In this story: Properly applied pre-emergent herbicides can form the backbone of the weed-control effort in your lawn. It’s an effective tool — if you know the secret.

All summer you’ve stared angrily at those patches of crabgrass. And you’re still not over this spring, when so many dandelions sprouted that every time the wind blew it looked like snow.

Never again, you vow, and go to the garden store for weed-killer … only to find a massive array of confusing herbicide choices.

In this story, we’ll walk you through one of the two major classes of weed-killing chemicals — pre-emergent herbicides. You spread or spray pre-emergent products on your lawn, taking the fight directly to weed seeds before they can grow. (If you already see weeds in your grass, see our discussion of the other major class, in the story “Applying Post-Emergent Herbicides to Your Lawn.”)

Pre-emergence herbicides form the backbone of weed control programs,” says the University of Georgia Extension Service’s guide to weed control. “They do not control all weeds that may be present in a lawn, but they are effective for many of the most common lawn weeds.”

Timing is critical

Rebecca Grubbs-Bowling
Rebecca Grubbs-Bowling, Texas A&M. Credit: Beth Ann Luedeker.

You may see nothing funny about your weeds, but controlling them has something in common with joke-telling: The secret is timing.

“The timing is a little tricky,” says Dr. Rebecca Grubbs-Bowling, assistant professor and turfgrass specialist at Texas A&M University. She is the author of the newly published “A Homeowner’s Guide to Herbicide Selection for Warm-Season Turfgrass Lawns.”

Pre-emergent herbicides don’t actually kill weed seeds before germination. Instead, they fatally interrupt their growth process in some way to prevent the seed from getting all the way through the seed germination stage. For that to work, it has to be in the soil at the right time — at the right temperature.

“The soil temperature should be in the 50-55 degree range,” Grubbs-Bowling says.

First Application: When the Soil Warms

Exactly when your soil turns that temperature will depend on your local climate, and what the weather is like this season. Mid-winter? Early spring? Late spring? Maybe, maybe and maybe.

For example, with crabgrass, one of the most-loathed grassy weeds, a pre-emergent can be applied as early as January in Florida and at the southern tips of states that border the Gulf of Mexico. But in the northern U.S., those who want to kill crabgrass (and its summer annual brethren such as barnyard grass, foxtails and goosegrass) might have until the end of May.

Crabgrass seed germination dates
Crabgrass seed germination dates

You can find localized soil temperature readings online or from your county’s Extension Service. For the most localized data of all, plunge a gauge into your own turf. Simple soil temperature gauges can be found online or at garden shops for $8-$15.  A meat thermometer with a 3-inch probe will serve the same purpose.

Grubbs-Bowling says pre-emergent weed preventers are best suited for grassy weeds and annual weeds that reproduce by seeds. “They don’t work as well on perennials.”

Second Emergent Applications

Chickweed, filaree and poa annua (annual bluegrass) and filaree are among the weeds that germinate in winter and have a delayed growing season in spring. For them, a second application of pre-emergence herbicide in early fall, late fall or early winter — depending on your climate — will prevent them from taking hold. They’ll kill clover seed, too, but you may not want to: Clover is making a comeback as a nitrogen-rich, pollinator-friendly companion for lawns.

Choosing a Pre-Emergent Herbicide

Here are some things to consider before choosing a pre-emergent herbicide:

Is It a Selective or Nonselective Product?

A selective herbicide is formulated to kill certain kinds of weeds and leave other plant life alone, or at least not damage your grass so much it can’t recover.. A nonselective herbicide will try to kill everything it touches — including your grass.

Which Weeds Will It Kill?

Not all herbicides will kill all type of weeds. For example, selective herbicides made to kill broadleaf weeds will not kill the dreaded crabgrass — because it’s a grass.

Herbicides that contain isoxaben, simazine, or oxyfluorfen, for example, kill some broadleaf weeds but are ineffective against others and against invasive grasses. Herbicides classified as a dinitroaniline, napropamide, metolachlor and dichlobenil will kill invasive grasses and some – but not all – broadleaf weeds.

Other popular pre-emergent chemicals include:

Prodiamine – The active ingredient found in the popular Barricade brand pre-emergence herbicide, which tackles about 30 different broadleaf and grassy weeds, including the dreaded crabgrass and annual bluegrass (poa annua).

Oryzalin – This chemical is used in Surflan and several other brands as a broadleaf weed killer also effective against spurge. In turfgrass, it gained popularity for pre-emergent weed control on established, warm-season turf (including Bahiagrass, Bermudagrass, buffalograss, centipedegrass, St. Augustine grass).

Dithiopyr – Effective on about 45 grassy and broadleaf weeds, Dithiopyr is the active ingredient in Dimension and several other brands. It is one of the few pre-emergent lawn care products that has some effectiveness against weeds that have already sprouted. But it needs to be used at maximum strength for that to happen. Effective for about four months after application.

Could It Damage Your Lawn?

Some herbicides can hurt your grass. Pre-emergent herbicides can damage new lawns and shouldn’t be used until the grass has settled in for a few months. Products containing dicamba can damage St. Augustine and carpetgrass if applied at the wrong time. Also, methylated seed oil, often used in treating crabgrass, should never be used on St. Augustine, carpetgrass, Bermuda, or centipede grass.

Reseeding Ater Using a Pre-Emergent

Pay particular attention to the label of the pre-emergent product if you have a new lawn, or intend to reseed. If you pick a variety that kills grassy weeds, likely it will kill any new desirable grass seed as well. Most pre-emergence products lose effectiveness after about six to eight weeks, so wait at least that long before reseeding.

Do You Use Granular or Liquid Pre-Emergent?

This is a matter of personal preference. Liquid pre-emergent products must be applied with a properly calibrated sprayer. It’s essential to water in granular products applied by a spreader.

Most of the herbicides you’ll find at the garden center are going to be selective, Grubbs-Bowling says, but choose carefully.

Whichever product you select, it’s vital to apply it thoroughly and evenly to gain the best weed prevention. You will need to know how many square feet in your yard to mix the right amount. A pre-emergent must cover your target area completely to serve as a barrier against weed growth. Missing a spot could mean trouble because if you give a weed an inch — it’ll take a yard!

Read Labels Thoroughly

By law, herbicide labels must contain specific information on the ingredients, proper application, and dangers of the product. Although it’s hardly provocative prose, read the labels thoroughly.

“People just don’t read labels,” says Karey Windbiel-Rojas, a pest management specialist with the University of California. In her community outreach programs, she stresses the importance of reading and understanding labels.

Don’t Expect Miracles

Don’t expect a miracle. You will not kill all your weeds. The herbicide will not reach all the seeds buried in the soil and weed seeds can sit dormant for years before sprouting. Others will arrive by air in your lawn, sprouted from weeds from your neighbor’s yard or a random patch of weeds miles upwind.

“If you have soil and sunlight and water, weeds are going to grow,” Windbiel-Rojas says.

Selective herbicides are ineffective on unwanted perennial grasses which will continue to grow through the winter. The old hoe and spade is the best solution for those.

Weed control does not deliver instant gratification.

“Be realistic and reasonable,” Grubbs-Bowling says. “The best defense against weeds is a healthy and competitive yard.”

Proper watering, fertilizing, aerating and mowing makes your yard competitive in the war on weeds – along with patience and persistence.