Applying Post-Emergent Herbicides to Your Lawn

Post-emergent herbicide

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They’re b-a-a-c-k!  The weeds.

Being a good steward of your yard, you’re determined to get rid of them. Before you head to the garden center, do a little homework.

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. Don’t be put off by the challenge.

“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.”

These are the terms you need to look for and understand if you want to kill those weeds.

Want the Best Post-Emergent Herbicides?

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.

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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 sprout.

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.

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 broad-leaf 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 applications 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.

Topical (Contact)  Vs. Systemic

The next terms you need to look for on the labels 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, Windbiel-Rojas recommends using systemic products when possible. “Systemics get into the whole plant,” she said.

Liquid Vs. Granular Herbicides

Herbicides 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 spray, and concentrate it onto a troublesome area.

Granular herbicides: These will require a spreader — also a relatively inexpensive piece of lawn equipment. With 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.

Weed, 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 safety 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 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.

Main image credit: Flickr

Lynn Walker

Lynn Walker

Lynn Walker has been writing for radio, TV and newspapers for more than 50 years, and has expertise in news, features, humor, history, weather, genealogy, science, archaeology and government.