The Perfect Time to Apply Pre-Emergent Herbicide


Oct 05, 2019

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Ridding nutrient draining weeds from your yard is one of the most critical steps in maintaining a full, vibrant lawn.

When weeds crop up, you can always go pick up an on-the-spot or another post-emergent weed killer to get rid of it. The issue is that at this point, the weed has already done some damage, and killing it could leave an unsightly brown patch in its wake.

This is where pre-emergent herbicides can help. These special chemicals will do nothing to existing weeds, but they are specialists in seeking out weeds in their sprouting phase and killing them before they can break through the soil.

Because pre-emergent herbicides attack the weed before it can grow, timing is key to this weed killer’s effectiveness.

Here’s how to figure out when to apply pre-emergent herbicide to get the best results.

What is Pre-Emergent Herbicide in Plain English?

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There are many long names in the lawn care game, and one of the longest and frequently misunderstood is pre-emergent herbicide.

What in the world is this stuff, and how does it help your lawn?

Put into simple terms, it is a weed control system that does not prevent the weed from germinating. Instead, it holds off its attack until that seed turns into a young, healthy sprout with a bright future of thieving your lawn’s precious nutrients ahead of it.

It’s at this vulnerable sprout phase that the big bad pre-emergent herbicide strikes the weed dead, preventing it from ever feeling the sun’s warm rays.

OK, maybe it sounds terrible when we put it that way, but this is one of the most reliable ways to permanently kill that weed so you aren’t frustratingly pulling or spot treating it every year.

Types of Pre-Emergent Herbicide

You should just roll into your local store and snag any old pre-emergent herbicide and get to spreading, right?

We love your enthusiasm, but there are several types of pre-emergent herbicides, and only certain ones will work on the weeds you need to kill.

Regardless of what type of weed you need to kill always verify the pre-emergent herbicide you choose is a selective type. If you mistakenly apply a nonselective herbicide, well… this is like using a sledgehammer to kill that fly on your window. Sure, the fly is dead, but you smashed your window doing so. Same principles apply here.

Non-selective herbicides kill everything (even the good grass) in sight.

Most pre-emergent herbicides will be selective, but it is always best to verify that via the label or asking an expert. You’ll also want to pick up a few extras to make your herbicide more effective and make it easier to apply.

Here are the main types of pre-emergent herbicides:

Spring Pre-Emergent Herbicide

Spring pre-emergent herbicides are those you apply in the springtime to kill weeds that generally crop up in the summertime. These are typically your grassy and broadleaf weeds like crabgrass or dandelions.

While you may not see these weeds until the summer, their seeds are just starting to germinate and sprout in the mid to late spring. Therefore, you’d use a spring pre-emergent to kill them in the sprouting phase.

Fall Pre-Emergent Herbicide

Got an issue with hardy wintertime weeds like prickly lettuce or deadnettle?

You need a fall pre-emergent herbicide. These weeds are at their strongest when the weather gets cold, so spreading this herbicide in the fall kills the weed before it can ever cause an issue.

Determining the Type of Pre-Emergent Herbicide You Need

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Determining the type of pre-emergent herbicide you need is as easy as identifying the types of weeds your lawn struggles with and finding the weed killer that manages it.

Watch your lawn closely each summer and winter and notate the types of weeds you see.

If your lawn is generally weed-free in the summer but riddled with deadnettle or henbit in the winter, you know you need a fall pre-emergent herbicide. If your lawn is the other way around and is clear in the winter but packed with grass-killing weeds in the summer, you need to pick up a spring pre-emergent herbicide. If you have year-round issues, congratulations, you need them both.

Realistically, though, predicting weeds each season based on previous years is a crapshoot. Weeds aren’t suburbanites who find a comfy place to set up and call it their forever home. Birds, the wind, and the kiddos playing in the yard can introduce seeds from weeds you never had an issue with before. Next thing you know, your beautiful summertime lawn looks like a dandelion patch.

This uncertainty is exactly why it is best to plan on applying spring and fall pre-emergent herbicides every year.

When is the Ideal Time to Apply Pre-Emergent Herbicide?

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Now that we broke the news that you should plan to apply spring and fall pre-emergent weed control, you’re likely wondering when you should apply each product. There are two valid schools of thought here.

The Scientific Method

If you have the time to monitor weather patterns and get soil temperature readings in your area, you can pinpoint the exact time to apply your pre-emergent herbicide. A quick Google search for “soil temperatures in [your state]” will show you the info you need.

For springtime application, you’ll want to wait until the local soil temperatures rise to around 55 degrees F for at least 36 hours. This gives you a few weeks to get your pre-emergent barrier in place before the seeds germinate, and the sprouts begin their skyward path.

Wintertime weeds start their germination process around the time daily high temperatures dip under the 70-degree mark, which is when soil temps fall to around 50 degrees F. You want to get ahead of the game a bit here and apply your fall pre-emergent herbicide when the high temperature remains in the mid-70s for five days in a row.

The Earlier-is-Better Method

While the scientific method is best for pinpointing the best time to apply your pre-emergent herbicide, we understand your busy life may not afford you the luxury of monitoring weather patterns and soil temperatures. This is where the earlier-is-better method makes sense.

Pre-emergent herbicides pay no mind to when you apply them. They do their job no matter what and will continue working throughout the season.

So, as long as you get your herbicide applied early in the season before the seeds germinate – the first few weeks of the season should suffice – it will still stop root and shoot growth and kill the weed before it ever sees the light of day.