeved shirt, shoes and socks, gloves, and protective glasses.” He also recommends showering after application, and washing the clothes you wear during application in a separate load. The effects of the ingredients vary among products, so be sure to read and follow all cautions regarding eye exposure, skin exposure, and possible toxicity to pets or children.

Final Thoughts

Avoid the need for pre-emergents or their repeated use by keeping lawns healthy and full, and by adding a 2- to 3-inch layer of mulch to garden beds in the spring to prevent weed seed growth. Plan timing of pre-emergents based on the type of weed, when it typically appears, and weather (both temperature and expected rainfall). For the best coverage, apply pre-emergents throughout a lawn or bed, not just where you have seen weeds grow. Using liquid formulas and watering after applying any pre-emergent herbicide will help spread the product’s coverage.  

 

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