How and When to Apply Post-Emergent Herbicides 

Home is where the heart is, not the weeds. But sometimes homeowners end up sharing their space with these annoying plants. Keep reading to find out how and when to apply post-emergent herbicides to give your yard the makeover it deserves. No matter what kind of weed is growing in your yard, there’s a post-emergent herbicide to help you get rid of it. 

In this article:

• When to Apply Post-Emergent Herbicides
• How to Apply Post-Emergent Herbicides
• What Are Post-Emergent Herbicides?
• How Do Post-Emergent Herbicides Work?
• Selecting the Right Post-Emergent
• FAQ

When to Apply Post-Emergent Herbicides

The best time to apply post-emergent herbicides is when weeds are young and actively growing. This is when they are most vulnerable to the active ingredients.

Time of Year: Applying in the fall or late spring is best. It is easier to manage perennial weeds and several types of annual weeds during these times of year. Perennial weeds with deep root systems are best treated in the early spring. Homeowners should be aware that herbicides are most effective on weeds that are actively growing.

Temperature: Apply post-emergent weed control when the air temperature is less than 85 degrees Fahrenheit and the soil temperature is higher than 55 degrees. You can purchase a soil thermometer online or at your local garden supply store for less than $10 to check the soil temperature.

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When Not to Apply Post-Emergent Herbicides:

• If your lawn is in a drought-like state or very dry
• When the grass is in the green-up stage after winter dormancy
• Windy days, as the chemicals can be dispersed to other plants

Post-emergent herbicides before rain? Read the product label to determine how soon a treated area can be exposed to water. Some herbicides require water to be activated and kill the plant. On the contrary, rain or watering can render other herbicide products ineffective. Instead of activating ingredients, the water can wash them away.

How to Apply Post-Emergent Herbicides

Refrain from mowing the lawn about three days before and after using herbicides so weeds can grow enough to absorb the chemicals. Be sure to protect yourself, your family, and your pets. Wear the proper clothing and keep children and pets away from the treated area until it is dry.

Protective Apparel to Wear While Applying Herbicides:

• Goggles
• Gloves
• Closed-toe shoes
• Cover as much of your skin as possible. Wear long sleeves and pants. Use garments that cover your head and face, like a hat and mask.

Method 1: Spot Treatments

This method allows you to target weeds specifically, not your grass or surrounding plants. It’s a good option when there is not a large infestation and/or when you’re using non-selective herbicides that can kill plants other than weeds.

How to Prepare the Herbicide: Read the label and combine the post-emergent herbicide with water (and a surfactant, if needed) as instructed. A surfactant increases the effectiveness of herbicides by helping the chemicals absorb into the weeds better. If the herbicide does not have a built-in surfactant, the label will inform you that you must add it. 

How to Apply Spot Treatments: With your mixture, spray each weed individually and thoroughly. Do your best not to get herbicide on the surrounding grass and plants, especially if you’re using a non-selective formula. 

Method 2: Blanket Treatment

When you have many weeds that have taken over your yard, a blanket treatment will allow you to treat the area more efficiently than spot treatments.

How to Prepare the Herbicide: You can use a liquid post-emergent or granular post-emergent herbicide. Mix the liquid product with water as per the label’s instructions. Use a sprayer to apply. For granules, apply using a granular spreader.

How to Apply a Blanket Treatment: Go back and forth over the entire lawn with the sprayer or spreader, applying the herbicide evenly. For granules, it will take about half an inch of water for the herbicide to be activated, so be sure to water your lawn after application.

For any post-emergent treatments, reapply as per the product’s label to maintain weed control.

What Are Post-Emergent Herbicides?

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Post-emergent herbicides are used to treat weeds post-growth or after they are sprouting and growing.

If you get to the weeds before they sprout, a pre-emergent herbicide can be used to prevent the weed seeds from germinating in the first place.

Dithiopyr is a chemical that acts as both a pre and post-emergent herbicide, preventing weeds from germinating and killing them after they are actively growing.

How Do Post-Emergent Herbicides Work?

Unlike pre-emergent herbicides that target weed seeds, post-emergent herbicides attack plant parts, like stems, leaves, and the plant’s root system. Not all post-emergent weed killers work the same way. There are two types: contact-based and systemic.

Contact-Based Post-Emergent Herbicides: This type of herbicide kills only the plant parts it comes into contact with. It may take several applications. Contact-based herbicide applications typically only target the parts of plants above ground, like the weed’s foliage. It can kill the targeted parts of the plant quickly.

Systemic Post-Emergent Herbicides: Systemic herbicides kill weeds completely, even if the herbicide only comes into contact with part of the plant. The chemicals are absorbed and travel throughout the entire plant, attacking its vascular system. This method allows the active ingredients to reach and kill different parts of the plant, like the stems, leaves, and root system, killing the pesky weed. 

It’s also important to know that not all herbicides work on all weeds. There are both selective and non-selective post-emergent herbicides. 

Selective Post-Emergent Herbicides: This type of herbicide targets specific weeds without harming other plants. Selective products list on their labels whether they kill grassy weeds, like crabgrass and goosegrass, or broadleaf weeds, like chickweed and dandelions. You must accurately identify the weed in your yard to choose the right selective herbicide. 

Non-selective Post-Emergent Herbicides: These herbicides kill all plants they touch, so a sprayer is usually used to apply non-selective herbicides to give weeds a targeted spot treatment that affects the weeds and not other plants.

Selecting the Right Post-Emergent

Make sure you are correctly identifying the weeds in your yard. Certain weed killers are for specific weeds and may be ineffective on other types of weeds. The type of herbicide you choose should also be labeled for use on your turfgrass type. Otherwise, it may kill your grass along with the weeds.

Here are some common active ingredients in post-emergent herbicides, along with the weeds they kill and the grass types they’re safe for. 

Glyphosate (Roundup):

• Weeds it works on: Controls several types of perennial and annual weeds.
• Grass: Not safe to spray on grass. Will kill the lawn. 

Imazaquin (Image): 

• Weeds it works on: Labeled for use to control yellow nutsedge, annual sedges, and many annual winter weeds.
• Grass: Labeled for use on established Zoysiagrass, St. Augustinegrass, Bermudagrass, and centipedegrass.

Foramsulfuron (Revolver):

• Weeds it works on: Eliminates henbit and annual bluegrass.
• Grass: Can be used on Zoysiagrass and Bermudagrass.

Sulfentrazone (Dismiss):

• Weeds it works on: Controls broadleaf weeds and sedges.
• Grass: Labeled for use on the majority of major cool- and warm-season grasses. 

FAQ About Post-Emergent Herbicides

What is a “weed and feed” herbicide?

Weed and feed herbicides are pre-emergent and post-emergent products that also contain fertilizer. Several granular herbicides come with fertilizers. Make sure you are getting the weed and feed product that is best for your grass. 

But be aware of when it is best to apply the herbicide and when it is best to fertilize your grass. Because the two may not coincide, weed and feed products can be bad for your lawn.

Do pre-emergent herbicides kill weeds?

No, they do not kill weeds that have already sprouted. Pre-emergent herbicides prevent seeds from germinating.

What chemicals treat goosegrass?

Indaziflam provides pre-emergence control for goosegrass. Fenoxaprop is a post-emergence for goosegrass. The name-brand product Dismiss also inhibits the growth of goosegrass.

Are there any chemicals that can be used on cool and warm-season grass?

Prodiamine is a selective pre-emergence herbicide that can be used on most cool and warm-season grass types. Halosulfuron and sulfentrazone are post-emergent chemicals for use on most cool and warm-season grasses. 

Are there any pet-safe weed killers?

There are some DIY pet-safe weed killers. Some are non-toxic to humans and pets, like a salt solution or boiling water. A mixture of borax and water is considered a less harmful natural weed killer; borax is toxic in its concentrated form.

Final Thoughts 

The best way to treat lawn weeds is to prevent them before they occur with appropriate lawn care. This includes treating your lawn for diseases, providing the correct amounts of water and fertilizer at the recommended intervals, and mowing the lawn at advised heights. 

The next step is to catch weeds before you see them by treating them with a pre-emergent herbicide. But if you don’t have that kind of foresight and end up with weeds in your lawn, you can use post-emergent herbicides to kill them. 

Fortunately, you can treat weeds yourself using products from your local home improvement store or Amazon. But it can be a lot of work, especially if you end up having to hand-pull a yard full of weeds. If you’d rather leave your lawn care needs to professionals, we can connect you with lawn care pros in your area who can help with weed control and other lawn maintenance, such as mowing and fertilization. 

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