What Are Pre-Emergent Herbicides: Tips On Using Pre-Emergents


By: Bonnie L. Grant, Certified Urban Agriculturist

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.

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Using Pre-Emergent Herbicides the Right Way to Kill Your Lawn’s Weeds

In this story: Properly applied pre-emergent herbicides form the backbone of your lawn weed-control effort. It’s effective — if you apply it at the right time.

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:


MSU Extension

Available herbicides target a variety of weeds in red beet production.

Weed control in specialty crops like table beets is challenged by lack of herbicides. Bernie Zandstra of Michigan State University has conducted research to determine how existing products can best be used to control key weeds in red beets. Products can be chosen based on their ability to control problem weeds specific to a location, and include both pre- and post-emergence products. Note, all herbicides will injure beets to some degree, but they can grow out of the injury if rates are appropriate and conditions are right.

A partial list of pre- and post-emergence herbicides available for weed control in red beets, which research suggests provide excellent “E”, good “G”, fair “F”, poor “P” or no control “N” of select weed species. Efficacies and potential for crop injury will vary with environmental conditions. Shaded cells highlight weeds an active ingredient may be good or excellent at controlling. For more information, consult the MSU Weed Control Guide for Vegetable Crops (E-433).Download a printable/larger version of this table.

Products available for pre-emergence weed control

Dual Magnum and Nortron are available for pre-emergence weed control in red beets. Dual Magnum is most effective at providing a few weeks of control for grasses and broadleaves including redroot pigweed, nightshades and yellow nutsedge. Nortron can be used alone or in combination with Dual-Magnum to provide control of some additional broadleaf species including common lambsquarters, nightshades, smartweeds and mustards. Nortron is only available for purchase in pallet-size quantities. For sandy soils, Ro-Neet is another option that can be used to provide control of grasses and redroot pigweed. Note, an old standby, Pyramin 68DF (a.i. pyrazon), is no longer available. For more details on how to apply these products, consult Michigan State University Extension’s Weed Control Guide and the product label.

Products available for post-emergence weed control

Post-emergence herbicides can be applied after beets have emerged and can tolerate some herbicide injury. It is critical to use the lowest rate for these products the first time you apply them as they can all injure beets. Select Max can be used for grass control at any time when grasses are actively growing. Spin-Aid provides control of broadleaves including common lambsquarters, purslane, common ragweeds and mustards, and can be applied when beets have at least four true leaves do not apply if plants are under stress. Upbeet can provide control of velvetleaf and mustards, in addition to suppressing lambsquarter, pigweed, nightshades, ragweeds, smartweed and wild buckwheat, and can be applied when beets have two to six true leaves. Stinger can be used to control composites such as horseweed in addition to nightshades it should be applied after beets are at least 4 inches tall.

Choosing herbicides

  1. Consider which weeds are likely to be challenging in a particular field.
  2. Make a list of candidate herbicides for pre- and post-emergence control.
  3. Determine the appropriate time for application in relation to planting and crop growth stage.
  4. Keep track of weed pressure and crop development to time applications.

Note that efficacies and crop injury can vary with local environmental conditions. Consequently, if you decide to try a product you have not used before, consider using a low rate on a small percentage of your acres to minimize the impact of any crop injury and get experience with what your beets can tolerate.

For more information on herbicides for red beets and other crops, consult the MSU Weed Control Guide for Vegetables (E-433) and always remember to read the herbicide label.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).

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Herbicide Weed Killer for Edible Gardens

There are many options available when it comes to choosing the right weed killer for your edible garden plants. Pre-emergent products work by killing weeds before they grow above the ground by targeting weed seeds under the soil. They must be applied at the right time of year to be effective, which is usually in the spring when temperatures reach 55 degrees. Post emergent products can be applied to weeds already up and growing in your yard, and can be used throughout the year.

Make sure to read the label on products carefully to make sure the product can be used on or around your plants, and that the weeds you want to kill are also listed on the label. Apply products carefully, and take note if there is a time period from application to harvest, so you can be sure your produce is safe to eat.

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When To Apply Pre-Emergent Herbicide

April 15, 2020

Pre-emergent herbicide is an important tool in an effective weed management program, but properly timing the application can be tricky. In this article, we'll look at key principles in developing a weed control strategy, what pre-emergent herbicide can and cannot accomplish within a management program, and when to apply pre-emergent to different classifications of weeds.

Developing a Weed Control Strategy

When you're developing a weed control strategy for a property, an important detail to impress upon your customer is that it's not possible to completely eliminate weeds with a single application or cultural practice. There is no silver bullet that kills all weeds and an effective program adjusts to the needs of the property on a seasonal or annual basis.

All weeds have a survival strategy and cannot be completely eliminated because they have different life cycles and methods of reproduction. Seeds can lay dormant for years before they germinate, surviving drought, fire, and herbicide applications. Even if you were to completely clear a property of seeds, seed and vegetative propagules can easily be transported to the property by wind, water, animals or human activity.

As you're developing or adjusting your weed control strategy, it's important to ask yourself the following questions:

What weeds do you want to control?

Is the goal to prevent these weeds, to eradicate them, or both?

Are there cultural practices that could help reduce the presence of the weeds?

What are the life cycles of the weeds, and when is the proper timing for a pre-emergent herbicide application?

What desired plants are on the property, and are the herbicides you're considering safe/labeled for those plants?

How Pre-Emergent Herbicide Works

To get a better idea of how pre-emergent works, let's look at 3 key principles of pre-emergent weed control.

Principle #1: Pre-emergent herbicides are designed to control germinating weed seeds.

As its name suggests, pre-emergent is targeted towards weeds that have not yet emerged from the soil. To get the best results and to avoid wasting time and labor cost down the road, the weeds shouldn't be visible above ground at the time of application.

Important: Pre-emergent is not designed to control existing weeds or weed seeds.

The weed will only be killed when it begins to sprout from the seed and hits the herbicide barrier. It is possible for seeds to remain dormant and not be harmed by the pre-emergent herbicide application. This is why weed control is a constant process. There will always be seeds under the surface and a portion will germinate each season. Annual applications must be made to significantly reduce large infestations.

Remember, pre-emergent herbicide can affect desirable plants. That includes turf. Caution must be taken if you're applying pre-emergent and seeding the turf in the same season. Seed first, then apply pre-emergent at least 6 weeks later to allow for lawn establishment. Or seed at least 3 months after the pre-emergent has been applied.

Principle #2: Pre-emergent must be mixed correctly and applied evenly over the target area for best results.

Pre-emergent herbicides need to be mixed correctly for the spray solution to be at the appropriate strength. Take the time to read the manufacturer's recommendations and don't forget to calibrate your sprayer!

Thorough coverage is key. Think of pre-emergents like a blanket – you need to cover an entire area through which the weed seeds cannot germinate. Spot spraying achieves nothing, as there is plenty of open space for weeds to come through. Manufacturer instructions will indicate how much product to use “per 1000 square feet” or “per acre”, which determines how much herbicide to use for each gallon of water. Note that it usually takes 1 to 2 gallons of spray solution to cover 1000 square feet.

Principle #3: Pre-emergent herbicide must be watered in.

Watering in activates the herbicide, creating a barrier just below the surface. Most products call for 0.5 inches of irrigation or rain within 21 days after application.

If you're working with a non-irrigated area or a drip zone, apply the pre-emergent just before rain is anticipated.

Applying Pre-Emergent to Different Classifications of Weeds

To know when to apply pre-emergent herbicide, it is important to know how weeds are classified, namely by their life cycles.

Weed Classification: Summer Annuals

Most well-known example: Crabgrass (Crabgrass Germination Map below)
Other examples: Lambsquarters, Mallow, Pigweed, Spurge
Life Cycle: 1 year - Germinate in spring. They flower, produce seed, then die in fall.
Pre-emergent timing: Early spring (late winter for Southern & Coastal U.S.)

Weed Classification: Winter Annuals

Most well-known example: Annual Bluegrass (Poa Annua)
Other examples: Shotweed, Chickweed, Mustards.
Life Cycle: 1 year - Germinate in fall. Flower and produce seed quickly, then die in spring.
Pre-emergent timing: Late summer/early fall (rule of thumb is by September 15th)

Need help choosing the right pre-emergent herbicide for your Spring application?
Contact your local Horizon store. We're happy to help!


When to Apply Pre-Emergent Herbicide?

It’s important to know when to spray pre-emergent or apply granular herbicides. Using these chemicals in the wrong temperatures can render them ineffective.

Ideal Times of Year for Application:

Now, what is the right month to apply pre-emergent? While there are ideal application times, pre-emergents can be applied throughout the year. Fall application should take place in late summer or early autumn. Spring application should be early in the season. The key is to apply the product before unwanted seeds germinate.

Best Temperatures for Application:

Next, what temperature should it be to apply pre-emergent? Some landscape specialists claim that the temperature should be 60 or 70 degrees (F). But, a study from The University of Missouri College of Agriculture, Food, & Natural Resources found that the most effective time to apply pre-emergent herbicides was when the ground has been 55 degrees (F) for 5 days in a row. So, watch the weather and plan your application in advance.


The Don’ts: Things to Avoid when Killing Horsetail Weeds

Horsetail is a unique weed due to its membership in the fern family and its large root system. Several typical weed-killing tactics won’t work when trying to get rid of horsetail.

DO NOT attempt the following when trying to kill horsetail. These methods will cost you time and money without killing the plant.

  • Roundup: Roundup does not kill horsetail weeds. Not only do the waxy leaves of horsetail plants protect it from most topical herbicides, the plant is also very resistant to Glyphosate, the active ingredient in Roundup.
  • Sheet Mulching/Ground Covering: Horsetail thrives in the low-oxygen, low-light environment created by covering an area with plastic sheeting or thick mulch. Not only that, but because horsetail establishes such large root networks, it will find uncovered places to sprout. You may hold it back for a season with this method, but sooner or later, horsetail will push up through your mulch, around the edges of it, or even begin to grow underneath your ground covering.
  • Hand-Pulling: Don’t expect to get rid of horsetail by hand. Horsetail roots can grow to depths of five feet. You won’t kill the plant simply by pulling the exposed portion. If you follow this method you can expect to battle resurgent horsetail year after year.


Watch the video: When To Apply Pre Emergent Herbicide To The Lawn


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