When To Apply Pre Emergent Herbicide

When To Apply Pre Emergent Herbicide

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Rebecca
December 29, 2021
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Benjamin Franklin might be addressing fire-threatened Philadelphians when he famously said, “An ounce of prevention is worth a pound of cure,” but that statement is true for weed control, too!

Knowing when to apply pre-emergent herbicide can save you a lot of headaches.

This is especially true when dealing with weed species like crabgrass, goosegrass, and annual bluegrass.

It is a lot easier to prevent weeds than going through the hassles of killing them once they are established. This guide will show you the best times to apply pre-emergent herbicides.

But first, what exactly is pre-emergent herbicide?

What is Pre-Emergent Herbicide?

What is Pre-Emergent Herbicide?

In simple terms, a pre-emergent herbicide is a chemical that stops unwanted plants or grasses in your lawn, garden, flower bed, sidewalk, or any other part of your yard.

Here’s another way to say it: Pre-emergent herbicides prevent weeds from taking root!

Just so you know, pre-emergence herbicides don’t prevent seeds from germinating.

No! The seeds of undesirable plants will germinate all right. However, they won’t sprout – the herbicide won’t allow it.

Pre-emergent herbicides create a difficult-to-push-past layer that’s highly effective at killing weeds when they try to go past the layer.

All of this may be true, but if you get the application timing wrong, pre-emergent herbicides will do little to control weed growth.

After all, pre-anything means before something happens.

In other words, the success of weed control using pre-emergent herbicides depends heavily on applying the chemical before the weeds sprout and become visible.

If you already have full-blown weed growth in your yard, you might want to read this guide on Organic Lawn Pest Control to help you tackle the culprits.

When Should You Apply Pre-Emergent Herbicide?

When To Apply Pre Emergent Herbicide

Now, to the meat of the matter!

When should you apply pre-emergent herbicide? This will depend on a few different factors, like where you live and the temperature.

Generally, though, the spring and fall seasons are the ideal times to treat your lawn, garden, and other areas with pre-emergent herbicides.

A few of the popular weeds you can effectively control with pre-emergent products during these seasons include:

  • Crabgrass
  • Annual Bluegrass
  • Common Chickweed
  • Back Medic
  • Goosegrass
  • Prickly Lettuce
  • Dandelion
  • Foxtail

To know when to apply a pre-emergent herbicide, it is crucial to understand the lifecycle of weeds. Typically, this classification includes summer annuals and winter annuals.

Spring Application

Common summer annual weeds are crabgrass, pigweed, spurge, lambsquarters, and mallow. These have a 1-year life cycle, germinating in the spring and dying off in the fall.

If the weeds in your yard are summer annuals, you want to tackle them in the spring to prevent them from showing up during the summer.

The best time to apply a pre-emergent herbicide for summer annuals is early spring or late winter if you live in the Southern and Coastal U.S.

To get the best results, spring pre-emergence herbicide application should be done when the soil temperature is anywhere around 55 degrees or above for no less than 48 hours.

Most places in the U.S. will experience this soil temperature around March and April.

Still, it is usually a good idea to check with your local county extension service to get the most current soil temperatures for your location.

Early Spring Application

Your driveways, sidewalks, and patios are usually the first areas where weeds sprout in the spring.

That’s because the concrete slabs in these areas retain the heat from the sun. This keeps the ground warm for any seeds around these spots.

So, you’ll probably have early sprouts in these areas.

Keep an eye out for these early sprouts. Once you see them, it is high time you applied the pre-emergent herbicide to your lawn, garden, or yard.

Fall Application

Want a clean lawn or yard in spring? Fall pre-emergent herbicide application is the way to go!

This is meant to prevent winter annuals such as annual bluegrass, chickweed, mustards, and shotweed. These weeds have a 1-year lifecycle; germinating in the fall and dying off come spring season.

Depending on your location, late summer and early fall are the best times to apply fall pre-emergent herbicides.

When it comes to soil temperature, wait until daytime high is anywhere in the mid-70s for about 3 to 5 days straight before application.

Early Fall Application

For best results, you want to make the first fall pre-emergent application sometime around late August and early October.

Don’t stop at the first application, though. A second application will be necessary, and that should be made within 4 to 8 weeks from the date of the first fall pre-emergent herbicide application.

This should be around October through November.

If you ignore the second application, you might still have some unsightly weeds to deal with come spring season. This is especially the case with annual bluegrass.

The weed species are known to have resistance to pre-emergent applications, so a repeat application is often necessary to ensure full weed control.

Difference Between Granules and Liquid Formulations

You have two options when you go shopping for pre-emergent herbicide products. These are granules and liquid herbicides.

Both types work in essentially the same way, and the one you choose mostly comes down to personal preference.

Here are the major differences between the two.

Granular Pre-Emergent Herbicide

When To Apply Pre Emergent Herbicide

Granular pre-emergent herbicides come in tiny grain form.

They are best applied using a broadcast spreader and drop spreader, especially if you want to cover a large area.

Here’s the thing, though.

There’s no need to invest in any fancy garden equipment if you only need to treat a small area. You can simply sprinkle the granules around areas with established plants using a cup.

Just remember to work the granules into the soil surface for better weed control.

No matter the application method you choose, you want to water granular pre-emergent herbicides.

That’s because the active ingredient in the chemical in the product needs to penetrate the soil to be effective.

If you don’t water the granules, the herbicide will remain trapped in there.

Liquid Pre Emergent Herbicide

Liquid Pre Emergent Herbicide

Liquid pre-emergent herbicides are available in two different strengths.

You can get options that require diluting with water before use or products that come in full strength.

In either case, liquid options are best applied using pump-style prayers.

Remember how important watering is for granular pre-emergent herbicides? Well, it applies to the liquid types, too!

Grasses may trap the active ingredient in liquid products and stop it from fully penetrating the soil when you apply the herbicide.

Watering is crucial to help push the chemical past the grasses and other plants into the soil.

Active Ingredients in Pre-Emergent Herbicides and Best Use Case

Active Ingredients in Pre-Emergent Herbicides and Best Use Case

Some of the best pre-emergent herbicides are available here at GreenIQ. But just before you buy any product, you should know that they aren’t all created equal.

Pre-emergent herbicides are made with different active ingredients that target different types of weeds and plants.

Here’s a quick rundown of what to look for depending on the results you want:

Pendimethalin

This compound is the most widely used in pre-emergent herbicides for residential environments. Usually, products with pendimethalin as their main active ingredient are great for use in spring.

They are also comparatively inexpensive. But they are relatively short-lived. Meaning, you may have to apply the chemical more than once, and that could remove any cost-saving in the long run.

Also, the compound can hinder root growth if it comes in contact with existing plants. Plus, it may stain vinyl floors and concrete.

Oryzalin

When it comes time to control annual grasses and broadleaf, pre-emergent herbicides containing oryzalin are a great option.

They are very affordable, highly effective, and selective. That means they are formulated to control certain weed categories while being less toxic to others.

Still, you want to apply this carefully as it may injure shrubs and ornamental plants. Plus, it may stain concrete if not cleaned immediately.

Prodiamine

This is one of the least water-soluble compounds, so it lasts very long in the soil.

If you’re looking to prevent crabgrass from showing up during spring, you can apply prodiamine pre-emergent herbicides in the fall.

For the best results, avoid overseeding within four months of application.

Isoxaben

This works best on broadleaf weeds such as oxalis, bittercress, chickweed, and spurge.

It is a long-lasting compound and can prevent weeds for up to eight months. But it’s not the best pick for crabgrass.

Dithiopyr

Go for products with this active ingredient if you want the most forgiving pre-emergent herbicide.

It works great even with newly sprouted weeds, provided they are still in the 2 to 3-leaf stage.

Unlike pendimethalin and oryzalin, it doesn’t stain and won’t damage the roots of existing plants.

It is highly effective for pre-emergence control of goosegrass, crabgrass, chickweed, henbit, and more.

How to Apply Pre-Emergent Herbicide

How to Apply Pre-Emergent Herbicide

There is no one-size-fits-all when it comes to pre-emergent herbicide application.

The frequency, exact time, and application method will vary depending on location, product, treatment area, and type of weed or weed seed you want to control.

However, here are general tips and suggestions to guide you.

  • The herbicides must be mixed correctly for best results.
  • Pre-emergents work like blankets. They are more effective with thorough coverage of the entire treatment area.
  • Spot spraying is utterly useless! It leaves a lot of open space for weeds to grow.
  • For precise application of liquid herbicides, consider investing in a good-quality pump-style sprayer or drop spreader.
  • For effective and efficient coverage over large areas (such as a big lawn), consider using a broadcast spreader.
  • Regardless of your preferred spreader or sprayer, always follow the recommended rates on each product label.
  • Watering in will activate the herbicide. It is best to apply it just before the rains set in if you are using the pre-emergent on a drip zone or non-irrigated area.

FAQ

Should You Reseed the Lawn After Applying Pre Emergent Herbicide?

You shouldn’t reseed your lawn immediately after applying a pre-emergent herbicide.

That’s because the chemical will prevent grass seeds from sprouting.

If you must reseed, it is best to wait until the herbicide is no longer active. In many cases, this can take up to four weeks or more.

Should You Fertilize the Lawn After Applying Pre Emergent Herbicide?

Should You Fertilize the Lawn After Applying Pre Emergent Herbicide?

Yes, tackling a weed problem should come first before fertilizing your lawn.

If you don’t get rid of the weeds or prevent them from growing, they can choke your beautiful lawn grasses and compete for nutrients with your plant roots.

The time between the pre-emergent herbicide application and fertilizer application will vary depending on the specific products.

In some cases, you can apply lawn fertilizers within a few hours of treating the area with the herbicide. With other products, you may have to wait for a couple of weeks.

Should You Aerate The Lawn After Applying Pre Emergent Herbicide?

Consider aerating your lawn before pre-emergent herbicide application. The standard recommendation on most herbicide labels is to aerate your lawn before applying the chemical.

The reason for this is simple. You don’t want to disrupt the chemical barrier that’s created over the soil by the herbicide.

But here’s the thing. No research has shown any adverse effect of aerating the lawn after applying per-emergent herbicide.

So, you can go ahead and aerate the site if doing so will encourage the growth and development of turfgrass.

However, if this isn’t the case, then it is best to avoid disturbing your lawn after applying pre-emergence herbicides.

Final Thoughts

Knowing when to apply pre-emergent herbicide is crucial since these products don’t just stop only weeds from sprouting.

Many of them will prevent all seeds from growing!

If you have beneficial plants or flowers where you want to control weed growth, it is best to hold off applying the chemical until the desirable plants sprout and start to grow.

Remember, there is no room for assumptions with pre-emergent herbicides. Always follow the recommended rates on product labels.

Resources:

Rebecca

Rebecca Vargas is an experienced gardener and landscaper and has been rendering professional services for many years. Her services cover both private homes and commercial properties. Leveraging that rich experience, Rebecca Vargas now dedicates a chunk of time to show just about anyone how to maintain their garden and yard, whether at home or workplace. GreenIQ is his way of reaching and teaching millions of homeowners across the globe about proper gardening and lawn care practices.