Applying Post-Emergent Herbicides to Your Lawn

Post-emergent herbicide

They’re b-a-a-c-k!  The pesky weeds.

Being a good steward of your yard, you’re planning on applying post-emergent herbicides to your lawn. But where to start?

You’ll find a bewildering lineup of herbicides at the store, but if you’ve learned what kind of grass and what kind of weeds you have, weeding through the products will be easier.

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When to Apply Post-Emergents

According to the University of Florida, “It is best to apply herbicide to weeds that are still young because they will absorb chemicals more readily than their mature counterparts.” Post-emergents are chemicals that kill weeds after they’ve sprouted in your lawn.

First and foremost, follow the directions on the chosen herbicide. The exact timing to apply the product will vary depending on the climate and the weeds’ rate of growth. 

Follow these tips for the best time to apply post-emergent weed control:

  • Spray in good weather: Herbicides applied in rainy or windy conditions risk drifting to nearby desirable plants like flowers and grass.
  • Let moisture evaporate: Moisture dilutes the herbicide rendering it less effective. Wait until after the morning dew has evaporated for best results.
  • Don’t spray in hot weather: Spray when the temperature will be less than 85 degrees for the day. Spraying when the temperature is too high will reduce efficacy.
  • With the right pH level: Herbicides are most effective when the soil has the right pH level. Use a soil test kit to be sure your herbicide is ready to apply to your lawn.

How to Apply Post-Emergents

For safety and for optimal results, it’s crucial to follow manufacturer directions. How you apply will also depend on the type of herbicide.

  • Liquid: Usually mixed with water and applied using a handheld pressure sprayer
  • Granular: Typically mixed with a carrier like lime or fertilizer and spread using a fertilizer spreader.

Here are best practices for how to apply post-emergent herbicides:

  • Protect yourself: Use gloves, a face mask, long sleeve shirt, long pants, a hat, and closed shoes (rubber if you have them). Throw away disposable protective wear and wash reusable wear immediately after use.
  • Calculate herbicide: Use the manufacturer’s recommendations when calculating the amount needed for your lawn.
  • Calibrate: Whether using a spreader or a prayer, calibrate according to the product label.
  • Spay at two feet: When using a sprayer, the ideal spray height is two feet for maximum coverage and to reduce product drift.
  • Clean: Clean any equipment used, including the sprayer, spreader, and measuring tools. Be sure to clean yourself too/
  • Note the wait time: Most herbicides have recommended drying times before it’s safe for children and pets. Read closely.
  • Heed warnings: Some herbicides, like Roundup, have been under scrutiny lately due to long-term health concerns. This is one of the reasons it’s so critical to follow directions exactly and to use personal protective equipment.

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How to Select an Herbicide

Now that we understand when and how to apply post-emergent herbicide, how do you choose the one that’s best for your space? There are so many herbicide options available that it’s hard to choose. Read on to decide which herbicide is best for your lawn.

Post-Emergent vs. Pre-Emergent

Post-emergent: If your weeds have already sprouted, your first choice is easy: You need a post-emergent herbicide. It kills weeds that have already popped out of the ground to invade your turfgrass, as opposed to a pre-emergent that kills them before they begin germinating.

Weeds start to germinate after soil temperature reaches about 55 degrees. That means a post-emergent product is your choice from spring through fall.

Pre-emergent herbicide: They’re a preventer. They kill weeds as the seeds begin to sprout, come warmer temperatures.

This type of herbicide must be applied before lawn weed seeds germinate and weed growth begins. That means an application in the early spring before soil temperatures reach 55 degrees.

Spraying on plants
Photo Credit: Cade Martin / Pixnio / CC0

Selective vs. Nonselective

Post-emergent herbicides will also have one of two other terms on their labels — selective or nonselective.

Selective herbicide: These weed killers are formulated to take out specific types of weeds while leaving others alone. On their product labels, they will list what they kill — and the grasses that will tolerate them. 

Some types are designed to kill grassy weeds, others will take on broadleaf weeds. Often, selective herbicides will leave warm-season grasses alone but can harm cool-season grasses, or vice versa.

Nonselective herbicide: A nonselective herbicide can wipe out everything it touches. They’re best for spot treatments in places where you don’t want any vegetation, such as the cracks in your sidewalk or driveway, or along a fence. Nonselective herbicides kill weeds, but can also easily take out desirable plants like grass and flowers.

Topical (Contact) vs. Systemic

The next terms to look for are topical and systemic.

Topical herbicides: These contact herbicides kill only that part of the weed that the herbicide touches — the part above the ground. They work best on small, annual weeds.

Systemic herbicides: They pass through the plant and control weeds by taking the entire plant out, roots and all. They’re effective on perennial weeds.

While some topical post-emergents may show ground-level results within hours, Karey Windbiel-Rojas, a turf specialist with the University of California,  recommends using systemic products when possible. “Systemics get into the whole plant,” she said.

Liquid vs. Granular Herbicides

Herbicide products come in liquid and granular forms. Either can be effective, as long as the right amount reaches the right plant.

Liquid herbicides: These may require mixing with water and you need to have some type of sprayer. Simple pump or hose-end sprayers are inexpensive. You can also easily control the spray and concentrate it onto a troublesome area. 

Some liquid herbicides recommend mixing the solution with a surfactant (a chemical that helps the herbicide stick to the plant).

Granular herbicides: Granular post-emergents will require a spreader — also a relatively inexpensive piece of lawn equipment. Most granules will need a good watering in after application.

Dr. Rebecca Grubbs, a turf specialist at Texas A&M University,  said it’s up to the user to decide which is best for them. “Just make sure you follow the instructions on the label,” Grubbs advised.

Best Post-Emergent Herbicide Products

“The earlier you spot, identify, and treat your weeds, the greater chance you have of controlling them,” says Windbiel-Rojas.

“Herbicides can reduce the infestation of weeds such as crabgrass, dandelions, and Johnson grass if applied when the plants are young, but once they mature, it will be harder, if not impossible.”

In a rush and just want to know what post-emergent is best?  Here are our 5 picks for the best post-emergent products on the market.

Weed and Feed Separately

Turf experts sound an alarm about one popular kind of product you’ll see prominently displayed — the “weed and feed” concoctions that combine herbicide and fertilizer.

“While they’re convenient, they’re not really recommended by weed scientists,” Windbiel-Rojas said.

Grubbs agrees. “The proper timing of fertilizer application doesn’t always coincide with that of herbicide application,” she said.

Also be aware that products containing glyphosate, such as the top-selling Roundup brand, have been mired in controversy and lawsuits because of potential cancer risks

While the Environmental Protection Agency in 2019 affirmed the chemical is “not likely to be carcinogenic to humans,” a long list of local and international agencies have decided otherwise, and have restricted or banned its use.

Use it, and any herbicide, strictly according to the instructions on the label.

Temper Your Expectations

Once you’ve picked your herbicide and applied it, don’t expect miracles. You might not see results right away.

“It might take days or weeks,” says Windbiel-Rojas, who notes that over time weeds develop a resistance to chemicals. “The idea that weed control can be quick and easy is not reality.”

Grubbs agrees. “Be realistic and reasonable” in your expectations,” she says.

Both experts agree the best defense against weeds is to keep your yard healthy through proper watering, fertilizing, mowing, and aeration. Healthy, dense yards will do a lot of the work in warding off weeds.

Herbicides can help, but their success depends on you following a wise regimen of lawn care. Patience and persistence will pay off. 

For more information on how to achieve a healthy, dense lawn, check out our other lawn care articles: 

FAQ About Post-Emergent Herbicides

Can I Use One Post-Emergent for all the Weeds in My Lawn?

It’s not recommended. Most post-emergent herbicides are selective, targeting specific weeds, which is good for your lawn. Selective herbicides only target the weeds that they are formulated to eliminate. Read the product labels carefully to ensure the weeds you want gone are covered.

If you’re thinking, what about Roundup? It’s not recommended to use non-selective herbicides on your lawn because they will kill your grass, introducing a whole new problem to your lawn.

Should I Mow my Lawn Before Applying Herbicide?

No. Mowing right before or right after applying herbicide will decrease effectiveness. 

Mowing temporarily puts weeds into a state of dormancy, and therefore, will not be able to soak in the herbicide well. Mowing right after will fling the herbicide around, not giving the herbicide enough time to be effective as well as potentially spreading the herbicides to desired plants.

Can I Use Both Pre-Emergent and Post-Emergent Herbicides?

Yes, you can use both pre- and post-emergent herbicides.

Pre-emergents are designed to prevent weed growth and should be applied in early spring (varies by climate area) and fall, if needed. Post-emergents are designed to kill weeds in their seedling stage (young plant) and should be applied in late spring (varies by climate area).

Keep in mind that you may need to reapply pre-or post-emergent herbicides to tackle winter weeds like chickweed. Be sure to follow manufacturer directions carefully to ensure safety and the best results.

Herbicide Help

What’s your favorite aspect of having a lawn? Whether it’s backyard barbecues or lounging peacefully, everyone can agree that having a lush lawn is glorious. Weeds ruin turf tranquility and quickly become the bane of lawn care for many homeowners. Dealing with weeds can seem tedious and never-ending.

But you don’t have to be rooted by weeds, and you don’t have to handle them alone. There are local, experienced, highly rated lawn care professionals that will take care of your lawn, letting you enjoy your space without the hassle and at a reasonable price. A free and easy quote is just a call or click away.

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Main image credit: Mike Mozart / Flickr / CC BY 2.0

Nicki DeStasi

Nicki DeStasi

Nicki DeStasi is a writer, author, and teacher who grew up in western Massachusetts and currently resides in the Austin area. She enjoys flower and vegetable gardening, reading, cooking, listening to true-crime podcasts, and spending time with her husband, two children, dog, and cat.