How Long Does it Take for Weed Killer to Work?

When you use a weed killer spray, you want results. However, when taking out hardy dandelions and resilient ground ivy, it can be difficult to know if the weed killer you’ve used is working. How can you tell if it’s time to re-apply weed killers? How long should you wait for results after spraying for weeds? How long does it take for weed killer to work?

To choose the best weed killer for your lawn, it’s important to know about the different weed killers on the market and their effects. In this article, we’ll discuss different types of weed killers, the best time to spray for weeds, and how long it will take before you see results.

How long does it take for weed killer to work

How Does Weed Killer Work?

If your lawn is all weeds, you may need to use more than one type of weed killer in order to kill all the different weed species you’re dealing with. The important questions that help determine how to use weed killers are What, When, and Where:

  • What types of weeds are you killing? Are you battling broadleaf weeds or grassy weeds? Annuals or perennials?
  • When are you beginning your weed-killing efforts? Season matters in the weed-killing game. For instance, pre-emergent weed-killers are only effective if they are applied before seeds sprout.
  • Where are the weeds you’re killing located? Are you tackling weeds in your lawn, your garden, or pesky weeds coming up through seams in a concrete driveway? You may choose different products based on these locations.

Types of Weed Killers

Roundup and other Contact Weed Killers

Roundup makes some of the strongest and fastest-acting weed killers on the market. They are often non-selective weed killers that kill emergent weeds.

Additionally, some long-term weed control Roundup formulations contain pre-emergent weed control measures. These specialized formulations even prevent plants from sprouting in the area of application for up to one year.

Pre-Emergent Weed Killers

Pre-emergent weed killers are designed to stop weeds in their infancy. They work by killing seedlings, which prevents new weeds and grasses from sprouting up in your yard. Pre-emergent herbicides won’t kill established or mature plants.

Pre-emergent weed killers can be applied anywhere in your lawn and garden, but they don’t discriminate. These weed killers will stop the growth of any grass seeds or other plants. Don’t apply to any areas with any desirable seedlings or any areas that you plan to seed within the next 3 weeks.

Emergent Weed Killers

Emergent weed killers target established weeds, killing mature plants. They will shrivel up dandelions and kill clover. Keep in mind, weed seeds in the soil won’t be affected by an emergent weed killer.

Note: The types of weeds targeted by spraying weeds with emergent weed killer depend on the active ingredients included in the formula. Be sure to read the product description to find out if you are using a non-selective or selective weed killer, as well as how to use weed killer.

Non-Selective Weed Killers

Typically, the fastest weed killers are non-selective weed killers. These weed killers will kill broadleaf weeds, woody plants, and grassy weeds, along with desirable grasses. So, while these will take out your enemies, they’ll also kill any other plants they touch.

Apply non-selective weed killers only in areas that will not impact desirable plants or grasses. These weed killers are best for dandelions, ivy, or isolated tufts of pest grass. They are not viable solutions when weeds are mixed in with grass, such as with some clover infestations.

Selective Weed Killers

Selective weed killers are designed to kill weeds and leave turf grass unharmed. Sounds pretty perfect, doesn’t it? This way, you get to kill the pests and keep what you want.

Selective weed killers are most commonly found as broadleaf weed killers. These will target dandelions, chickweed, and clover while leaving grass unharmed. Even better, there are selective weed-killing formulations that kill broadleaf weeds AND other pest grasses, such as crabgrass. There are also selective weed killers designed specifically to kill sedge and nutgrass.

Note: Check all product information to verify the weed killer you use is safe for your type of grass.

When to Apply Weed Killers

Man spraying weeds in grass on front lawn near a large tree

Timing is everything when it comes to killing weeds. You can have the right product and tools, but if you apply the weed killer at the wrong time it can have a diminished effect or no effect at all. For instance, excessively cold or hot weather can hinder the effectiveness of weed killer.

When to apply weed killer to a lawn depends on the product you’re using and on the climate you live in. If you want to see results, follow these simple guides when you spray for weeds in your lawn.

  • Roundup and other Contact Weed Killers: Contact weed killers are any sprays designed to kill weeds quickly. There are some great tips for how to use Roundup and similar weed killers.
    • Spray Roundup when temperatures are between 60°and 85° Fahrenheit to ensure maximum effectiveness.
    • Apply Roundup early in the day so they are absorbed by the leaves during daytime photosynthesis.
    • Do not apply Roundup if rain is expected within the next 6 hours. Rain can wash away the active ingredients before they completely infiltrate the plant’s system.
    • Don’t apply Roundup on an excessively windy day. Wind can cause Roundup to dry too quickly, which hinders it from entering the plant’s leaves.
  • Pre-Emergent Weed Killer: Apply in spring (typically March/April) when soil temperatures reach 50° Fahrenheit for 3–5 days. This is when many seeds begin to sprout and is the effective window for pre-emergent weed killers.
  • Emergent Weed Killer: If you’re applying an emergent weed killer to an entire lawn, there are two times of year that provide the best results. The first time is late-May through late-June. The second is between mid-September and early-November.

How Long Does it Take for Weed Killer to Work?

Once you have chosen the correct weed killer for your lawn and decided how to use your weed killer, the good news is you should begin to see results soon. Some herbicides work extremely quickly, even causing weeds to wilt within a few hours after application. In contrast, some special formulations designed to control specific weeds may require a bit more patience.

Roundup Weed Killers

If you’re asking, “how long does it take for Roundup to work?” you won’t have to wait long for an answer. Once standard Roundup has been sprayed on weeds, you will often see results within 2–6 hours. The plants you’ve sprayed will begin to yellow and wilt.

Keep in mind that although the weed is already dying, it may take up to 2 weeks for Roundup to kill the weed’s entire root system.Also, it’s important to note that Roundup variations designed for long-term weed control act more slowly. These may not show results for up to 12 hours.

Broadleaf Weed Killers

If you want to kill broadleaf weeds in your grass, you will usually end up using a selective weed killer designed to kill broadleaf weeds, or you may choose one of the best weed and feed products on the market. Weed and feed formulas often combine broadleaf herbicide with grass fertilizer, to get rid of weeds and improve grass health.

Whether you’re using a weed and feed product or a herbicide-only solution, patience is necessary. After applying your weed control solution, you will typically begin to see weeds begin to die within 5–7 days. Most weeds should be completely eliminated 2–4 weeks after application.

Grassy Weed Killers

If you’ve chosen a grassy weed killer with Glyphosate, a popular and powerful weed-killer used in Roundup and other products, you will see fast results in killing grasses—wilting within hours and complete death in 1–3 weeks. Fast results, but you have to be careful. Glyphosate is a non-selective herbicide that will kill any plant it is applied to.

If you choose to use a selective grassy weed killer, your results will vary based on the product and the type of grass you’re combatting. Different products work best to kill different grasses.

  • Crabgrass killers containing quinclorac kill weeds and grasses within 2–4 weeks.
  • Selective sedge and nutgrass killers such as Sedgehammer require 2–4 weeks to kill weeds completely.
  • If you are using Turflon Ester to control and kill bermudagrass, it may require repeat applications every 30 days throughout the growing season. It may take several months to see full results, as bermudagrass is very hardy.

Pre-Emergent Weed Killers

Pre-emergent weed killers work quickly. However, because they kill seeds just as they begin to sprout, this process is often invisible. Don’t worry though, pre-emergent solutions can begin killing weed seedlings mere hours after application.

When late spring rolls around and you see no new tufts of crabgrass or sprouting chickweed, the pre-emergent weed killer has done its job. If you do see annual weeds returning to your lawn, this may be because the pre-emergent weed killer was applied too late or washed away by excessive rain.

How Long Does Weed Killer Stay in the Soil?

If you’re considering using one of the most popular weed removal solutions on the market, you may find yourself asking, “How long does Roundup last in the soil?” The answer depends on the specific formulation, but here are some general guidelines:

  • Weed killers containing Glyphosate (the active ingredient in standard Roundup) typically break down within 1–6 weeks after application.
  • Long-Term weed control options (such as Roundup’s Max Control 365) are specifically designed to stop weed and plant growth for up to a year. They remain in the soil for up to 12 months.
  • Selective weed killers such as broadleaf herbicides containing Quinclorac, MCPP, Dicamba, or 2,4 D typically remain in the soil for 2-4 weeks.
  • Pre-emergent weed killers remain active for 3 weeks.

There are other factors that determine how long weed killer remains in the soil. Climate and environmental factors play a large role. If you water frequently or if your climate experiences frequent rain, weed killers will be flushed from the soil more quickly. Conversely, in dry areas weed killer remains in the soil for a much longer period of time.

Man spraying weed killer herbicide on weeds in garden

Working with Weed Killer for Fast Results

When you’re using a weed killer, you want to see weeds die fast. There are several products that deliver quick results to kill weeds. Whether you choose a contact weed killer like Roundup to kill weeds today or a pre-emergent weed killer to prevent weeds from appearing in the future, you will see the greatest impact if the weed killer is applied at the right time and under the correct conditions.

Always make sure to read all product labels and warnings to avoid damage to desired plants or turf grasses. When you spray for weeds, test the spray first in a grassless area to make sure the spray pattern won’t reach any plants that should not be exposed to the weed killer.

When you choose the right weed killer, you can truly see the results of your hard work. Sometimes, in just a few hours! Plus, it only gets better with time. After you’ve applied weed killer to your lawn, you’ll be amazed at how different your yard looks in just a few weeks.


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