8 Common Questions About Pre-Emergent Weed Control

Written By: Todd Thomasson

Weeds are definitely some of the worst things to find in your lawn. They ruin your perfect lawn picture. And who wants a blemish or multiple blemishes in their gorgeous, green lawn?

Did you know that weeds aren’t only controlled after they emerge from the ground? There are actually multiple ways to tackle weeds, and one of the best ways is to do it before they burst forth from the soil. This is done using a pre-emergent weed killer, and it’s an essential tool in an effective and comprehensive weed management strategy. 

But pre-emergent weed control has some specific requirements to work best at keeping those weeds at bay. 

We bet you have some questions. We completely understand. 

Here are some of the most common pre-emergent weed killer questions and answers to ensure you have all of the information you need to keep your lawn weed-free.

About Pre-Emergent Weed Killer

Weeds are survivors. Sometimes their seeds will sit dormant for years before the right conditions enable them to erupt. This is one of the characteristics that makes weeds so annoying.

Weedy lawn

This is why when you’re developing a weed control strategy for your lawn, it’s best that you have all of the options available to you. Pre-emergent weed killer is one of those tools you don’t want to leave out of your arsenal. 

These 8 questions and answers will give you everything you need to know about this important weed fighting weapon.

1. What is Pre-Emergent Weed Control?

As its name suggests, pre-emergent weed killer for lawns is targeted toward weeds that have not yet emerged from the soil. 

lawn care team spreads fertilizer

When a weed starts growing after the pre-emergent herbicide application, it will hit the herbicide barrier and the roots won’t be able to continue growing.  

2. How Does Pre-emergent Weed Control Work?

Pre-emergent weed killers work by creating a barrier in the soil that prevents weed seeds from germinating.

lawn care expert pours fertilizer into spreader

Once a weed seed germinates, the initial or primary root emerges from the seed. At this point, if a pre-emergent weed control product has been applied, when the young root absorbs moisture from the soil, the pre-emergent will do its job.

3. When Should Pre-Emergent Weed Control Be Applied?

Timing is critical when one is using a pre-emergent weed killer. 

Ideally, you want to target your application before the soil temperatures reach a constant 55 to 58 degrees Fahrenheit. This is when the weeds will begin to germinate. 

lawn care expert applies weed control

The timing of when this ideal temperature occurs can vary from year to year and based on the sun exposure on your site. For instance, a hill with southern exposure to the sun will reach that temperature threshold much sooner than a lawn with northern exposure to the sun. 

Turf’s Up likes to get all of its pre-emergent herbicide applications in Northern Virginia done in March and early April to ensure the product is down before this temperature window arrives.

4. How to Apply Pre-emergent Weed Control

Pre-emergent weed killer for lawns provides a blanket of protection on your lawn to limit as many of these grassy weeds before they grow. 

Read The Ultimate Guide to Controlling Lawn Weeds
This means coverage is key. The product must be mixed correctly and applied at the right rate to cover your lawn thoroughly, providing this “blanket” of control.

5. What Weeds Does Pre-Emergent Weed Killer Target?

Pre-emergent weed killer targets grassy weeds best. 

Crabgrass with seedheads

The biggest menace to the Northern Virginia lawn is crabgrass, and that’s what pre-emergent weed control products work hardest to squash. If you have crabgrass problems, pre-emergent herbicide will help you alleviate them. Crabgrass germinates in the spring, so these weeds would be best targeted in March with pre-emergent. 

Annual bluegrass is another one that pre-emergent weed control works well to prevent. But since this weed germinates in the fall, you’d want to target it with pre-emergent herbicides in late summer/early fall.

6. How Long Does Pre-Emergent Weed Killer Last?

While every product reacts differently after it is applied based on different factors, such as weather conditions in Northern Virginia, every product also has an efficacy or expected length of control.

lawn care expert inspects grass

Turf’s Up likes to apply pre-emergent weed control in two split applications to lengthen the control and maximize the effectiveness of the product. This helps catch areas that unexpectedly get warmer faster, for instance. 

Since some weed seeds can sit dormant in the ground for years before the perfect conditions force them to emerge from the ground, we found this strategy works best to ensure we control the most weeds we can before they grow and ruin a great looking lawn.

7. Is Pre-Emergent Weed Killer Safe?

Pre-emergent weed killer is not harmful to your lawn, your pets, or anyone going onto your lawn after the application dries. 

family playing on healthy green grass

If you’re planting a new lawn, you shouldn’t apply pre-emergent weed control products to the area beforehand since it could impact new lawn seed growth. 

And once pre-emergent weed control products are applied in a liquid form, we usually recommend waiting about 45 minutes for the product to dry so that you don’t disturb it or limit its ability to do its job.

8. What Happens If I See Weeds After a Pre-Emergent Weed Killer Application?

While the goal is to eliminate as many weeds as possible with a pre-emergent weed killer, sometimes weeds find a way to sneak through into your lawn. If they were already above ground, for instance, but you didn’t see them, then it's likely the pre-emergent herbicide won’t get them in time and they’ll continue growing. 


Other factors can sometimes impact the product, too. An extremely wet spring, for example, can cause some weed breakthroughs. 

There are also some weeds pre-emergent herbicides don’t help, such as winter annuals. This is when post-emergent weed control products can be used to save the day and take care of any other weed troubles you’re having.

Get Ahead of Weeds in Northern Virginia  

Ready to tackle weeds before they take over your lawn? Then pre-emergent weed control could be a great part of your overall weed fighting strategy. 

Still feeling lost as to how pre-emergent herbicides work? We understand. It can be pretty confusing to target weeds at the right time to ensure they don’t make an appearance in your well-kept lawn. 

Turf’s Up can help you by providing a complete, proactive lawn care program that includes everything you need to keep your lawn strong – proper fertilization based on soil test results, pre-emergent weed killer and post-emergent weed control treatments, lime applications to get your pH in the right range, and aeration, overseeding, and topdressing services. 

When you have a professional eye on your property who can recognize common lawn weeds in Northern Virginia right away and know how to address them, you can keep weeds in check before they get out of hand. 

Ready to learn why Turf’s Up could be your totally awesome choice for lawn care services in Northern Virginia? We’re stoked to learn more about you and help you have the best lawn on the block. Get started today with a free quote. Together, we can prepare a customized plan that is perfect for you and your lawn. 
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