When to Apply Pre-Emergent Herbicide – Preemergent Weed Killer

Imagine your once lush green lawn overrun with weeds with all your efforts to kill them non-effective. This, right here, is the plight of a lot of gardeners.

There are several ways to control weeds but using pre-emergent weed killers is the best of them all. 

While using this class of weed killers may be effective, using them at the wrong time may give you adverse results ranging from non-effectiveness to damage to your plants. To avoid this, you need to apply at the right time. Before I show you the right time to apply this weed killer, let’s first see what a pre-emergent weed killer is.

What is a Pre-Emergent Herbicide?

Pre-emergent herbicides do not stop the germination of grass seeds in the ground. They work by forming a mat-like barrier above the seed and just below the soil surface that stops the seed from sprouting – that moment after germination when the weed is getting ready to raise its head and overrun your yard; that is when pre-emergent herbicide strikes.

Most weeds like crabgrass are best handled before they sprout.

Pre-emergent herbicides are so effective because they tackle the weed before they can lay a foothold in your yard.

Now that you’ve seen what pre-emergent herbicide is, you also need to know why it is important you apply this effective weed killer at the right time.

Why it is Important to Apply Pre-Emergent Herbicides at the Right Time?

Because this class of herbicide works by stopping the sprouting of already germinated weeds in the ground, there is only a small window for it to work. Getting the application date wrong by a few days will result in weeds filling up your garden as fast as possible.

Pre-emergent weed killers will only be able to work when they are applied before the weed surfaces. When the weed is visible, there’s hardly anything this herbicide can do anymore.

How Does Preemergent Herbicides Work?

When to Apply Pre-Emergent Herbicide

Pre-emergent herbicides work by forming a barrier just below the surface of the ground. This barrier prevents the newly germinated seeds from reaching the soil surface.

Bear in mind that pre-emergent weed killers can affect your lawn if applied at the wrong time. So, you should wait at least 6 weeks after planting your lawn before applying pre-emergent or plant your lawn 3 months after applying a pre-emergent herbicide.

When to Apply Pre-Emergent Herbicides?

The best time to apply this class of weed killers is not set in stone; it is dependent on the type of weed you’re looking to control.

Although they can be applied all through the year with decent results, you should apply them during the grass’s sprouting season. For most grasses, spring and fall are when they sprout.

Understanding how the weed works and its growing circle will tell you its sprouting period is when you want to apply this weed killer.

Preemergent weed killer

When to Apply to Kill Summer Annuals Weeds?

First of all, which weeds are summer annuals?

Summer annual weeds are weeds that blossoms during the summer and last for just one year before they die. These weeds include crabgrass, lambs-quarters, mallows, pigweed, among others.

The best time to apply pre-emergent weed killers to control annual summer weeds is during early spring or late winter if you’re in the US’s southern or coastal parts. 

Make sure to wait until the soil temperature has risen to 55 degrees Fahrenheit for at least 36 hours before applying this herbicide. This is when summer annuals start to germinate.

When to Apply to Kill Winter Annuals Weeds?

As their name implies, winter annuals are annual weeds that blossoms during the winter. These weeds include chickweeds, mustards, shotweed, and annual bluegrass, among several others.

If the weeds you’re trying to prevent is a winter annual, the best time to apply the pre-emergent herbicide is by late summer or early fall. Mid-September is the generally accepted period. Remember, the aim is to stop the weed from sprouting. When in doubt, apply earlier.

Winter annuals start to germinate when the temperature falls below 70 degrees Fahrenheit, usually closer to 50 degrees. Apply when the temperature has stayed around 75 degrees Fahrenheit for at least 5 days.

Herbicides come in two forms, liquid and granular herbicides. Granular pre-emergent herbicides need to be activated by watering them down. This is done because its active ingredients are in the granules and must be set free to start working. You can make use of a broadcast spreader to apply the granular pre-emergent herbicide.

3 Things to Consider When Selecting the Pre-Emergent Herbicide to Use

Before you choose your pre-emergent weed killer, here are three factors to consider.

Apply Pre Emergent weed killer

1. Type of Weeds You Want to Kill

Some herbicides can kill broadleaf weeds, while others are more effective when used on invasive grasses like crabgrass. 

The type of weeds you have in your garden will determine the pre-emergent herbicide you choose. You want to get a weed killer that is effective against the weeds in your garden.

Herbicides that will kill broadleaf weeds but are almost ineffective on invasive grasses contain simazine, isoxaben, and oxyfluorfen. In contrast, herbicides effective against invasive grasses and a few broadleaf weeds have dichlobenil, dinitroaniline, napropamide, and metolachlor.

2. Granular or Liquid Pre-Emergent

They are both effective herbicides. Choosing between them is based on personal preference and perhaps materials you already have at your disposal. Not many gardeners have sprayers lying around in their garden shed. Still, many have broadcasters that can be used to spray granules.

Granular or Liquid Pre Emergent

Remember, if you decide to go with the granular pre-emergent, it needs to be activated with water after spraying.

3. Is the Preemergent Selective or Nonselective?

Selective herbicides are selective of the grasses they kill. They kill the weeds while allowing your lawn to regrow after. On the other hand, Nonselective herbicides will kill everything it comes in contact with, including your lawn.

Choosing the type of pre-emergent is important if you still want your lawn to regrow.


Pre-emergent herbicides are perhaps the most effective weed killers as they tackle the weed before sprouting from the ground. They are so powerful that they can also inhibit your lawn’s growth if it is not applied at the right time.

Reseeding your lawn after applying pre-emergent needs to be done months later when the herbicide has lost its effectiveness, and the barrier must have broken down.

About Jennifer Igra

Growing up in Brooklyn, New York City known for it’s green gardens. Jennifer, a 30 year old gardener and green living fanatic started Igra World to share her gardening journey and increase gardening awareness among masses. Follow Igra World to improve your gardening skills.

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