When to Apply Pre-Emergent Herbicide

Pre-emergent herbicide is one of the best ways to fight weeds. When should you apply it?

When it comes to proper lawn care practices, preventative maintenance is the best way to ensure that your lawn remains healthy and lush. In almost every case, if you can solve a problem before it becomes a problem, you’re not just winning the battle. You’re also winning the war.

The same can be said for the numerous broadleaf weeds that always show up uninvited to your healthy lawn’s party. Unfortunately, these opportunistic weeds will never be completely eradicated, and they will find a way to establish themselves on your lawn in some way. Even the healthiest lawn on the planet is not immune to their presence. However, if you use the proper preventative measures, you can keep their unwanted party crashing to a minimum.

A healthy application of a quality pre-emergent herbicide is the best preventative maintenance you can give these invasive plants. Herbicides that are applied before seeds germinate are known as pre-emergent herbicides. These herbicides will keep weeds at bay as long as you apply them at the right time. This type of herbicide works in one of three ways to keep weeds at bay:

  1. Inhibiting the growth of the weed’s roots
  2. Inhibiting the ability of the weed’s cells to divide
  3. Inhibiting the presence of certain enzymes essential to the growth of the weeds

Nobody wants to have an unsightly weed invasion on their lawn. To avoid succumbing to their onslaught, read this article from beginning to end. You’ll learn everything you need to know about applying pre-emergent herbicides to your lawn and beating those pesky foreign invaders to it.

Overgrown grass in the yard with growing flowers

When is the Ideal Time to Apply?

The timing of a pre-emergent killer application is critical to the herbicide’s overall effectiveness. Pre-emergent herbicides are most effective in the early spring and late fall. If you apply during these times, you will prevent the majority of seeds from germinating, as weeds try to take root at the greatest rate during these times.

Although you can apply pre-emergent all year, focusing your efforts in early spring and early fall will prevent the most weed growth.

Should You Apply When the Lawn Is Dormant?

If you apply these herbicides in the fall as recommended, you should not need to apply them during the winter months when the lawn is dormant. In fact, if you apply them in September, October, or November, the herbicide should keep the weeds at bay until spring.

Weed seeds that have not germinated have likely gone dormant when your grass goes dormant. They are waiting for the temperatures to rise sufficiently to allow the fragile, newly germinated seeds to grow. A winter dormancy herbicide application won’t hurt, but it won’t help because the weed seeds aren’t attempting to germinate.

Should You Apply Before or After You Mow Your Lawn?

Applying pre-emergents after mowing your lawn is the best practice. Watering in these herbicides is required to activate them. Mowing your lawn before applying herbicide will benefit you because it will allow the product to reach the soil more easily. If you don’t mow before applying weed killer, the herbicide may become entangled in the taller grass blades. You can’t use a pre-emergent if you’re stuck on the grass blades.

The package contains information that will tell you how long you have to water it in before it becomes ineffective. The duration can range from a week to two months.

However, just because you can wait up to two months does not mean you should. Water in your herbicide as soon as possible to get the most out of it. Watering it in immediately after application activates it while it is at its most potent and effective.

Wait at least three days after applying the herbicide to your lawn before cutting it. Mowing your lawn too soon after applying the herbicide can reduce its effectiveness. While the lawnmower is still trying to do its job, you will remove the product.

A lawnmower mowing the overgrown grass in the yard

Should You Apply Before or After Fertilizing Your Lawn?

The order in which you apply your pre-emergent weed killer and fertilizer is critical to the overall health and productivity of your lawn. Apply your pre-emergent herbicide before fertilizing to prevent invasive plants from growing.

Weeds that appear on your lawn will compete for water and nutrients with your desirable grass. So, getting rid of as many weeds as possible before fertilizing your lawn will result in less unwanted competition for water and nutrients.

Allow enough time for your weed killer to do its job before fertilizing your lawn. Wait one week after applying your pre-emergent before applying fertilizer.

Should You Apply Before or After Seeding Your Lawn?

Pre-emergent herbicides can interfere with the germination and growth of your preferred grass seed. As a result, the herbicide must not come into contact with newly seeded lawns. It is entirely up to you whether you seed before or after applying your herbicide, but there are different waiting periods for each scenario.

If you apply a pre-emergent before sowing your grass seed, wait three months before laying down new grass seed. Three months is enough time for the herbicide to kill the weed seeds and break them down in the soil.

If you intend to seed your lawn before applying herbicide, allow enough time for the seed to germinate, take root, and establish itself. Between 6 and 8 weeks, your new seed should be established in the soil.

The “tug test” is the most effective way to determine whether your new grass is established. This is accomplished by grasping a tuft of fresh grass and pulling on it. If the grass offers little to no resistance and easily pulls from the soil, you should give it more time. If, on the other hand, it is firm and requires some effort to come out of the ground, your grass is well-established enough to withstand a pre-emergent herbicide application.

A shovel full of seeds to be planted in the garden

Guide to Applying Your Pre-Emergent Herbicide

Available Forms of Pre-Emergents

You need to determine which type of herbicide that you’ll be applying to your lawn:

  • Granular
  • Spray Solution

Other than the physical form, there is no discernible difference between the two products. Spray solutions require slightly less effort because they are watered in automatically. To activate the weed killer in granular products, you must water them in after application.

The only real advantage of using granular fertilizer is if you need to wait for the herbicide to take effect for some reason. These products can remain inert on the soil until you decide to water them in. So, if you need to delay activating your fertilizer for any reason, go with the granular product.

Cover Your Entire Lawn

These items are not intended to be used as spot treatments. Instead, spread them evenly across your entire lawn.

Weed seeds can travel long distances and end up in places you wouldn’t expect. Covering the entire lawn evenly gives you the best chance of preventing weeds from growing among your desirable grass and other plants.

A gardener manually spraying or watering the grass

Check the Soil’s Temperature

The temperature of the soil is another factor to consider when using a pre-emergent product. You may hear some people say that the soil temperature should reach 60°F, while others believe it should reach 70°F.

Both of these temperatures are incorrect. The soil temperature must be at least 55°F for five consecutive days for a pre-emergent to be effective. If you wait until the soil temperature reaches a consistent 70°F, the conditions for seeds to germinate and grow may occur while you wait for the soil to warm up. Those seeds could be growing by the time you reach your desired temperature, and you’ll have missed your window of opportunity.

Wait One Year to Re-Sod

If you used a pre-emergent on your lawn, you must wait one year before laying down new sod. The presence of the weed killer on the soil can interfere with how well the sod takes root and establishes itself. If you intend to use sod, lay it down before applying the weed killers and allow it to establish itself. You can apply your herbicide to the new turf once the sod has established roots in the soil.

A man laying grass turf in for garden landscaping

Calibrate Your Spreader or Sprayer

Calibrate your spreader or sprayer to ensure even, consistent coverage of your herbicide. Taking the time to do this maintenance will make your life much easier because even coverage allows your herbicide to work as effectively as possible. Some areas will go untreated if your herbicide is applied unevenly. Weeds have a better chance of germinating and taking root when there is less coverage.

Reasons Your Pre-Emergent Doesn’t Work

  • Drought: These weed killers form a barrier that weed seeds cannot penetrate in order to take root and grow. Drought can cause fissures and cracks in the soil, allowing weeds to pass through. Weeds are tough and don’t need much water to grow. If a seed can get into a crack in the soil, it will grow and work its way up through that crack.
  • Uneven Application: As previously stated, uneven coverage results in herbicide-free spots on your lawn. Areas that do not receive pre-emergent treatment will become weedy, necessitating the application of a post-emergent.
  • Disturbed Soil:  If you dig up the soil or otherwise cultivate it for any reason, you are removing the layer of protection provided by your pre-emergent. Weeds will take advantage of any gaps in the barrier.
  • Heavy Rainfall:  Before applying herbicide to your lawn, check the weather. If rain is expected in the coming days, postpone the application until dry weather is predicted. Heavy rain will cause the product to degrade and become inert sooner than expected. This allows late-season weeds to emerge.
  • Poor Timing:  This is the most common error people make. They miss their window of opportunity. When the product is applied too soon, it is broken down before the seeds can germinate. When it comes time for them to germinate, the weed killer is rendered ineffective. When the pre-emergent is applied after the weed has emerged, it becomes ineffective.
Jeffrey Douglas
Jeffrey Douglas own a landscaping company and has been in the business for over 20 years. He loves all things related to lawns or gardens and believes that proper maintenance is the key to preventing problems in the first place.
More ArticlesDiseases and Pests