How to Use a Pre-Emergent Herbicide in the Fall

How to Use a Pre-Emergent Herbicide in the Fall

How to Use a Pre-Emergent Herbicide in the Fall

How to Use a Pre-Emergent Herbicide in the Fall

The heat of summer is slowly coming to an end and the cool temperatures of fall are finally among us. When it comes to the beginning of fall, a lot of people may think that this signifies the end of lawn care. Although time spent outside on your lawn may be greatly reduced, the time that takes place between summer and fall is still important for preparing your lawn for winter and next spring. Applying a pre-emergent herbicide to prevent weeds is one of the best things you can do for your lawn during this time for weed control in the cooler seasons.

Depending on your geographic location, certain annual weeds can continue to grow during the winter. Some of these weeds include Poa annua (also known as annual bluegrass), common chickweed, purple deadnettle and henbit. Annual weeds typically emerge in the fall and winter and continue to grow actively in spring. After they flower in spring and disappear during the summer, they often return again in the fall or winter when seeds germinate.

Pictured above from left to right: Poa annua grass, common chickweed, purple deadnettle and henbit.

What is a pre-emergent herbicide and what is its purpose?

A pre-emergent herbicides, as its name suggests, is a category of herbicides that prevents weeds from appearing in the first place. The difference between a pre-emergent and post-emergent herbicide is that a pre-emergent herbicide should be applied before a weed grows while a post-emergent herbicide should be applied if you already have weeds in your lawn. The idea behind a pre-emergent is to apply it before weeds have time to germinate from seed. This prevents them from growing during the cooler months and reduces any time or effort spent removing them.

Pre-emergents do not kill existing weeds. Instead, they prevent them from appearing in the first place. They work by forming a barrier in the soil that prevents weeds from sprouting or germinating as illustrated in the image below. It can be hard to get rid of weeds once they are already growing, so get ahead of the game by applying a pre-emergent herbicide in the fall and save yourself time and money—especially if you already know which weeds tend to come around in the winter every year.

How a Pre-Emergent Herbicide Works

Photo Credit: Horizon Online.

When should I apply a pre-emergent herbicide in the fall?

Although the window for pre-emergent applications varies, it’s best to apply pre-emergent herbicides twice a year, which will be during the spring and fall. In the spring, you’ll want to apply a pre-emergent herbicide before the ground temperature reaches 55 degrees Fahrenheit. But in the fall, pre-emergents are typically applied between the months of August and November depending on your location. Most of us apply them around the beginning of September. This applies to both warm and cool season grass types. Regardless of where you live, the application window for fall pre-emergents should be when temperatures are below 70 degrees and dropping.

It should be noted that a pre-emergent herbicide that is applied before you overseed in the fall will keep your seed from germinating. Your common pre-emergents will kill the seed that you put down 100 percent of the time. You can read more about overseeding in the fall in How to Keep Your Lawn Green During the Winter or by visiting our Grass Seed Planting Guide.

Which pre-emergent herbicide should I use?

When it comes to selecting a pre-emergent herbicide, choices can be daunting. Narrow down the selection process by looking for an herbicide that works on the type of weed you’ve seen in previous years and for your type of grass. Most herbicides will say on the back of the product which grasses they can and shouldn’t be used on as well as which weeds they will kill.

Once you have simplified your choices of herbicides, the last thing to choose between may be a granule vs. a liquid herbicide. Liquid herbicides are usually mixed with water in a tank and then sprayed. These should be mixed in accordance to the product label instructions. Granular, or dry herbicides, are small pellets coated with herbicides that can be applied with a broadcast or drop spreader at the rate specified on the product label.

Either choice of granular or liquid herbicides will perform well on your lawn. The most important thing to look for is that it kills the weeds you usually encounter during this time of year and that it works with your type of grass. A few of the best pre-emergent herbicides include Crabgrass Control Plus 0-0-7 with 0.37% Prodiamine, Tenacity Herbicide, Prodiamine 65 WDG and Dimension 2 EW. Be sure to read product labels thoroughly before application.

Product Comparisons

Crabgrass Control Plus 0-0-7 with 0.37% Prodiamine Herbicide

  • Coverage: 50 lbs. covers about 12,500 sq. ft.
  • Active Ingredient(s): Prodiamine 0.37%.
  • Ease of Use: Requires granular drop or broadcast spreader for application.
  • Best Used On/For: Established warm and cool season turfgrass before weeds appear.

Tenacity Pre- and Post-Emergent Herbicide

  • Coverage: Covers one to two acres when mixed properly.
  • Active Ingredient(s): Mesotrione 40%
  • Ease of Use: Requires tank mixing and application with sprayer.
  • Best Used On/For: Difficult to remove outdoor weeds.

Prodiamine 65 WDG Pre-Emergent


Coverage: 5 lbs. covers between 50,000–80,000 sq. ft.

Active Ingredient(s): Prodiamine 65%.

Ease of Use: Requires tank mixing and application with a sprayer.

Best Used On/For: Established turfgrass before weeds appear.

Dimension 2EW Pre-Emergent

Dimension 2EW Half Gallon

Coverage: 0.5 gallon covers between 87,000–228,000 sq. ft.

Active Ingredient(s): Dithiopyr 24%.

Ease of Use: Requires tank mixing and application with a sprayer.

Best Used On/For: Established turfgrass before broadleaf weeds appear.

Although fertilizer isn’t going to help kill your weeds, a good fertilizer program will help your lawn stay healthy and thick so that it chokes them out. Be sure to check out the LawnifiTM Fall Fertilizer Box and Lawnifi Foundation to help give your lawn the nutrition it needs this fall. Lawnifi is a fertilizer program powered by Catalyst TechnologyTM that matches seasonal changes with combined fertilizer boxes for spring, summer and fall. The Fall Fertilizer Box contains one bottle of Boost, Maintain and Recover, which work together to help your lawn get over the stressful heat of the summer as well as prepare for winter dormancy. To learn more about Lawnifi, visit Why Use Lawnifi Fertilizer?.

If you are interested in being able to recognize and name weeds, or to understand the difference between grassy and broadleaf weeds, read our Identifying Common Lawn Weeds blog.

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