Should You Spray Weed Killer on Your Lawn Before It Rains? What to Know to Make It Work

• Rain washes off weed killers, making them less effective.
• Every product has a specific “rainfast period” during which it should sit untouched by rainfall or watering.
• The proper timing of herbicide application will depend on the product you’re using, but is either in spring or fall.

Timing is crucial when using herbicides to eradicate weeds. Not all products are applied at the same time. Pre-emergent herbicides are exactly that: applied before weeds emerge from the ground but after they have germinated. Post-emergent herbicides are sprayed on the actual weeds, but if it rains afterwards, the herbicide cannot do its job.

If your forecast says rain, here’s everything you need to know about spraying weed killer—plus easy substitutes for herbicides.

Can You Spray Weed Killer Before It Rains?

Rainfall can make weed killers less effective by washing them off. Herbicides have a “rainfast period”—the amount of time a product needs to sit without rainfall or watering. The rainfast period varies from product to product, ranging from “when dry” to several hours.

There is no standard for how long it takes for the herbicide to dry. Large, branched weeds with lots of foliage take longer to dry than low-growing, small weeds. The severity of the weed problem also plays a role in the time window you have before a rain. Sufficiently covering all the weeds in a dense patch requires more herbicide and therefore more time to dry afterwards.

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Can You Spray Weed Killer After Rain?

When using broadleaf herbicides, wait until the leaves are fully dry before spraying weed killer. Applying herbicides on wet foliage is ineffective because the product is diluted and not fully absorbed.

The exact opposite applies to pre-emergence herbicides, which are applied onto the soil and need moisture to reach the weed seeds in the soil.

When Is the Best Time to Spray Weed Killer?

The timing depends on the type of weeds you are trying to kill and the product you are using.

Late spring and fall are the best times to spray broadleaf weed killer. Fall is ideal for perennial weeds such as dandelion, thistle, plantain, and creeping Charlie because the product reaches the roots when the plants move energy to their roots before winter dormancy.

Annual turfgrass weeds such as crabgrass are best controlled with a pre-emergent herbicide. The timing for those weed killers is very different; they are applied in the spring, 10 to 14 days before the earliest expected germination. 

When spraying herbicides, choose a day with moderate temperatures and no wind to avoid drifting. Herbicide application is best when temperatures are between 60°F and 70°F, and not advised above 90°F. Not many weeds actively grow in cool temperatures below 50°F, so the herbicide is less effective.

Alternatives to Weed Killer

• A dense, healthy lawn gives weeds less room to grow. Select a type of grass suitable for your climate, fertilize it regularly without overfertilizing it, aerate and dethatch as needed, and overseed bare spots.
• If you water your lawn, do it less frequently but longer for deeper root growth. Abstain from watering lawn that goes naturally dormant in hot weather; it will spring back to life when conditions improve, and if you keep watering it, you inadvertently also encourage weed growth.
• Use corn gluten as a pre-emergent organic herbicide. It can be used against crabgrass and other lawn weeds, but like most pre-emergent herbicides, it requires repeated applications to control lawn weeds.
• Set your mower so the grass is at least 3 to 4 inches high, which shades weed seeds and keeps them from sprouting.
• Leave clippings on the lawn because they act as mulch, preventing the germination of weed seeds while adding nutrients to the soil.
• Remove weeds manually, preferably after a rain when the ground is soft and before they go to seed. Use special tools such as a stand-up dandelion puller that make the job easier.