The best time depends on the weed you’re targeting and whether you want to prevent it or treat it after it appears. Apply a pre-emergent before the target weed germinates—often in spring for summer annuals and in fall for some winter annuals. For weeds already growing, a suitable post-emergent treatment is generally most useful while they are small and before they flower or set seed. Your location, lawn grass, and the product label determine the right timing.
Choose the timing by weed and treatment
“Weed killer” can mean a pre-emergent product that prevents specified weeds from establishing, or a post-emergent product that treats weeds already growing. They have different timing windows, so identify the weed and the product type before applying anything.
| Approach | When to apply | Typical target |
|---|---|---|
| Pre-emergent | Before the target weed germinates | Annual weeds, with spring applications commonly aimed at summer annuals and fall applications at selected winter annuals |
| Post-emergent | After the weed is growing; treat while it is small and before it flowers or sets seed | Existing weeds listed on a product labeled for your lawn grass |
When to apply pre-emergent
Spring: before summer annual weeds germinate
Spring pre-emergent applications commonly target summer annual weeds such as crabgrass. The University of Delaware Cooperative Extension recommends applying approximately 10 to 14 days before the expected germination period. That interval is a general timing guide, not a fixed date for every region; use local conditions and the target weed’s expected germination to plan the application.
Fall: before selected winter annual weeds germinate
A fall application may help prevent selected winter annual weeds, but the appropriate timing varies with climate and the weeds present. Do not assume that the same product or application schedule is suitable for every lawn or every winter annual.
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- WEED KILLER SPRAY FOR LAWNS: With a simple hose-end application, kill weeds in your lawn, including dandelions, chickweed, clover, crabgrass and foxtail with Ortho WeedClear Lawn Weed Killer Ready-to-Spray
- FAST-ACTING WEED CONTROL: This weed killer hose sprayer kills listed weeds like crabgrass, dandelion, clover, chickweed and creeping charlie down to the root and gets results with 1 application
- COVERAGE: This 32 fl. oz. container of Ortho WeedClear Lawn Weed Killer Ready-to-Spray provides up to 5,000 sq. ft. of lawn care
- HOW TO USE: Apply to listed lawn weeds that are young and actively growing by connecting your garden hose to the weed killer sprayer, sliding the switch to "ON" and spraying evenly until the lawn is wet
- WHERE TO USE: This spray weed killer for lawns won't harm lawn grasses when used as directed - use on northern lawn grasses including Bermudagrass, Buffalograss, Fescues, Kentucky Bluegrass, Ryegrass and Zoysiagrass
Follow-up applications
Some programs may require a follow-up for season-long control. UF/IFAS Extension describes a possible follow-up 6 to 9 weeks after the initial application; this is guidance from its Florida lawn-care context, not a universal interval. Follow the product label for whether repeat applications are allowed and when they may be made.
When to apply post-emergent weed killer
If weeds are already visible, pre-emergent prevention may be too late for those plants. A selective post-emergent herbicide may be appropriate if its label lists both the target weed and your lawn grass. UGA Extension advises treating weeds while they are small and before they flower and set seed. The label’s directions and restrictions govern the application.
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- WHERE NOT TO USE: This weed and dandelion killer for lawns and lawn care product should not be used on Dichondra, Carpetgrass, 'Floratam' St. Augustinegrass, Creeping Bentgrass, or turfgrass where desirable clovers are present
- LAWN WEED KILLER: Scotts Weed Control for Lawns is a weed killer for grass that starts working on contact to kill even the toughest weeds down to the root
- KILLS LISTED WEEDS: This broadleaf weed killer for lawns controls over 200 listed weeds, including dandelion, clover, dollarweed, chickweed, poison ivy, henbit, oxalis, ground ivy, and purslane
- WHEN TO USE: Apply weed killer granules in the spring or early fall when broadleaf weeds are actively growing
- COVERAGE: One 14 lb. bag of Scotts Weed Control for Lawns granular weed killer covers 5,000 sq. ft.
Why there is no universal calendar date
A date that works for one lawn may be mistimed for another. Weed germination varies by region and conditions, while turfgrass species affect which products are suitable. Seeding or overseeding plans also matter because herbicide labels can restrict when grass seed may be planted or when an application may be made relative to seeding.
Use local extension guidance to refine the timing for your area, then confirm the product label covers your weed and turfgrass. UF/IFAS warns that high temperatures can injure or kill turf after herbicide application, so do not assume an application is safe just because weeds are present.
Quick Recap
Best Value
- LAWN WEED KILLER: Use Ortho WeedClear Weed Killer for Lawns Concentrate weed killer spray to kill over 250 listed weeds including dandelion and clover down to the root, without harming your lawn (when used as directed)
- HOW TO USE: To use, dilute with water following label directions and apply using in a tank sprayer or apply with the Ortho Dial N Spray hose-end weed sprayer to small, actively growing weeds when daytime temperatures are below 90°F
- WEED SPRAY COVERAGE: This 1 gal. container of Ortho WeedClear Weed Killer for Lawns Concentrate treats up to 64,000 sq. ft.
- FAST-ACTING WEED CONTROL: This concentrated weed killer controls listed weeds like dandelion, clover, and chickweed down to the root; it's rainproof in 1 hour and begins to show visible results in hours
- WHERE TO USE: Use broadleaf weed killer for lawns on St. Augustinegrass (not Floratam), Bermudagrass, Centipedegrass, Zoysiagrass, Buffalograss, Bahiagrass, Kentucky Bluegrass, Ryegrass, Tall Fescue, and Fine Fescue lawns
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- KILLS 460+ WEEDS AS LISTED: Kills all types of listed broadleaf weeds including dandelion, chickweed, clover and yellow nutsedge.
- VISIBLE RESULTS: This product will give you visible results in 3 hours.
- KILLS BY CONTACT: Spectracide Weed Stop For Lawns Concentrate2 kills by contact and kills weed roots.
- WHERE TO USE: Use this product on lawns – treats bluegrass, fescue, zoysia, Bermuda and perennial ryegrass.
- Do not apply to zoysiagrass just emerging from dormancy.
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- LAWN FRIENDLY FORMULA: Our lawn friendly weed killer spray formula contains three proven ingredients that specifically target common lawn weeds, so it won't harm your grass (when used as directed)
- EASY APPLICATION: The continuous spray Comfort Wand weed killer sprayer applicator makes it easy to apply for lawn care, allowing for precise spray control and maximum accuracy without any pumping required
- LAWN WEED CONTROL: Use Ortho WeedClear Weed Killer for Lawns with Comfort Wand, a lawn weed killer, to kill weeds down to the root without harming your grass (when used as directed), with visible results in hours
- KILLS OVER 250 WEEDS: This broadleaf weed killer for lawns targets over 250 common broadleaf lawn weeds, including dandelions, clover, poison ivy, chickweed, and dollarweed (see product label for complete weed list)
- FAST VISIBLE RESULTS: See visible results in hours; this lawn care formula that is rainproof in 1 hour after drying, and people and pets may re-enter the treated area after the spray has dried
Check before you apply
- Identify the weed you want to prevent or treat.
- Confirm whether you need a pre-emergent or post-emergent product.
- Check that the label lists both the target weed and your lawn grass.
- Follow label directions for application, watering, repeat treatments, heat exposure, and seeding or overseeding restrictions.
- Use regional timing guidance rather than relying on a calendar date from another climate.
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