A weed killer (or "herbicide") is designed to target weeds and later disintegrate. It is important that it not remain potent for too long, since its presence raises health concerns if you or your pets walk in the area.
The makers of many of these products claim that they last on your lawn only for a day or two, after which it is safe to use your lawn again. However, the length of time you should wait varies depending on the specific weed killer.
Learn how long weed killer stays on your lawn, how it affects your soil, how long you should stay off your lawn after the application, and how to avoid having to use weed killers at all.
How Long Does Weed Killer Last on Your Lawn?
How long a weed killer is effective in keeping weeds from coming back varies depending on the weed and the herbicide, but rarely will you get away with applying weed killer just one time. Let's use a typical "weed and feed" product as an example.
You will have to reapply weed and feed every year. It is usually enough, during the course of one year, to apply it in spring. Occasionally, people will have to apply it a second time later that year.
Another example is the post-emergent weed killer, Roundup (glyphosate), a non-selective herbicide that you may use (with caution) to kill an isolated weed here and there on your lawn.
How long Roundup "lasts" depends on how persistent the weed is: Some will be killed immediately, others will spring back to life over and over again. Reapply Roundup on an as-needed basis.
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How Weed Killer Affects Soil
The chemicals in some weed killers can harm earthworms, beneficial fungi, and other microorganisms. All of these are important for soil health. The best way to restore them is to work compost into the lawn, little by little.
Tip
You can fight weeds without resorting to weed killers if you hand-pull them. However, people are more reluctant to pull weeds in lawns than in gardens because they're afraid of spoiling the well-manicured look of their lawn by putting holes in it. Luckily, there are ways to pull weeds without making a mess, including using a dandelion puller.
How Long Should I Stay Off the Lawn After Applying Weed Killer?
Always read the information on the package that the weed killer comes in. Some will say that, after the application, you can walk on your lawn again in one to three days, with one day being common.
It is always a good idea to wait a little longer and err on the side of caution, though.
Pre-Emergent vs. Post-Emergent Weed Killers
Pre-emergent herbicides are designed to fight weeds proactively. After a seed germinates, they inhibit the growth of the first roots the weed strikes down. Post-emergent herbicides are applied to weeds that have already emerged.
4 Tips for Reapplying Weed Killer
Usually, you will have to reapply weed killer at some point (probably multiple times). Weed seeds that are already present in the soil are constantly germinating, creating new weeds. New seeds are constantly being brought in by birds, the wind, etc.
In the case of perennial weeds, many are so tough that one application of weed killer simply isn't enough: Reapplying the herbicide when they reemerge is the standard operating procedure.
Choose the right selective herbicide for your lawn if you are treating a large area (non-selective weed killers, if used carefully, are acceptable for spot treatments).
Selective vs. Non-Selective Herbicides
Selective herbicides target particular weeds or classes of weeds, leaving other plants unharmed. Non-selective herbicides will kill just about anything. Extensive weed control on lawns requires selective herbicides so that you don't harm your grass.
As much as we crave one-and-done weed control, expect to have to reapply weed killer:
- Whenever you detect lots of new weeds in your lawn, it is time to reapply your post-emergent weed killer. But if you walk your lawn regularly to detect new weeds, you can pull them before they get out of hand enough to require spraying.
- You may even need to reapply it before that: If it rains just after your application, the herbicide will be washed off before it can do any good. That's why it's a good idea to always check the weather forecast before applying weed killer.
- Some reapplications are done on an annual basis simply because it's the most practical way to fight a weed. If your neighbors have crabgrass, then you'll soon have it, too unless you take preventive measures. Apply a pre-emergent weed killer on the lawn every spring to prevent crabgrass.
- Products labeled "weed and feed" are also applied annually to lawns. Their great benefit is that they kill two birds with one stone.
4 Tips for a Healthy, Weed-Free Lawn
One way to avoid obsessing over the persistence of weed killers is to take proper care of your lawn. A healthy lawn will not be invaded by weeds (at least not to any great degree). Here are some steps to take:
- Do not scalp your lawn. If you mow your grass higher, it will shade out many weeds. Never cut down the grass by more than 1/2 its height. Ideally, remove only 1/3 of a grass blade's height at any one time.
- Instead of removing your grass clippings, use a mulching mower and leave them in place. This helps fertilize the lawn, and it's free.
- Supplement that free fertilizer with store-bought fertilizer or compost. Stick to a regular fertilizing schedule.
- Water the lawn properly.
