When to Apply Pre-Emergent Herbicides

By Jane Purnell | Updated:
Lawn Care Tips
When to Apply Pre-Emergent Herbicides

Apply pre-emergent herbicides in spring when soil temperatures reach 55°F (typically February-April) to prevent summer weeds like crabgrass. Apply again in fall when soil temps drop to 70°F (August-October) to stop winter weeds. Timing varies by location—Southern homeowners should apply earlier than Northern regions.

Pre-emergent herbicides create a barrier in soil that stops weed seeds from germinating. Missing the application window means weeds establish roots before the herbicide can work. If you’re short on time, Lawn Love’s local lawn care pros handle pre-emergent applications with precision timing and professional-grade products.

Key Takeaways
• Apply spring pre-emergent when soil temperatures consistently reach 55°F.
• Apply fall pre-emergent when soil temperatures drop to 70°F.
• Most pre-emergents last 8-12 weeks, so 2 applications per year (spring and fall) provide season-long protection.
• Timing varies by region: Southern states apply in February for spring and late September-October for fall; Northern states apply in April for spring and August for fall.
• Never apply pre-emergent to new grass seed—it prevents all seeds from germinating.

Spring pre-emergent application

graphic for grassy weeds broadleaf weeds
Infographic by Juan Rodriguez

When to apply in spring

Apply spring pre-emergent when soil temperatures consistently reach 55°F for several consecutive days. Use a soil thermometer inserted 1-2 inches deep (even a meat thermometer works). This temperature triggers summer annual weed germination, so you need the barrier in place beforehand.

Most pre-emergents last 8-12 weeks, giving you flexibility in your application window. “Apply your spring preemergence herbicide when soil temperatures reach approximately 55 degrees for several days,” says Becky Grubbs, turfgrass specialist at Texas A&M University AgriLife Extension.

Spring application by region

Northeast

Southeast

Midwest

Northwest

Southwest

Note: Hawaii’s tropical climate doesn’t require pre-emergent applications on the same schedule. Timing within each region varies by 2-4 weeks depending on elevation and local climate. Use soil temperature (55°F) as your guide.

Is April too late for pre-emergent? In Northern states, early April is ideal. In Southern regions, April is too late—weeds will have already germinated.

Summer weeds prevented

Spring pre-emergent targets these common lawn invaders:

Fall pre-emergent application

When to apply in fall

Apply fall pre-emergent when soil temperatures drop to 70°F for several days. This typically occurs between August and October, depending on your climate zone. Use the same soil thermometer method—insert 1-2 inches deep to get accurate readings.

“Fall applications can be made several months later when soil temperatures decrease to approximately 70°F for several days,” Grubbs explains.

Fall application by region

Northeast

Southeast

Midwest

Northwest

Southwest

Note: Hawaii’s tropical climate doesn’t require pre-emergent applications on the same schedule. Timing within each region varies by 2-4 weeks depending on elevation and local climate. Use soil temperature (70°F) as your guide.

Can you apply pre-emergent in summer? Yes, if you missed spring application. Late summer (when temps drop toward 70°F) targets winter weeds emerging in fall.

Winter weeds prevented

Fall pre-emergent stops:

  • Annual bluegrass
  • Henbit
  • Common chickweed
  • Hairy bittercress
  • Wild garlic
  • Wild onion
  • Annual ryegrass
  • Carolina geranium
  • Shepherd’s purse
  • Corn speedwell
  • Lawn burweed
  • Purple deadnettle
  • Winter grass

FAQs

Can you apply pre-emergent too early?

Yes. Applying too early reduces effectiveness because the herbicide breaks down before weeds germinate, leaving your lawn unprotected. Pre-emergent herbicides typically last 8-12 weeks. If you apply in January when soil temps are 40°F, the product will degrade by March—before crabgrass germinates in April. While some early weeds may be stopped, most will germinate after the pre-emergent degrades. Stick to soil temperature guidelines rather than calendar dates—wait until soil consistently reaches the target temperature (55°F for spring, 70°F for fall).

Can I use pre-emergent on new grass seed?

No. Pre-emergent herbicides prevent all seeds from germinating—including grass seed. Wait until new grass is fully established (typically 2-3 months with multiple mowings) before applying pre-emergent. If you need to overseed, use a selective pre-emergent safe for seeding or skip the application.

Should I apply pre-emergent in summer if I missed spring?

If you missed spring application and crabgrass has already emerged, pre-emergent won’t help with existing weeds. However, you can apply in late summer (as temps approach 70°F) to get ahead of fall-germinating winter weeds. For existing summer weeds, use post-emergent herbicide.

Get professional pre-emergent application

Pre-emergent herbicides require precise timing, proper products, and correct application rates. Missing the window by a few weeks can mean a lawn full of crabgrass or winter weeds. Lawn Love’s vetted local professionals monitor soil temperatures, apply commercial-grade pre-emergents, and provide year-round weed prevention—so you get a weed-free lawn without the guesswork.

Main Image: Gardener spraying herbicide in the lawn. Image Credit: plo / Adobe Stock

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Jane Purnell

Jane Purnell is an artist, writer, and nature lover. She enjoys teaching readers about the importance of eco-friendly lawn care, integrated pest management, biodiversity, and sustainable landscaping.
Posts by Jane Purnell