A comprehensive guide to lawn weed killer

Key points

• Lawn weed killer can be preemergent, postemergent, selective, nonselective or systemic.

• Misapplication of weed killer can damage your grass and other desirable plants.

• The best time to apply weed killer depends on the season, weed type and average temperatures.

While a nuisance, weeds in your yard or flower beds can provide valuable insight into your lawn’s health. They can reveal underlying problems such as soil compaction, poor drainage, nutrient deficiencies or even inadequate sunlight. 

But when weeds begin to overstay their welcome, consider a lawn weed killer. Finding the best weed killer starts with identifying and targeting the specific weeds growing in your lawn or garden. Common lawn weeds such as dandelions, clover and Oxalis are just some of the varieties you may encounter.

We spoke with Kody J. Ketterline, founder of K-IT Products, and Marty Teresi, grass expert and host of The Grass Station Podcast, to learn more about how to find the right lawn weed killer for a lush, green lawn sooner rather than later (and without wasting resources).

What are the best lawn weed killers?

Weed killers, also known as herbicides, are designed to eliminate and control weeds in lawns, gardens and agricultural areas. While there are multiple ways to attack weeds, many people prefer weed killers because of their efficiency, precision and long-term control. If you’ve decided to treat your yard with a weed killer, you have several types to choose from.

Residual weed killer

Also known as preemergent herbicides, residual weed killers create a barrier in the soil that stops weed seeds from germinating and sprouting. One example is Prodiamine, which tackles a wide range of annual grassy weeds, such as crabgrass, annual bluegrass and foxtail.

While residual weed killer provides long-lasting weed control, if applied incorrectly, it can also prevent desired plants from germinating.

Contact weed killer

Contact weed killers, also known as postemergent herbicides, are designed to kill weeds on contact. They are fast-acting and ideal for actively growing weeds, providing visible results nearly overnight.

These herbicides are generally nonselective, so you’ll want to apply them cautiously to avoid damaging healthy grass or other desirable plants. It’s great for driveways and other isolated areas.

One example of a contact weed killer is glyphosate, commonly known as Roundup. It’s effective against broadleaf weeds such as dandelions, plantain, chickweed and thistle. It’s also ideal for annual bluegrass and perennial ryegrass.

Systemic weed killer

Systemic weed killers work as their name suggests. The plant absorbs the weed killer, which circulates throughout its system, killing it from the inside out, including its roots. These products are effective against annuals, perennials and woody plants such as poison ivy and brambles. 

Systemic weed killers provide long-lasting control while minimizing the chance of regrowth. However, they take time to work and may also affect nearby plants.

Selective weed killer

Selective weed killers target specific types of weeds while sparing other plants, including grasses. These specialized herbicides offer a tailored approach to weed control. 

Accurately identifying the type of weed you’re dealing with is crucial for choosing the right weed killer. Try using a weed identification app or this online weed list with images.

Nonselective weed killer

Nonselective weed killers don’t play favorites; they target everything they come into contact with. While nonselective weed killers are helpful in clearing areas of unwanted vegetation for other purposes, they are rarely suitable for use in lawns, where they’ll kill healthy grass.

Thick, healthy grass

“The best way to attack weeds is with a lush lawn,” said Teresi. Thick grass can help keep weed seeds from germinating, stopping them before they even start. 

While herbicides can help you treat a weed-infested yard and get it back to its radiant self, the best way to treat weeds is to prevent them in the first place with a healthy lawn. Regular maintenance practices, such as proper watering and mowing, can make all the difference.

Will weed killer affect your grass?

Weed killers can affect your grass, but you have more control over the outcome than you think. First, let’s discuss why weed killers can negatively affect your lawn’s appearance.

• Misapplication: If you use a sprayer to apply your weed killer during windy conditions or don’t follow the application rate instructions on the product label, you risk overspray or drift, which can damage desirable plants, including grass.
• Nonselective herbicides: You can choose between selective and nonselective herbicides. Selective weed killers target one or two types of weeds and leave other plants, including grass, unharmed. However, a nonselective herbicide will affect every type of foliage it comes into contact with.
• Residual activity: Some weed killers remain active in soil for weeks or even months. This can make it nearly impossible to grow new grass or other vegetation in an area treated with a nonselective preemergent, which acts as both a weed and grass killer.

It’s also worth noting that certain types of grass, including fine fescue and centipede grass, are more susceptible to herbicide damage than others. But if you follow the label instructions and focus on spot-treating weeds rather than broadcasting herbicide over your entire lawn, you’re more likely to end up weed-free than grass-free.

When is the best time to use a weed killer?

The best time to use weed killer is before you spot a weed. Otherwise, it depends on what’s growing in your lawn and the time of year. Among several types of ready-to-spray weed killers, two main categories are key: 

• Preemergent weed killer: Apply this before weed seeds germinate to reduce the need for postemergent treatments. However, you must use it at the correct time — which we’ll explain below — for it to be effective.
• Postemergent weed killer: Apply this to actively growing weeds to kill them quickly. You may need to apply it multiple times to complete weed control. If your applications are more frequent or heavier than recommended, you run the risk of damaging desirable plants.

Beyond the category, you must also consider the weed type you’re attacking. If you’re tackling warm-season weeds, such as crabgrass, goosegrass and spurge, you’ll want to use your preemergent in the early spring. A late-summer or early-fall application is ideal for winter weeds, such as henbit and chickweed.

When to apply preemergent herbicides

Using a preemergent in early spring to control summer weeds can prevent them from germinating and becoming established. However, there are a few considerations to keep in mind, especially in southern regions where temperatures may rise earlier than in other areas.

• Temperature: Apply preemergent herbicides when soil temperatures consistently reach 55–60 degrees Fahrenheit. If you don’t have a soil thermometer, using a soil temperature map can help.
• Timing: Temperatures can fluctuate during early spring, making timing difficult. Since preemergent forms a barrier above the soil, it won’t be effective if weeds are already sprouting.
• Rainfall: Preemergent isn’t rainproof. Excessive rainfall immediately following an application can wash it away before it can form a barrier.
• Soil moisture: Soil should be moist but not waterlogged when applying preemergent granules.
• Label instructions: Finally, follow any brand-specific instructions on the label, including proper calibration and settings for your spreader, to ensure your preemergent’s effectiveness and efficiency.

According to Ketterling, for optimal weed prevention, preemergent application should occur when outdoor temperatures range between 50 and 55 degrees Fahrenheit. “We find that mid-March is usually a good time to apply the preemergent to the soils and the turf,” he said. 

Additionally, Ketterling advised against treating newly seeded areas, as preemergent can inhibit seed germination if the seeds have not yet sprouted. When figuring out when to plant grass seed, consider your weed-killer application schedule as well.

If you missed the preemergent window or are struggling with weeds later in the season, you always have other options. Don’t forget about postemergent weed killer, which is designed to control and eradicate actively growing weeds. 

Apply postemergent herbicide when weeds are actively growing but before they mature and produce seeds. Choose a calm, dry day without rain in the forecast. Weeds can be stubborn, so be prepared to make follow-up applications as needed.

What’s next?

While applying weed killer may seem like the last step in eliminating weeds from your yard, you’re actually just beginning. Keeping your yard weed-free is an ongoing process, not a one-time event.

Once you’ve applied your weed killer, wait for it to absorb. Depending on your chosen product type and your local weather conditions, this can take a few days or up to a week. Next, monitor the treated area. If the active ingredients in your weed killer are working, you’ll notice wilting, yellowing or dieback. Be patient, as signs don’t always appear overnight.

If your weed killer is successful, consider fertilizing and overseeding your lawn to fill in any bare or thin areas. But if your weeds are still strong, you may want to seek the help of a lawn care company to help you gain ground (or, in this scenario, healthy grass). 

Either way, remember that weeds are designed to survive. Even if you end the growing season with a lush, green lawn, you’ll still have to be ready to battle again come spring.

Teresi advised keeping an application log so that you don’t forget what you applied and when. “Did you apply preemergent? Was there fertilizer? Write it down to avoid exceeding maximum nitrate rates,” he said.

Take time to note your products, application processes and results to make each subsequent season easier and more successful.

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