Applying Post-Emergent Herbicides to Your Lawn

Post-emergent herbicide

They’re b-a-a-c-k! Those pesky weeds. Now that they’re making a mess of your lawn, it’s time to apply a post-emergent weed killer. But to have the greatest success — while protecting yourself, your family, and the environment from any potential dangers — it’s critical to know everything you can about applying post-emergent herbicide to your lawn.

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What is a Post-Emergent Herbicide

A post-emergent herbicide controls weeds in lawns and gardens after germinating and emerging through the soil. They are the second prong in a fundamental two-pronged approach to weed control.

When to Apply Post-Emergents to Your Lawn

spraying weed killer herbicide
Photo Credit: JJ Gouin / Canva Pro / License

Post-emergent weed killers can be used in your lawn anytime during the growing season. As mentioned, the only requirement is that the weed be above ground so the product touches the foliage.

That being said, there are better times to apply a post-emergent product than others.

For example, The University of Florida says, “It is best to apply herbicide to weeds that are still young because they will absorb chemicals more readily than their mature counterparts.” 

“The earlier you spot, identify, and treat your weeds, the greater chance you have of controlling them,” says Karey Windbiel-Rojas, a turf specialist with the University of California.

“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.”

Realistically, you want to follow these weed spraying tips to determine the best time to apply post-emergent weed control:

  • Always spray weed killers in calm weather. If you apply herbicides in windy conditions, you risk the product drifting to nearby desirable plants like flowers and grass.
  • Let the moisture evaporate from the lawn. For the best results, wait until after the morning dew has evaporated. Moisture dilutes the herbicide and keeps it from sticking to the plant, rendering it less effective.
  • Don’t spray in hot weather. Applying an herbicide when the temperature is too high will reduce efficacy. Ideally, you want to spray weeds when it’s below 85 degrees Fahrenheit for the day. 
  • Apply herbicides when the soil pH is in the recommended range. Herbicides are most effective when the soil is at the right pH level. Use a soil test kit to check the soil pH and make necessary pH adjustments before attacking weeds. 

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How to Select an Herbicide

There are many types of post-emergent herbicides available. How do you choose the best option for your space and the weeds you’re fighting? First, it’s essential to understand the different types of products and identify the weeds you’re fighting. Then, you can decide which herbicide is best for your lawn.

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.

Post-Emergent vs. Pre-Emergent Herbicides

Obviously, this article is geared toward post-emergent herbicides, but let’s briefly touch on the differences between them and pre-emergent products.

Post-Emergent Herbicides

If your weeds have already sprouted, your first choice is easy — you need a post-emergent herbicide. These products kill weeds that have already popped out of the ground to invade your turfgrass instead of a pre-emergent that kills them before they emerge through the soil.

Weed seeds start to germinate once soil temperatures hit about 55 degrees Fahrenheit. That means a post-emergent product is usable from spring through fall.

Pre-Emergent Herbicides

Pre-emergent products are preventers. They kill weeds as the seeds begin to sprout and keep the seedlings from pushing through or emerging from the soil. 

These herbicides must be applied before lawn weed seeds germinate and weed growth begins. That means they’re typically applied in the early spring before soil temperatures reach 55 degrees Fahrenheit.

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

Selective vs. Nonselective Herbicides

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

Selective Herbicides

Selective weed killers are formulated so their active ingredients take out specific types of weeds while leaving others alone. They list target weeds on their product labels and the grasses (and other plants) that will tolerate them without any adverse effects. 

Some types are designed to kill grassy weeds like crabgrass, quackgrass, and nutsedge; others will take on broadleaf weeds like dandelions, spurge, and henbit. 

Often, selective herbicides will leave warm-season grasses (Bermudagrass, Zoysia, St. Augustine) alone but can harm cool-season grasses (Kentucky bluegrass, tall fescue, perennial ryegrass) or vice versa, making it essential you know your grass type.

For information on the different types of turfgrasses, check out these helpful articles:

Nonselective Herbicides

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

Topical (Contact) vs. Systemic Herbicides

The next terms to look for are topical and systemic. They relate to how the herbicides work in the plant.

Topical Herbicides

Contact herbicides kill only the part of the weed that the herbicide touches — the part above the ground it comes in contact with. Once they kill the foliage, the weed won’t be able to photosynthesize and should die. They work best on small, annual weeds.

Systemic Herbicides

Systemic herbicides are absorbed through the plant’s tissue and move through the plant’s vascular system from the top down to the root system. They pass through the plant and control weeds by taking the entire plant out, roots and all. A systemic post-emergent is 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.

Synthetic (Inorganic) vs. Organic Herbicides

Like other lawn care chemicals, post-emergent herbicides are classified as inorganic or organic based on the ingredients or compounds they contain. Both types have their advantages and disadvantages.

Inorganic Herbicides

Inorganic herbicides contain synthetic products created in a lab or manufacturing facility. These products are not found naturally. Synthetic weed killers tend to be very effective but can pose many human and environmental risks.

Organic Herbicides

Organic herbicides, on the other hand, are composed of naturally occurring ingredients. They tend to have fewer potentially adverse effects but are not as effective. Natural weed killers may require numerous applications and take much longer to see results.

Liquid vs. Granular Herbicides

Herbicide products come in liquid and granular forms. Either type can be effective if the correct amount is applied to the right plant.

Liquid Herbicides

Liquid herbicides come in a concentrated form. Before application, they must be mixed with water, which means you must have a sprayer. You also can easily control and direct the spray onto a troublesome area. 

Some liquid weed killers come in a hose-end sprayer that attaches to the end of your garden hose; when you turn the water on, it automatically pulls the product out of the bottle, diluting it as it sprays. Or you can purchase a handheld pump sprayer; they’re inexpensive and readily available. Simple pump or hose-end sprayers are inexpensive. 

To improve efficacy, 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 come in a dry pelleted form, making them incredibly easy to work with and apply. You will need a fertilizer spreader, either a handheld or a push-behind broadcast or drop spreader. In most cases, they are a relatively inexpensive piece of lawn equipment. 

Pro Tip: Your fertilizer spreader can pull double duty! I use mine for fertilizer, weed killer, and grass seed. Just make sure to clean it thoroughly after you’re done using it so it doesn’t have any lingering granules that may affect your next application.

Best Post-Emergent Herbicide Products

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

How to Apply Post-Emergent Herbicides

For safety and optimal results, it’s crucial to follow manufacturer directions. Always keep in mind that how you apply the product also depends on the type of herbicide you’re using.

  • Liquid herbicides must be mixed with water and applied using a handheld sprayer.
  • Granular herbicides are applied using a fertilizer spreader.

Here are some best practices to follow when applying post-emergent herbicides:

  • Protect yourself: Wear gloves, a face mask, a long-sleeved shirt, long pants, a hat, and closed-toed shoes (rubber if you have them). These items protect your skin, eyes, and lungs from exposure and potential irritation. Throw away disposable protective wear and wash reusable wear immediately after use.
  • Calculate herbicide amount to apply: Use the manufacturer’s recommendations when calculating the amount needed for your lawn. Do not apply more than the label recommends!
  • Calibrate: Whether using a spreader or a sprayer, calibrate the application rate according to the product label.
  • Spay at 2 feet: When using a sprayer, the ideal height is 2 feet from the plant(s) for maximum coverage and reducing product drift.
  • Clean up after: Clean any equipment used, including the sprayer, spreader, and measuring tools. Be sure to clean yourself, too, including washing your hands well or even taking a shower. 
  • Note the wait time: Most herbicides have recommended drying times before they’re safe for children and pets to reenter the application area. Read closely and follow the directions carefully.
  • Heed product warnings: These products are chemicals and do have potential risks. Some herbicides, like Roundup, have been scrutinized lately due to long-term health concerns. It’s critical to follow directions strictly and to use personal protective equipment.

Considerations When Applying Post-Emergent Herbicides

Gardener horticulturalist spraying weed killer herbicide on lawn
Photo Credit: Henfaes / Canva Pro / License

Weed and Feed Separately

Turf experts sound an alarm about one popular product prominently displayed — the “weed and feed” concoctions that combine herbicide and fertilizer. These products are designed to cut down the time and effort required for lawn care, but they have shortcomings and should be avoided.

“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.

Be Mindful of Glyphosate

You also should 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 affirmed in 2020 that 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.

If you choose to use it, use it and any other herbicide strictly according to the label instructions.

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, thick lawns will do a lot of the work in warding off weeds.

Herbicides can help, but their success depends on you following a wise lawn care regimen. 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. Many post-emergent herbicides are selective, targeting specific weeds and leaving the grass alone, which is good for your lawn. But it doesn’t mean they target all weeds! Read the product labels carefully to ensure the weeds you want gone are covered.

If you’re thinking, what about Roundup? Does it kill everything? Using non-selective herbicides on your lawn is not recommended because they will kill your grass, introducing a whole new problem to your yard.

Should I Mow My Lawn Before Applying Herbicide?

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

Mowing temporarily stresses weeds, so they don’t absorb the herbicide well. Mowing right after will fling the herbicide around, not giving it enough time to be effective and potentially spreading the herbicides to nearby 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 seedlings from emerging through the soil 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) after emergence.

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

Need Post-Emergent 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, however, are a pain in the grass. They ruin turf tranquility and quickly become the bane of lawn care for many homeowners. Dealing with them 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! Reach out to LawnStarter, and we’ll put you in touch with local, experienced, highly-rated lawn care professionals who 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

Amanda Shiffler

Amanda Shiffler

Most comfortable with soil under her fingernails, Amanda has an enthusiasm for gardening, agriculture, and all things plant-related. With a master's degree in agriculture and more than a decade of experience gardening and tending to her lawn, she combines her plant knowledge and knack for writing to share what she knows and loves.