When To Apply Preemergent Herbicide for the Best Results - USA TODAY

Keeping your lawn's grass green and healthy doesn't come easy, but preemergent herbicides can help.

A preemergent herbicide is a weed killer applied to soil before weed seeds germinate, preventing them from sprouting and establishing roots. Knowing when to apply preemergent is crucial for success. Spraying too early or late in the growing season can result in wasted resources and triumphant weeds.

The best time to apply preemergent herbicide is typically in early spring before soil temperatures consistently reach 55 degrees Fahrenheit. But with so many types of weeds and climate variations, there's no one-size-fits-all approach to applying preemergent.

We reached out to Craig Elworthy, founder of DIY natural lawn care company Lawnbright, to learn about all things preemergent. Below, we explain the several types of preemergent and how to apply them for a greener lawn this year.

In what month should you apply preemergent herbicide?

In many U.S. regions, you should apply preemergent herbicide in March or early April,  just as the grass begins to sprout. While no one month applies to all areas because of varying climates and conditions, you can use the following as a general guideline:

• Early spring: Preemergent is ideal for early spring applications and can prevent weed germination. Aim to apply your herbicide after your grass and flowers begin to wake up but before weeds germinate.
• Late spring and early summer: A second late spring or early summer application may be necessary for stubborn weeds or an overgrown patch, typically around late May to early June.
• Early fall: From August to September, a round of fall preemergent can help prevent fall-emerging weeds, such as henbit and chickweed. Consider this application even if your yard appears weed-free.
• Late fall: A final preemergent application in October or November can inhibit winter weeds from popping up in early spring, though the effectiveness of late fall applications may vary.

What is the optimal temperature range for applying preemergent?

According to the Michigan Turfgrass Foundation, 80% of crabgrass germination occurs when soil temperatures are consistently between 60 and 70 degrees Fahrenheit. Getting a preemergent treatment beforehand — when the soil temperature measures between 50 and 55 degrees Fahrenheit — allows enough time for the application to be effective.

There are several ways to check soil temperature. One simple method is to insert a soil thermometer into the top inch of the soil (this is where most weed seeds germinate, so don't go much deeper) and leave it for a few minutes to get an accurate reading. A kitchen meat thermometer can work in a pinch if you don't have a soil thermometer.

Another simple way to determine soil temperature is to refer to a weather website or gardening app that provides soil temperature data for specific locations.

What happens if you put preemergent down too early?

Since preemergent prevents seeds from germinating, applying it too early can stop the germination of desirable plants, such as grass or flowers.

An early application can also be ineffective since preemergent creates a temporary barrier lasting only a few months. "If you are too early with your application, it can wear off before weeds actually get going," said Elworthy.

It's best to time your preemergent with soil temperature. This way, you allow your grass and flowers to take root yet catch weeds just as they're beginning to develop.

How do you apply preemergent herbicides?

Your preemergent application process will depend on the type you choose — liquid or granular. If you're not sure which is best, here are a few considerations to keep in mind.

Granular herbicides:

• Cost less than liquid products
• Involve minimal mixing or measuring
• Require a spreader
• Are recommended for smaller yards

Liquid herbicides:

• Allow for a more precise and even application (especially for larger yards)
• Provide faster results
• Require less watering
• Need more prep time and research

Once you determine which type to use, you can plan out your approach. Here are simple guidelines for both varieties.

How to apply liquid preemergent herbicides

First, gather your supplies:

• Chemical-resistant gloves
• Liquid herbicide of your choice
• Protective clothing (long sleeves, socks and close-toed shoes)
• Protective eyewear
• Sprayer

Mix your liquid preemergent with water according to the label and add it to your sprayer. Using the same technique you use to mow your lawn, apply your preemergent to the target area in a row-by-row pattern. Make sure to turn your irrigation system off for the recommended amount of time — typically at least 12 hours.

How to apply granular preemergent herbicides

You'll need a few supplies:

• Broadcast spreader (manual push or hand-held models are both fine)
• Chemical-resistant gloves
• Granular herbicide of choice
• Protective clothing (long sleeves, socks and close-toed shoes)
• Protective eyewear

Prepping granules starts with reading the label carefully, including its safety tips, application rates and precautions. Once you have the application rate, fill your spreader and adjust the settings accordingly, keeping the size of the treatment area in mind for uniform coverage. Before you begin, choose a crank speed that covers the appropriate distance.

When applying a granular preemergent, walking in opposite directions on both sides of the area you're targeting is best. Pass over the same area a few times to prevent an uneven application. Do your best to keep a steady pace. Once you're done, follow the product's watering instructions to activate the herbicide.

Should you apply preemergent before or after rain?

Preemergent herbicides typically need to be watered in after application to activate correctly. However, the time window you're working with depends on the type of herbicide, which is why reading the product label is so important.

For liquid preemergent, you'll likely need to wait several hours for the solution to become rainproof. Otherwise, a sudden shower could dilute the product before it has time to get into a plant's vascular system.

On the other hand, granular preemergent herbicides typically come with specific instructions regarding how much water they require for activation and a suggested timeline.

"Liquids get into the soil immediately. Granular takes longer to break down and get into the root zone," said Elworthy. "It needs to be watered in immediately, or it will gas off into the environment," he continued. 

When using granular preemergent, Elworthy suggested timing your application just before a rainstorm or making sure your irrigation is turned on.

Should you mow before applying preemergent?

Mowing is not a factor in a preemergent application because the grass isn't typically growing when you apply it. Soil temperatures must be between 50 and 65 degrees Fahrenheit for cool-season grasses (and, as the name would suggest, even warmer for warm-season grasses) to germinate.

If you're applying preemergent within the appropriate time window in early spring, you're unlikely to have grass tall enough to mow just yet.

However, as the growing season goes on, you may find it easier to achieve an even application if you mow beforehand when applying additional herbicide, such as a fall preemergent. 

It's also important to note that should you apply an herbicide later in the season (after weeds have sprouted), you don't want to mow directly after an application. It takes at least 24 hours for broadleaf weeds to absorb herbicide, so if you cut the lawn too soon after applying it, you're not letting it get into the plant.

How long does preemergent last?

A preemergent can last anywhere between one and eight months. Professional products last six to eight months, while DIY products usually last between three and four months with the proper application rate.

While everyone's needs are different, most lawns can benefit from two to three preemergent applications a year.

What are the different types of preemergent herbicides?

Weeds respond differently to different preemergents. Choosing the right product for your yard starts with understanding what each variety is designed to accomplish.

• Benefin: A member of the dinitroaniline chemical class, benefin is not ideal for broadleaf weeds. However, it is effective against annual grassy weeds, including crabgrass, ryegrass, chickweed, pigweed, foxtails, annual bluegrass, sandbur and goosegrass.
• Dithiopyr: While not ideal for spurge weeds, dithiopyr is a great crabgrass preventer. It also prevents dandelions, bittercress and poa annua from sprouting up.
• Oxadiazon: If your lawn tends to house chickweed, spurge and pearlwort, oxadiazon isn't the best herbicide for the job. However, it works great to prevent crabgrass, clover, dandelions, knotweed and thistle.
• Prodiamine: When you have several types of weeds in your yard, prodiamine can deliver outstanding results. It's ideal for targeting both grassy and broadleaf weeds, including chickweed, witchgrass, clover, creeping bentgrass and thistle. Although it's less effective on nutsedge, it's safe for established lawns.
• Trifluralin: Choose another type of preemergent if you're targeting thistle, clover or daisies. But trifluralin is an excellent option if you want to prevent redroot, annual bluegrass, red-dead nettle or chickweed.

How do I choose the best type of preemergent for my lawn?

You should ask yourself four questions when choosing a preemergent for your lawn:

1. What type of weeds do I want to control? Different herbicides are designed to control specific types of weeds. You'll want to know what weed types you're targeting so you don't end up with an ineffective product.
2. What is the square footage of the area I want to treat? Any type of herbicide can treat a small or large area. However, you may want to choose a more cost-effective option for a larger lawn compared to a small garden. Also, the size of your yard may make one type of application more appealing than the other, narrowing down your options.
3. What time of year will I be applying the herbicide? Preemergent products are designed to target weeds before they germinate. So, if you're applying herbicide early in the season, you'll want to purchase an herbicide designed for summer weeds. But when applying a preemergent in the fall, you'll want to choose a product targeting winter weeds.
4. Do I want a selective or nonselective herbicide? Some herbicides are selective and target specific types of weeds while sparing grass and ornamentals. On the other hand, nonselective options can prevent the germination of weeds and desirable plants. You may want to stick with selective varieties for lone-range weeds and nonselective if you're looking to start fresh in a weed-infested area of your yard.

Are there any risks to using preemergent herbicides?

Preemergents are generally considered safe for people and pets. However, there are a few risks to be aware of:

• Preemergent herbicides can remain active in the soil for several months, potentially affecting the growth of desirable plants if not applied properly.
• Heavy rainfall shortly after a preemergent application can cause the herbicide to run off into nearby water bodies, potentially contaminating groundwater or harming aquatic life.
• Relying on preemergent too much can contribute to the development of herbicide-resistant weeds.

What's next?

Preemergent herbicide is only one piece of the lawn care puzzle. You can take several other actions to complement preemergent herbicides, including regular mowing, proper watering and overseeding.

Keeping your lawn at the appropriate height helps shade the soil and discourages weed seeds from germinating. Deep and infrequent lawn watering encourages deep root growth in your grass while filling in bare patches by overseeding with grass seed promotes a dense lawn resistant to weed invasions.

Combining preemergent herbicides with proper lawn maintenance techniques lets you keep weeds at bay and maintain a beautiful and healthy lawn year-round.

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