What Are Pre-Emergent Herbicides: Tips On Using Pre-Emergents
Even the most vigilant gardener will have a weed or two in their lawn. Herbicides are useful in the battle against annual, perennial, and biennial weeds, but you have to know when to use them and which ones are most effective against a particular weed problem.
Pre-emergence weed killers are used on established lawns as part of an annual effort to combat plant pests. What are pre-emergent herbicides? These chemical compositions are used before weeds take hold to kill off infant root systems and keep them from growing. Learn how pre-emergent herbicides work so you can decide if they are the right method for you.
What are Pre-Emergent Herbicides?
Pre-emergence weed killers are used before you see the weeds to prevent them from showing up in the garden or lawn. This doesn’t mean the chemicals interfere with germination but rather they stop the formation of new root cells in baby weed plants.
Without weeds, the seedlings cannot continue to feed and grow and they just die back. This whole process happens at the soil level under the blades and thatch of the grass so you don’t ever have to see the sprouted weeds. Timing, weather, and the type of weeds that are problematic in the garden will dictate the exact formula and application for using pre-emergents.
How Pre-Emergents Work
The chemicals in pre-emergent weed killers are not effective on vegetative buds that sprout from existing roots or rhizomes. They also cannot be used on a prepared grass seedbed because their root stunting action in young plants will also affect sprouting grass.
Established plants have nothing to fear, as their root system is already developed and the plant is hearty and healthy. Pre-emergent info indicates that it is the sensitive root tissue of newly germinated seedlings that is killed off, resulting in complete plant death.
Perennial weeds develop thick persistent adult roots that re-sprout in spring, which makes them difficult to control with a pre-emergent formula. Annual weeds are in two classes: winter and summer annuals. The timing of a pre-emergence weed killer for each must match the germination period for the variety of weed. Biennial weeds, like dandelions, are not controlled by a pre-emergent because they produce seed that germinates nearly year around.
Pre-Emergent Info for Applications
As with most plant chemicals, the weather and type of weeds will affect the application method. When using pre-emergents for winter annuals, apply in fall because that is when the seeds germinate. Summer annuals germinate in spring and that is the correct time to apply a pre-emergent. If you are unsure what type of weed is the most troublesome, it is a safe bet that a springtime application will control the majority of the pests.
Pre-emergent weed killers require water to activate them and carry the chemical down to the root systems of newly sprouted weeds. Never apply an herbicide spray when there is a wind to prevent injury to other plants. The ambient temperature must be above freezing and the soil should be workable. Consult the manufacturer’s label for the varieties of weeds the product is effective against and the method and timing of application.
The Best Pre-Emergent Herbicides of 2022
For most gardeners, pulling weeds is a fact of life, but by applying the best pre-emergent herbicide, time spent weeding can be significantly reduced.
By Glenda Taylor | Updated May 24, 2022 10:13 AM
BobVila.com and its partners may earn a commission if you purchase a product through one of our links.
Stopping weeds early—before they can emerge from the soil—is the goal when applying a pre- emergent herbicide. A pre-emergent prevents the sprouting of undesirable weed seeds, so it’s a beneficial partner in the war against weeds in lawns, flowerbeds, and even vegetable gardens.
The best pre-emergent herbicide product will vary, depending on the size of the area requiring treatment and the type of weeds the gardener wants to kill. Ahead, learn what to look for when shopping for a pre-emergent weed killer, and find out why the following products can all help prevent unwanted weeds this year.
- BEST OVERALL:Quali-Pro Prodiamine 65 WDG Pre-Emergent Herbicide
- BEST BANG FOR THE BUCK:Miracle-Gro Garden Weed Preventer
- BEST FOR FLOWER BED:Preen Garden Weed Preventer + Plant Food
- BEST FOR PESTS:Preen Weed Preventer Plus Ant, Flea, & Tick Control
- BEST FOR LAWNS:Scotts Turf Builder Halts Crabgrass Preventer
- BEST NATURAL:Preen Natural Vegetable Garden Weed Preventer
What to Consider When Choosing the Best Pre-Emergent Herbicide
Pre-emergent herbicides are well suited for lawns and gardens where desirable grass and plants are already established. However, gardeners should not apply these products where they plan to plant beneficial seeds, such as starting flowers or vegetables from seeds or overseeding a lawn. These products vary in form, strength, and in the type of ingredients they contain. Many are labeled as “weed preventers.” Read on to discover more about these and other important factors to consider when selecting the best pre-emergent herbicide.
Pre-emergent herbicides are available in two primary forms: liquid and granules. While they both work in the same manner—by preventing weeds from emerging from the ground—groundskeepers and gardeners may prefer applying one form over another. Either type will help reduce the need for manual weeding.
- Liquid: Available either concentrated or full-strength for use in a pump-type sprayer, liquid pre-emergent herbicides are easy to apply and are well suited for treating lawns to protect them from crabgrass and other weeds. Concentrated liquids are the form of choice for many commercial applications, such as golf courses and parks.
- Granules: Granules resemble tiny grains and can be applied by a drop spreader, a broadcast spreader, or simply shaken out of a cup onto the soil to control weeds. The granules dissolve and seep into the soil after watering to form a weed-germination barrier.
Rather than targeting different types of plants, as many post-emergent herbicides do, pre- emergent herbicides target plants in different stages of growth. A pre-emergent will stop a seed from developing roots or a shoot, but it will not harm a larger plant’s roots. Also, a pre-emergent herbicide will not kill the roots of perennial weeds, such as bindweed or witchweed, that may lie beneath the soil. This can be a source of confusion for gardeners who see weeds emerging after the application of a pre-emergent herbicide. To eliminate perennial weeds, it’s best to wait until they emerge from the soil and then treat them directly with a post-emergent herbicide.
While many pre-emergent herbicides will prevent the germination of a majority of seeds, some weed seeds, such as crabgrass, may survive some of the weaker types of pre-emergent herbicides. For this reason, manufacturers often combine two or more of the following types of pre-emergent herbicides in a single product.
- Siduron: Siduron is commonly found in products designed to stop the seeds of weedy annual grasses, such as foxtail and crabgrass, from germinating.
- Trifluralin: Trifluralin is chiefly found in pre-emergent herbicides for use in flower and vegetable gardens as it prevents the germination of a wide variety of seeds.
- Isoxaben: Isoxaben is mainly used in agricultural crops to control broadleaf weeds, such as clover and bindweed.
- Dithiopyr: Dithiopyr is found in many types of pre-emergent lawn products—it controls crabgrass, a common lawn annoyance, in addition to other weeds.
- Pendimethalin: Pendimethalin is the only pre-emergent herbicide that also features some post-emergent properties. It’s used on lawns and agricultural crops, such as cotton and corn.
- Benefin: Benefin is found in herbicides used in vegetable gardens. It’s also useful for preventing grassy weeds in fields of legume-type crops, such as clover and alfalfa.
- Corn gluten: Corn gluten is the only natural pre-emergent herbicide currently on the market. An organic weed killer, it will stop many weed seeds from germinating. If a product contains corn gluten, it probably does not contain any additional chemical herbicides.
Pre-emergent herbicides form a barrier in the soil that stops successful weed seed germination. The average product will protect an area for 1 to 3 months, but some offer even longer control. Many manufacturers recommend applying pre-emergent herbicide products in the spring at about the time forsythia blooms are beginning to fade and then applying once more in early fall to prevent blown-in weed seeds from germinating. While the use of a pre-emergent may not stop all weeds from germinating, it will eliminate the majority of them, even if used only once per year.
Safety and Application
Most pre-emergent herbicide products are safe when used as directed. The key to maximizing safety is planning ahead and applying when children and pets are not around.
- Read the label. Unlike other products that come with safe-use instructions, herbicides bear this warning: “It is a violation of Federal law to use this product in a manner inconsistent with its labeling.” When misused, chemical herbicides can pose a danger to residents of the home, to animals, and to the environment.
- Protect the body. If they are inhaled or make contact with skin, some pre-emergent herbicide products may cause respiratory symptoms or skin irritation. Wear gloves, long sleeves, long pants, a dust mask, and eye protection during application.
- Apply on a calm day. This is more important when applying a liquid spray than granules. Wind can carry the spray, reducing good coverage in the desired area and perhaps contaminating an area where the gardener wants to plant seeds.
- Water after application. If rain isn’t in the immediate forecast, most pre-emergent herbicide products call for watering to encourage the herbicide to seep into the soil and form a barrier against seed germination.
- Wash afterward. Even with care, some of the product may inadvertently land on skin or clothing, so change clothes and thoroughly wash up after application to be on the safe side.
Our Top Picks
To qualify as a top pick, a pre-emergent herbicide should prevent a wide variety of weeds from sprouting and offer easy-to-follow instructions. While the best pre-emergent herbicide will vary depending on the treatment location, such as a lawn or a vegetable garden, it should stop the type of weeds most likely found in those specific areas. All of the following products will reduce hand-weeding and help avoid the need for additional post-emergent treatment of weeds.
Quali-Pro Prodiamine 65 WDG Pre-Emergent Herbicide
Those looking for an effective pre-emergent herbicide to prevent crabgrass in lawns, flowerbeds, and other planted beds and borders need look no further than Quali-Pro Prodiamine 65 WDG Pre-Emergent Herbicide. This professional-quality product is available in a 5-pound granular concentrate. It is designed to dilute and apply with a pump-type sprayer to lawns, under trees, and around bushes and shrubs.
In addition to controlling crabgrass, this pre-emergent controls several other troublesome weeds, including henbit, chickweed, and spurge. Prodiamine is the active ingredient; for the best results, apply the product both in the spring and in the fall.
- Large 5-pound container
- Suitable for use in spring and fall
- Prevents several weed types from sprouting
Get the Quali-Pro herbicide on Amazon.
Best Bang for the Buck
Miracle-Gro Garden Weed Preventer
Reduce weeding tasks without spending a lot of money with the Miracle-Gro Garden Weed Preventer. This granular pre-emergent comes from a well-known manufacturer, and best of all, it’s affordable. Available in a 5-pound jug, the convenient shaker top makes it simple to sprinkle the granules around established plants.
The Miracle-Gro weed preventer works best when applied early in the growing season, and it will prevent weed seeds from germinating for up to 3 months. It can be used in flowerbeds, around shrubs, and in the vegetable garden, but it’s not recommended for controlling weeds in lawns.
- Easy-to-use container
- Impressive coverage
- Does not kill existing weeds
Get the Miracle-Gro herbicide on Amazon.
Best for Flower Bed
Preen Garden Weed Preventer + Plant Food
Prevent weeds in the rose garden or the poppy patch while boosting plant health. The Preen Garden Weed Preventer + Plant Food creates a barrier in the soil that doesn’t allow weed seeds to germinate, while simultaneously providing nutrients that encourage robust plant growth and large bountiful blooms.
The active ingredient is trifluralin, and it comes in granular form in a hefty 16-pound container—enough to last the average gardener all year. This weed preventer dramatically reduces the number of weeds for up to 3 months, and it includes a handy shaker scoop that allows users to scoop up the granules and then uniformly disperse them on the soil.
- Feeds plants while preventing weeds
- Great for large gardens
- Reduces weeds for up to 3 months
- Container is heavy
Get the Preen herbicide at Amazon, The Home Depot, and Ace Hardware.
Best for Pests
Preen Weed Preventer Plus Ant, Flea, & Tick Control
Prevent weeds in flowerbeds and perennial borders with Preen Weed Preventer Plus Ant, Flea, and Tick Control. This product is not suitable for lawns or vegetable gardens, but it will stop unwanted weed seeds from sprouting in ornamental beds and around bushes and shrubs for up to 4 months using dithiopyr as the active ingredient.
As an added bonus, this herbicide product contains a pesticide that kills ants, fleas, and disease-carrying ticks. It comes in a 4.25-pound jug with a shaker top to easily dispense the granules. For the best results, sprinkle the product and then thoroughly water the area. Keep pets and children away from the area for 24 hours.
- Deals with ants, fleas, and ticks
- Protection for up to 4 months
- Good dispersion from the shaker nozzle
- Not suitable for lawns or vegetable gardens
- Not safe around pets or children for 24 hours
Get the Preen herbicide on Amazon and at Walmart.
Best for Lawns
Scotts Turf Builder Halts Crabgrass Preventer
Scotts Turf Builder Halts Crabgrass Preventer stops crabgrass before it can start while protecting the lawn from other weeds for an entire growing season. At the same time, this pre- emergent herbicide feeds the grass to help it develop a deep, healthy root system. The product features pendimethalin as the active herbicide and comes in a 1-pound bag for distribution with a drop spreader or a broadcast spreader.
For the best results, apply this pre-emergent early in the season, soon after the snow thaws, to create a weed-free barrier and a lush lawn. Once applied, it requires thorough watering or at least 0.25 inches of rain within 2 to 3 days.
- Huge coverage
- Doubles as effective lawn food
Get the Scotts herbicide at Amazon, Lowe’s, and The Home Depot.
Preen Natural Vegetable Garden Weed Preventer
Those looking for a natural pre-emergent for fruits and vegetables can check out Preen Natural Vegetable Garden Weed Preventer that contains no harsh chemicals, ensuring it’s safe for use around plants that produce foods. Corn gluten is the active ingredient in this pre-emergent, which creates a natural barrier to seed germination and makes it the most eco-friendly of all the herbicides in this lineup.
The product comes in granular form in a 5-pound bag, and it should be applied every 4 to 6 weeks to keep weeds at bay. This natural pre-emergent prevents crabgrass, clover, dandelions, and other weeds.
- Eco-friendly solution
- Suitable for vegetable gardens
- Handles most common weed types
- Requires frequent application
Get the Preen herbicide at Amazon, The Home Depot, and Walmart.
For a reliable herbicide that tackles a wide range of weeds and prevents them from sprouting, this Quali-Pro Prodiamine is a suitable option. Shoppers looking for a more budget-friendly solution may wish to consider the Miracle-Gro Garden Weed Preventer, which comes in an easy-to-use shaker bottle and provides up to 800 square feet of coverage.
How We Chose the Best Pre-Emergent Herbicides
We looked at the various types of pre-emergent herbicides on the market to determine our top picks in a set of categories, each of which is suited to specific shoppers looking for a particular characteristic for their yard.
While our research did find that there are two types of herbicides available—liquid and granules—we found that granules were much more effective at providing long-lasting weed prevention, and many liquid options were prone to over-spraying and burning the affected areas. Granules are more suitable for slowly diluting over time to provide a steadier protection. As such, our entire list of recommendations are varying types of granule products, all from respected brands.
Reducing the amount of weeding in a garden or flowerbed is the goal of many gardeners, and applying a pre-emergent herbicide can help. For those new to herbicides, however, some questions are to be expected.
Q: Will pre-emergent herbicide kill crabgrass?
A pre-emergent herbicide will not kill established crabgrass, but it will stop the germination of new crabgrass seeds.
Q: When should I apply pre-emergent herbicide?
For the best results, most pre-emergent herbicides should be applied at least once in the spring and once in the fall. Check the product label for additional instructions.
Q: What is the best temperature to apply pre-emergent herbicide?
Apply a pre-emergent herbicide in the spring when outdoor temperatures are still relatively cool—around the low 50s—before weed seeds begin to germinate.
Q: Will rain wash away my pre-emergent herbicide?
Rain will actually help the pre-emergent herbicide seep into the soil to form a barrier against weed seeds. Only a torrential rain would likely wash away the product.
Disclosure: BobVila.com participates in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for publishers to earn fees by linking to Amazon.com and affiliated sites.
Using Pre-Emergent Herbicides the Right Way to Kill Your Lawn’s Weeds
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.”
Looking for the best pre-emergent for your lawn? Here are our top 4 picks:
- Preen 2464221 Extended Control Weed Preventer, 10 lb. -Covers 1,630 sq. ft
- Quali-Pro Prodiamine 65 WDG Pre-Emergent Herbicide
- Snapshot 2.5 TG Granular Pre-Emergent Herbicide
Timing is critical
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.
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.
Lynn Walker has been writing for radio, TV and newspapers for more than 50 years, and has expertise in news, features, humor, history, weather, genealogy, science, archaeology and government.
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