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How to Use a Pre-Emergent Herbicide in the Fall

Person Spraying Herbicide On Grass Weeds

The heat of summer will slowly come to an end and the cool temperatures of fall will finally be 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 during the fall is still important for preparing your lawn for winter and spring next year.

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.

What you’ll find in this article:

Skip ahead to pre-emergent products.

What is a fall pre-emergent herbicide?

What is the difference between a pre-emergent and a post emergent?

A pre-emergent herbicide, 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 as illustrated in the image below.

Photo Credit: Horizon Online.

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.

What weeds do fall pre-emergents control?

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, common chickweed, purple deadnettle and henbit.

If you aren’t sure of which weeds you usually encounter, our blog on Identifying Common Lawn Weeds goes into tips for identifying many different types of weeds.

When should pre-emergents be applied?

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. Poa annua, a notoriously hard-to-control weed, will begin germinating as early as August in some areas as well.

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 growing. Your common pre-emergents will kill the seed that you put down 100 percent of the time. 

If you aren’t sure how to determine your soil’s temperature, use a soil thermometer like the one listed below. Pre-emergents should be applied again in the fall to prevent fall and winter weeds.

What is the difference between a spring pre-emergent and a fall pre-emergent?

Pre-emergents should be applied at least twice a year. As previously mentioned, they should be applied in the spring before the ground temperature reaches 55 degrees Fahrenheit. Then it should be applied again in the early fall after temperatures reach 70 degrees and continue dropping. 

Not much changes besides the timing from spring and fall and the types of weeds you’ll prevent.

Which pre-emergent 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.

Read product labels thoroughly and follow application instructions.

How do I apply a pre-emergent herbicide in the fall?

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 granular vs. a liquid herbicide. Liquid herbicides are usually mixed with water in a tank and then sprayed. 

Although this takes more time and effort, you usually get a lot more product. For example, a 50-pound bag of granular Crabgrass Control Plus 0-0-7 covers 12,500 sq. ft. whereas a half gallon bottle of liquid Dimension covers 87,000–228,000 sq. ft. Refer to the product label for mixing instructions and follow all safety precautions when mixing/applying chemicals.

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.

Should I use a pre-emergent upon sod installation, seeding or plugging?

Can I use a pre-emergent on new sod?

You shouldn’t need to use a pre-emergent on newly laid sod. If you’ve installed the sod properly, there should be few weeds that survive. Those that do survive are easily pulled by hand or controlled with a post-emergent later.

Pre-emergents may also disrupt the new sod’s establishment.

Can I use a pre-emergent on new grass plugs?

It’s actually recommended to use a pre-emergent during plug installation projects. In fact, when you plug your lawn without pre-emergents, it takes longer for the yard to fill in. Nick Radford with Tod Valley Farms discusses this with our interview with him on pre-emergents here.

Can I use a pre-emergent on new seed?A pre-emergent herbicide that is applied before overseeding in the fall will keep the seed from growing. Common pre-emergents will kill the seed that you put down 100 percent of the time.

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 Lawnifi® 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?.

Overall, pre-emergents are a great way to prevent weeds from taking over during the fall and winter seasons. Poa annua, a hard-to-control weed is hardly ever permanently killed with post-emergent herbicides or by hand-pulling. The most effective way to control poa annua is with the use of a pre-emergent herbicide in the fall.

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