When to Apply Pre-Emergent to Avoid a Weed Lawn

By Dan Simms

Weeds are the bane of every lawn, and dealing with them after they’ve laid claim to part of your lawn can be an uphill battle. The best way to keep your lawn healthy and weed-free is to get ahead of the problem by applying pre-emergent herbicide before the weeds can take root. Pre-emergent products prevent weed seeds from germinating so that they never get a chance to grow and spread.

Applying pre-emergent weed control is not as easy as other lawn treatments since timing is critical for it to have the most impact without damaging your grass. This guide explains why, how, and when to apply pre-emergent for the best results and covers spring vs. fall application and granular vs. liquid pre-emergent. If all of this sounds like too much for you, you can always hire a professional service to do it for you since all of the best lawn care companies offer pre-emergent weed control.

Best Time to Apply Pre-Emergent Weed Control

Generally speaking, the best time to apply a pre-emergent herbicide is in early spring or early fall, although the specific time depends on your local weather. Ideally, you should apply pre-emergent weed control when the soil temperature reaches 55°F. This usually happens in southern states like Florida and Georgia in early March, whereas it might not happen until June in northern states like Montana and North Dakota.

A good rule of thumb is to wait until the daytime temperatures stabilize in the mid-60s for several days since that usually means soil temperatures are around 55°F. If you want to be scientific about using pre-emergent, you can use a soil thermometer to measure your soil’s temperature. Most people find using a soil thermometer overkill, although it should theoretically give you the best results.

Early Spring Application

Applying pre-emergent products in the spring is your best defense against weeds that germinate in late spring, like crabgrass, foxtail, and sandbur. The trick is to apply the herbicide after your grass has started to sprout but before the weeds start to take hold. Pre-emergent herbicide is a weed preventer that stops seeds from germinating, making it effective only before germination starts. The trouble is that most pre-emergent treatments will prevent all seeds from germinating, including beneficial ones, so it is essential to get the timing right.

Timing is especially important if you want to treat your garden or flower beds with pre-emergent. Applying pre-emergent before the flowers sprout can prevent them from growing, wasting the money and effort you spent planting them. Instead, you should wait until your plants mature before applying pre-emergent.

Fall Application

Early fall is also a good time to treat your lawn with pre-emergent. Some invasive species like henbit and chickweed usually pop up in the fall when temperatures cool off, leading to a weed-choked lawn come springtime. You should still apply pre-emergent in the fall even if your lawn appears weed-free since many weeds will stay small and unnoticeable until the following year, and by then, it will be too late to control them with pre-emergent.

Fall pre-emergent application can interfere with overseeding, so you may want to forgo it if you need to overseed. Pre-emergent kills weed seeds and grass seeds alike, so you should avoid using pre-emergent herbicides whenever you plant new grass seed. Unfortunately, this probably means you’ll have to use post-emergent weed killer in the spring since any weeds that take root in the fall will already be too mature for pre-emergent treatments to be effective.

Apply When Soil Reaches 55º Fahrenheit

The best time to apply pre-emergent herbicides is when the soil temperature reaches 55°F, which varies substantially by location. If you live in USDA growing zones 1 through 5, your soil won’t reach the right temperature until late spring or early summer. Further south — in growing zones 6 through 13 — you’ll need to be ready to apply pre-emergent treatments right as the winter ends and spring begins. In warmer climates like southern California and Florida, many people get the best results from applying a pre-emergent herbicide in late February or early March.

If you’re concerned with timing your pre-emergent application based on the daytime temperature or don’t want to use USDA growing zone estimates, consider purchasing a soil thermometer. They’re relatively cheap and remove the guesswork involved in other methods. Most lawn care experts will tell you that a soil thermometer is unnecessary, but it can be invaluable for beginners while they learn.

Types of Pre-emergent Weed Killers

There are two main types of pre-emergent: liquid pre-emergent and granular pre-emergent. Which you choose to use is largely a matter of preference, although some practical considerations make each better in certain circumstances.

  • Liquid: Liquid pre-emergent is generally easier to apply over large areas like lawns since you can use a sprayer. Some liquid pre-emergent products are concentrates, meaning you have to dilute them with water before applying, so make sure you read your product’s label carefully.
  • Granular: Granular pre-emergent herbicide is less convenient to use for lawns and requires the use of a broadcast spreader or drop spreader. You can apply granular pre-emergent without a spreader, although doing so runs the risk of uneven coverage. Where granular products shine is in gardens and flower beds, where having extra control to distribute granules by hand makes it easier to apply the herbicide where you want it precisely.

If you already own a spreader, granular pre-emergent is the more flexible option, although most granular pre-emergent is more expensive than its liquid counterpart. As long as you apply both types correctly, both types of pre-emergent are equally effective at preventing weed growth.

Pre-emergent Chemicals

In addition to choosing liquid or granular pre-emergent, you also need to choose what chemical you want. There are three primary pre-emergent chemicals, each with its own use case.

  • Prodiamine: This pre-emergent works best against crabgrass and other broadleaf weeds, although it can be effective against certain species of invasive grass as well. Many crabgrass preventers contain prodiamine as one of the primary components.
  • Oxyzalin: This is less common in consumer-grade pre-emergent products, although it is sometimes used by homeowners who have intrusive vines like morning glory. It is also one of the best treatments for yellow foxtail.
  • Dithiopyr: If your lawn is plagued by crabgrass or ornamental grasses, dithiopyr is the best choice. It inhibits root growth in certain weeds and some types of turfgrass, making it a safe choice to use in mature lawns. It is an excellent choice if you’re trying to eliminate poa annua or annual bluegrass invasions.

Unfortunately, there aren’t many great all-natural or organic alternatives to chemical pre-emergent weed treatments.

How Long to Wait Before Planting Grass Seed

Reseeding or overseeding your lawn after applying pre-emergent herbicide can be tricky. If you plant grass seed too soon after a pre-emergent weed treatment, the seeds won’t sprout, and you’ll have to try again during the next seeding window. Most experts recommend waiting at least eight weeks to plant grass seed after a pre-emergent application. This gives the pre-emergent enough time to work and settle to lower concentrations before introducing the grass seed.

One complicating factor is the type of grass you’re seeding. Cool season grasses, like most kinds of fescue, require cooler soil temperatures to germinate, making it impossible to wait eight weeks after applying pre-emergent since the soil temperature will be too high. In this case, your best bet is to overseed or reseed in the fall and skip the pre-emergent fall application. Keep in mind that this will most likely lead to more weeds the following spring, but it is the best option.

Professional Lawn Care

If all of this sounds like too much work for you, hiring a professional lawn care service like Trugreen is a great idea. All of TruGreen’s lawn care plans include pre-emergent weed control except for the all-natural TruNatural plan, making it easy to get top-notch pre-emergent weed treatments, whatever your budget may be. Using TruGreen means you don’t have to worry about finding the perfect time to apply pre-emergent in between overseeding and fertilizing your lawn, and it is not much more expensive than doing it yourself.

The company also offers an impressive array of other lawn care services, including customized fertilization programs, targeted herbicide application, grub control, and tree and bush landscaping. It also offers standalone services like soil amendment services, mosquito defense treatments, and flea and tick prevention outside of its lawn care packages. You can read more about what the company has to offer in our TruGreen review.

Frequently Asked Questions

Can you apply pre-emergent too early?

Absolutely. Applying pre-emergent too early in the spring means that you won’t have peak concentrations when springtime weeds start to germinate, making the pre-emergent useless. Similarly, applying it too early in the fall will cause similar problems since many weeds that grow in the fall don’t crop up until the weather gets cooler. In general, you need to wait until the soil temperature reaches 55°F before applying pre-emergent to get the best results.

What temperature do you apply pre-emergent?

Most pre-emergent weed control products work best when the soil temperature is around 55°F. If you don’t have a soil thermometer, waiting until the average daytime temperature hits 60°F–70°F will ensure you aren’t applying the pre-emergent too early.

Do you water after applying pre-emergent?

Granular pre-emergent doesn’t need to be watered in, although most products work best with at least a quarter-inch of rainfall or water within 2–3 days. Watch the weather after applying pre-emergent and break out your sprinkler if you don’t get at least a quarter-inch after three days.

Liquid pre-emergent is a different story and needs to be watered in to reach the soil. Once again, a quarter-inch of water is all it takes, so you could get lucky if there’s rain in the forecast. Otherwise, running your sprinkler for 15–20 minutes for 2–3 days after applying liquid pre-emergent should do the trick.

Should I cut grass before applying pre-emergent weed killer?

Mowing your lawn before applying pre-emergent can help make it more effective, although it’s not strictly necessary. Regular mowing can insulate your lawn against weed invasions, and a healthy lawn is less likely to be affected by a pre-emergent application. Make sure your mower’s blades are sharp and cut your lawn to a height of two or three inches before applying pre-emergent.

Can you overseed after pre-emergent weed killer application?

Yes, you can, but you should wait at least eight weeks after applying pre-emergent to overseed. Pre-emergent herbicides are non-selective, which means they will prevent all seeds from germinating, including grass seeds. If you overseed shortly after using pre-emergent, the grass seed won’t take, and your lawn won’t benefit at all.

Will heavy rain wash away pre-emergent?

No. On the contrary, if you get a heavy rainfall after applying pre-emergent, the water will help it absorb into the soil more readily. The only caveat is that you need to apply pre-emergent to dry grass for it to be effective, so make sure your lawn isn’t wet before applying. After applying pre-emergent to wet grass, rainfall can wash it away, so wait until you have a few dry days before applying.

Should you dethatch before pre-emergent?

It’s probably a good idea, but you don’t have to. If your lawn has a lot of thatch and needs to be dethatched, you should do so before applying pre-emergent. Dethatching stirs up the soil, and pre-emergents like prodiamine need to bond with the soil to protect your lawn from weeds and invasive species. Dethatching after applying pre-emergent can reduce the pre-emergent’s ability to protect your lawn or possibly even make it not work at all.

How long does pre-emergent last?

Pre-emergent lasts around 3–5 months, so most people apply it twice per year. Spring pre-emergent is perfect for protecting your lawn against crabgrass, dandelions and other types of weeds that grow during late spring and early summer. Fall pre-emergent is effective against late-season weeds like henbit and chickweed and can help you avoid a post-emergent herbicide application the following spring.

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