There’s a secret to tell. Using pre-emergent herbicides the right way will kill your lawn’s weeds.
Okay, it’s not a secret. But it seems that too many homeowners stare angrily at sprouting patches of crabgrass or tear their hair out when dealing with summer weeds like dandelions.
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.
What is a Pre-Emergent Herbicide?
According to North Carolina State University, pre-emergent herbicides are chemicals that prevent weed roots, shoots, or both from establishing.
What does that mean? It means that pre-emergent herbicides don’t actually kill weed seeds before germination.
Instead, they fatally interrupt their growth process in some way (often it’s in the cell division stage) 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.
(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.”
When to Apply Pre-Emergents
“The soil temperature (not air temperature) should be in the 50-55 degree range,” says Dr. Rebecca Grubbs-Bowling, assistant professor and turfgrass specialist at Texas A&M University and author of “A Homeowner’s Guide to Herbicide Selection for Warm-Season Turfgrass Lawns.”
Because climates vary, application dates will be different depending on where you live.
Spring 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?
For example, with crabgrass (and its summer annual brethren such as foxtails, goosegrass, and barnyard grass), a pre-emergent can be applied as early as January in Florida, but in Michigan, homeowners might utilize a spring application.
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.”
Annual weeds are either winter annuals or summer annuals. When to use pre-emergent will depend on the weed type.
Summer Emergent Applications
Chickweed, filaree, and poa annua (annual bluegrass) are among the weeds that germinate in winter and have a delayed growing season in spring.
For them and other winter annuals, a second application of pre-emergence herbicide in early to late fall or early winter — depending on your climate — will prevent them from taking hold. Applying pre-emergents when the soil temperature is 70 degrees is ideal.
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.
How to Apply Pre-Emergents
Now that you know when to apply the pre-emergent, you’ll need to measure your lawn and determine how many square feet of product to purchase.
Then, you need to know how to apply it, which will depend on the type you choose. Choose between granular and liquid–which is an entirely personal preference.
How to Apply a Granular Pre-Emergent Herbicide
- You’ll need a spreader to spread the granules.
- Read the label thoroughly.
- Calibrate your spreader based on the label recommendations and then fill your spreader.
- Next, using your spreader, apply the granular pre-emergent as evenly as possible.
How to Apply a Liquid Pre-Emergent Herbicide
- For this job, you’ll need a sprayer.
- Most liquid pre-emergents come with a sprayer, but they can also be purchased in various sizes and styles.
- Next, mix the product according to the label. Not following this step carefully can lead to product ineffectiveness or a damaged lawn.
- Then, spray the pre-emergent evenly and systematically, row by row, just as you would mow your lawn.
Note: No matter which pre-emergent you choose, it will need about 1 inch of water to activate. Irrigate 3-5 days after application.
How to Choose Pre-Emergent Herbicide
Here are some things to consider before choosing a pre-emergent herbicide:
Selective vs. Nonselective
- Selective herbicide: 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.
- Nonselective herbicide: Will kill everything it touches — including your grass, flowers, and plants. It’s highly effective, just be careful when applying.
Most of the herbicides you’ll find at the garden center are going to be selective, but choose carefully.
Which Weeds Will It Kill?
Not all pre-emergent herbicide applications will kill all types of weeds. For example, selective herbicides made to kill broadleaf weeds will not kill the dreaded crabgrass — because it’s a type of grass.
- Herbicides that contain isoxaben, simazine, or oxyfluorfen, for example, kill some broadleaf weeds but are ineffective against others and against invasive grasses.
- Herbicides with the active ingredient 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 and is also effective against spurge. In turfgrass, it gained popularity for pre-emergent weed control on established, warm-season turf.
- 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 have 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.
A natural option: Corn gluten meal is a chemical-free pre-emergent that prevents weeds from sprouting about 70% of the time.
Granular vs. Liquid Pre-Emergent
This is a matter of personal preference. Here are a few things to consider when choosing granular vs. liquid pre-emergents.
- Liquid pre-emergent products must be mixed carefully while granular does not.
- Liquid requires a little less labor. You just have to spray, and move as needed to reach your entire lawn. Granular herbicide requires you to walk the entirety of your lawn with the spreader.
- Liquid is easier to distribute evenly than granular.
- Granular needs more water to become active than liquid.
- Granular is easier to apply for beginners.
- Granular is typically cheaper.
Whichever product you select, it’s vital to apply it thoroughly and evenly to gain the best weed prevention.
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. 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.”
For more weed information, check out our comprehensive “Guide to Weed Control in Your Yard.”
FAQ About Pre-Emergent Herbicides
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.
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.
For more seeding information, check out this overseeding guide.
Your pre-emergent, especially granular, will need about an inch of rain or water to activate. However, too much rain will dilute or wash away your herbicide. Before you apply, check your local weather forecast.
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