How and When to Apply Post-Emergent Herbicides

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close-up of dandelions being sprayed with a weed killer herbicide

Weeds are a pain in the grass, and post-emergent herbicides are your pain relief. We’ll show you when and how to apply post-emergent herbicides to get rid of dandelions, crabgrass, and other common invaders of lush, green lawns. 

How post-emergent herbicides work 

Post-emergent herbicides are part two of a one-two weed knockout punch. Apply pre-emergent herbicides to nip weeds in the seed underground and then spray or spread post-emergent herbicides when weeds spring up in your yard.

There are four types of post-emergent herbicides, and each works in different ways.

  • Systemic post-emergent herbicides are absorbed directly into the plant to kill from the root up. 
  • Contact post-emergent herbicides leave a coating on the plant to prevent photosynthesis. 
  • Selective post-emergent herbicides target specific weeds and won’t destroy nearby grass or plants.
  • Non-selective post-emergent herbicides destroy everything in the area of application and should be used with discretion. 

Post-emergent herbicides come in two forms — granular and liquid.

When to apply post-emergent herbicides 

When should you apply a post-emergent herbicide? The short answer is whenever you see weeds in your yard. The longer answer? Time of year and time of day are important factors when applying post-emergent herbicides to ensure they will be most effective. 

Time of year

The best time of the year to apply post-emergent herbicides is in early to late spring, when weeds are small but actively growing, as they will more easily absorb weed killer.

Peak time in spring to apply post-emergent herbicide is when the daytime temperature is under 85 degrees, and the soil temperature is above 55 degrees for at least three days. 

You can apply post-emergent products until late summer, but once the outside temperature exceeds 85 degrees, it usually takes more than one application to kill the weeds. 

Pro Tip: Wait until your grass has recovered from drought or disease before applying a post-emergent, as stressed-out weeds can struggle to take in any weed killer.

Time of day 

The best time of day to apply your post-emergent is morning when weeds are growing at their fastest rate. Weed growth slows later in the day and evening, so it will take longer for post-emergents to be absorbed and effective. 

Check the weather: If a rainstorm is predicted the day you plan to kill weeds, either apply the herbicide early enough to dry before the rain shower or wait until the next dry day. 

Pro Tip: Avoid spraying post-emergent herbicides on windy days, especially if you’re using a non-selective treatment for two reasons: You can end up with an uneven application, and you may kill plants you were trying to avoid. 

How to apply post-emergent herbicides 

There are two methods to apply post-emergent herbicides: blanket or spot treatment. 

Which one is right for your yard? 

Blanket treatments are excellent for yards covered in weeds. 

Spot treatments are great for lawns with a few weeds and work well along sidewalks and driveways.

Blanket treatments 

What you’ll need depends on whether you choose a granular or liquid post-emergent: To apply a granular post-emergent, you will need a spreader (similar to a wheelbarrow). To apply a liquid post-emergent, you will need a sprayer tool. 

Blanket application is best used with selective post-emergents, otherwise, you can easily kill your entire lawn. Also, because you’ll be applying a post-emergent across a wide area, you’ll need more herbicide, which can be a pain as blanket treatments tend to cost more than spot treatments.

How to apply a blanket post-emergent: Spread or spray the post-emergent herbicide by going back and forth over the entire area. The goal? An even application of the post-emergent across your yard.

Water in post-emergents: Granular and some liquid post-emergent herbicides will need to be watered in after application. Follow the directions on the herbicide label as the time frames for watering differ greatly. For example, you should wait at least 30 minutes and up to 8 hours to water in various herbicides.

Pro Tip: Add a spray indicator dye to your post-emergent herbicide to ensure an even application across your lawn. 

Spot treatments 

Spot treatments of liquid post-emergents require a steady hand but can save you time and money over blanket treatments. Spot treatments usually involve selective herbicides. If you see dandelions in your yard, for example, apply a dandelion killer.

What you’ll need: A sprayer tool and most liquid herbicides come with one. You also will need to read and follow label directions to mix the weed killer with water accurately.

How to apply spot treatments: Spray your post-emergent herbicide right on the weed and move on to the next one. If you use a non-selective post-emergent, apply it only on the weed, or you also will kill surrounding grass and plants. 

Pro Tip: Wait until your grass has recovered from drought or disease before applying a post-emergent, as stressed-out grass and weeds can struggle to take in any weed killer.

When to mow your lawn after applying herbicide

After applying your post-emergent herbicide, wait at least three days before mowing your lawn. This allows the weeds to grow a bit and fully absorb the herbicide before being cut. 

Weeds killed by post-emergent herbicides

Some weeds homeowners can eliminate with post-emergent herbicides include:

Warm-Season Weeds Cool-Season Weeds 
Thistle Clovers 
CrabgrassBramble
Nutsedge Henbit 
SpurgeThistle 
Dandelion Chickweed 

FAQ about post-emergent herbicides 

1. Is there any safety gear I should wear when applying a post-emergent herbicide? 

When applying herbicides, always wear long pants and shirts, closed-toed shoes, gloves, and protective eye gear. Herbicides contain chemicals you don’t want to get on your skin or in your eyes.

2. Can I use “weed and feed” post-emergent herbicides?

Try to avoid “weed and feed” products that combine post-emergents and fertilizer. While weed-and-feed sounds like it will save time and money, proper timing of post-emergents may not coincide with the best time to fertilize your lawn. 

3. Does it matter how much the weeds have grown before I apply my herbicide? 

Once weeds exceed 4 inches,  they are extremely hard to kill, so apply a post-emergent as soon as you see weeds break through the soil. 

4. Do I have to use a lot of post-emergent herbicides?

Post-emergent herbicides tend to be much more potent and concentrated than pre-emergents. What this means: You usually use less post-emergent herbicide to kill weeds after they’ve sprouted than pre-emergents to kill weeds before they have popped up in your yard. 

When to hire a pro to apply weed killers 

It’s easy enough to spray a weed killer yourself to get rid of occasional weeds in your yard or the cracks in your driveway or sidewalk, but if your whole lawn is covered in clover, hiring a pro may be the way to go.

Here’s why: You likely don’t have a spreader or backpack sprayer in your tool shed, and mixing pre-emergents and some post-emergents precisely can be a challenge for the mathematically challenged. 

If you’d rather hire a pro to handle your weed control, Lawn Love can help. Our crews keep your yard free of weeds by applying pre-emergent and post-emergent herbicides. 

Main Photo Credit: Shutterstock

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