If you have weeds in your lawn or garden, chances are you want to get rid of them as soon as possible. You know you have to give your weed killer enough time to work, but just how long does it take for weeds to die after you spray them?

The answer is that it depends on the weed and the type of weed killer or herbicide you use. You also have to consider whether a fast-acting formula is the best way to cope with weeds in the long run. Let’s take a closer look.

Post-Emergent Herbicides

If weeds unexpectedly popped up in your yard, and you are tired of attempting to pull them out by hand, consider a post-emergent weed killer. This type of herbicide kills the weeds that are already above the surface, and it can prevent future growth, too.

There are different types of post-emergent weed killers, and you have to know a little about the weeds you’re trying to kill to choose the right one. Some of these formulas work by entering the plant the leaves while others get absorbed into the roots.

Carefully read the label for whatever product you choose to make sure you apply it correctly, and so you know if there’s a risk of it harming your other plants.

Some herbicides are selective; some are not. Selective weed killers are appropriate if you’re trying to kill weeds scattered throughout your yard or in and around your garden. Non-selective weed sprays are better for when you’re trying to clear a large area of all growth.

How quickly post-emergent weed killers work depends on a few factors. The type of formula you use is one of the big ones. Chemical herbicides work much faster than organic ones. Other factors include the type of weed you’re dealing with and the rate of application.

When you apply the herbicide matters, too. Weed spray needs to dry for at least 30 minutes after application, so applying it on dry days is best. Avoid windy days, too, as wind affects how precisely you’re able to spray.

For best results, the temperature should be between 55 and 80 degrees Fahrenheit (13 – 27°C), and you must make sure the weeds are actively growing. Post-emergent weed killers only work during periods of active growth.

If applied correctly, you should see some results within a week, and the weeds should disappear completely in about three weeks. Again, this varies depending on the type of weed killer you’re using. Some brands promise visible results in as little as 24 hours.

Pre-Emergent Herbicides

To prevent weeds in the first place, use a pre-emergent weed killer. This type of herbicide is often used to treat large areas and established lawns in hopes of preventing weeds from showing up year after year.

Note that most of these formulas will also prevent new grass seed from sprouting and should only be applied to established lawns.

As with post-emergent formulas, how and when you apply pre-emergent weed sprays is important for getting results. The roots must be immature as established roots are tough and persistent enough that the herbicide won’t affect them.

Annual weeds can appear in the winter or summer, and applying the weed spray at the right time is key.

If you’re going after winter annuals, apply the weed spray in the fall when the seeds are germinating, and the roots are just getting started. For summer annuals, spray in the spring.

Pre-emergent spray usually needs to be watered into the ground to ensure that it gets down to the roots. That said, you should always read the instructions on the product that you choose just to make sure.

As with post-emergent weed killers, never apply pre-emergent weed killer on a windy day. The temperature should be above freezing, and the soil should be soft enough that the spray can be easily watered into the roots.

Pre-emergent weed killers stop immature weeds from growing roots, which means that everything happens below the surface where you can’t see it. So, it’s difficult to tell exactly how long it takes for plants to die.

But some things can help estimate this. For example, most pre-emergent weed killers are active for between six and eight weeks. But, the timing of the application is so important that it can take years to completely stop weeds from coming back if it’s not done just right.

As you can see, the timing of the application is essential. Make sure you wait until the weather isn’t too hot because this limits how effective the treatment is.

The Best Approach

If you have a difficult weed problem, the best approach is to use a combination of pre and post-emergent weed killers. This approach gives you relatively fast results while also making the task of weeding easier and easier every year.

Pre-emergent weed killers are very effective at preventing weeds from coming back year after year. But, it may take a few years to kill them all. This approach does not deliver quick results, but it is very effective in the long haul as long as you apply the weed spray correctly.

Post-emergent weed killers can help you manage the weeds that pop up in the meantime. These products tend to work quickly. Some formulas even promise results in a matter of days. But they’re not quite as effective as pre-emergent weed sprays for preventing weeds from returning.

So, why is a combination of the two weed killers the best approach? It’s pretty simple. As you probably already know, weeds are extremely difficult to kill. You have to use the right type of weed spray applied in the right way at the right time. If you don’t, you may not get the results you’re hoping for.

By using a fast-acting post-emergent weed spray to quickly kill the weeds above the ground and a pre-emergent formula to stop weeds from coming back next year, you have the best chance at having a weed-free yard.