When to Apply Crabgrass Preventers in Spring

Crabgrass is one of the most problematic weeds in lawns across North America. This annual grassy weed germinates in spring when soil temperatures reach 55 to 60°F, making early spring the optimal time to apply crabgrass preventers. Proper timing of crabgrass preventer application is key to effective control of this persistent weed.

Why Crabgrass is a Problem

Crabgrass is so difficult to control for several reasons:

  • It germinates early in spring, giving it a head start over desirable lawn grasses like Kentucky bluegrass, perennial ryegrass, and tall fescue. By the time the lawn greens up, crabgrass is already established.
  • A mature crabgrass plant can produce over 150,000 seeds that remain viable for up to three years in the soil. This means crabgrass infestations can worsen over time if not controlled early.
  • Crabgrass forms dense patches that crowd out lawn grasses. Left uncontrolled, it can take over large areas of the lawn.
  • It turns brown after frost, leaving unsightly dead patches in the lawn.

How Crabgrass Preventers Work

Crabgrass preventers are pre-emergent herbicides that create a chemical barrier in the soil that kills crabgrass seeds as they germinate. They do not kill existing plants that have already germinated. That is why timing of application is so important.

There are two main active ingredients in crabgrass preventers:

Dithiopyr – Found in products like Dimension, this herbicide provides early post-emergent control on newly sprouted crabgrass, in addition to pre-emergent control.

Prodiamine – The active ingredient in Barricade, this provides season-long control with one application. It binds tightly to soil particles for residual pre-emergent control.

Crabgrass preventers must be applied before the soil temperature reaches 55°F to be effective. This usually occurs 4-6 weeks before the average last spring frost date in your area.

When to Apply Crabgrass Preventer

The optimal window for applying crabgrass preventer is when forsythia shrubs are blooming in your region. This flowering shrub is one of the earliest to bloom each spring, making it a good visual indicator that soil temperatures are approaching 55°F.

Here are some general guidelines based on region:

Northern States and Canada

Apply crabgrass preventer between early to mid April when forsythia blooms and before the average last spring frost date. Northern regions have a shorter window of opportunity for application.

Transition Zone

Apply crabgrass preventer when forsythia blooms, usually in early to mid April. The transition zone has moderate winters and falls between the northern and southern regions.

Southern States

Apply crabgrass preventer 6-8 weeks before the average last spring frost for your area. Forsythia typically blooms in February/March in southern zones, signaling it’s time to apply preventer.

California and Desert Southwest

Apply crabgrass preventer in late January/early February before any crabgrass germinates. The warm climates in these regions allow crabgrass to sprout earlier than other areas.

Tips for Proper Application

Follow these tips to get the most effective control from your crabgrass preventer:

  • Read and follow label instructions for proper rate and application method. Using too little product is a common mistake.
  • Apply on a calm, dry day with no rain expected for 24 hours. Irrigation or rainfall shortly after application will wash away the preventer.
  • Consider split applications 8-10 weeks apart for season-long control in warmer climates. Early and late-germinating crabgrass will be controlled.
  • Water product into soil lightly after application. This activates the herbicide layer in the top 1⁄8-1⁄4 inch of soil where crabgrass seeds germinate.
  • Allow lawn to dry before any mowing, fertilization, dethatching, aerating, or seeding. Crabgrass preventer creates a protective barrier that should not be disturbed.
  • Avoid overlapping application that could lead to overly high amounts of product. This wastes product and risks turf injury.
  • Do not apply to newly seeded lawns. Crabgrass preventer will inhibit germination of grass seeds. Wait until new lawn is established.
  • Do not apply to zoysiagrass lawns when they are emerging from dormancy. Zoysia is sensitive to crabgrass preventers.

Effectiveness of Crabgrass Preventers

When applied correctly in spring before soil temperatures reach 55°F, crabgrass preventers provide 85-90% or better control of crabgrass in lawns. Efficacy can approach 100% with split applications.

Pre-emergent herbicides like Dimension can also provide early post-emergent control of crabgrass, catching plants that may have germinated early before the preventer was applied.

Crabgrass preventers do not kill existing plants, so combination with early post-emergent herbicide like quinclorac may be needed if any crabgrass has already germinated.

Early spring application timing is critical for success. Applying too late reduces effectiveness since some crabgrass will have already sprouted. Preventers cannot do their job on plants that are already visible.

While not 100% foolproof, following the tips above gives crabgrass preventers the best chance of delivering a clean, weed-free lawn through summer.

What if Preventer was Applied Too Early?

If crabgrass preventer was applied too far in advance of the optimal window described above, the protective chemical barrier can break down over time, leaving the lawn vulnerable.

Many crabgrass preventer products are labeled to provide 12-16 weeks of control. If you applied in late winter or early spring and it has now been more than 12-16 weeks, the product may no longer be active in the soil.

To ensure protection through the peak crabgrass germination period, it is recommended to reapply the same preventer you originally used at the label rate. No more than 2 applications per year are permitted.

This maintenance application should provide control through summer. Timing as forsythia blooms in spring is still ideal, even for a second application.

What if Preventer was Applied Too Late?

If crabgrass preventer was applied after soil temperatures exceeded 55°F for several days or weeks, some crabgrass seeds may have already germinated. The preventer will not control existing plants.

In this case, a multi-pronged approach is recommended:

  • Apply crabgrass preventer anyway to stop later flushes of crabgrass from emerging. Preventer applied too late is better than not at all.
  • Use early post-emergent herbicide like quinclorac to kill any young crabgrass plants. Look for crabgrass with 3 leaves or less.
  • Manually dig out larger crabgrass plants to prevent seed production and spread.
  • Overseed bare spots in fall to crowd out any remaining crabgrass and prevent reinfestation.

While late application reduces effectiveness for the current year, getting preventer down is still important to limit further crabgrass germination. Follow up with other control measures as needed.

How Long Does Crabgrass Preventer Last?

Most crabgrass preventers used by homeowners have a 3-4 month residual in the soil when applied at the label rate. This gives them staying power to control successive flushes of crabgrass through peak summer germination months.

Here are some common active ingredients and their longevity:

  • Prodiamine – Applied in early spring, this provides 3-4 months of control with one application. Some formulations claim “up to 5” months.
  • Dithiopyr – Also active for 3-4 months after early spring treatment. May provide slightly longer protection than prodiamine.
  • Pendimethalin – Shorter 1-2 month residual but often applied at split intervals for season-long control.

Warmer climates may fall on the shorter end of these control windows while cooler climates may achieve closer to 4 months protection. Reapplication intervals should be adjusted accordingly.

How Soon Can Overseeding be Done After Application?

Crabgrass preventers create a chemical barrier that prevents establishment of new grass seedlings along with weed seeds. As such, an interval of 2-4 months is recommended between application and seeding or overseeding lawns.

Here are some guidelines on reseeding intervals:

  • Northern Cool-Season Grasses – Wait at least 2 months after applying crabgrass preventer before overseeding. This allows the product to dissipate from soil.
  • Southern Warm-Season Grasses – Wait 3-4 months before seeding bermudagrass, zoysia, or other sensitive southern grasses. Delay seeding until after summer heat has passed.
  • When reseeding heavy traffic or shaded areas in either climate, allow 4 months before seeding for less risk of germination problems.

If reseeding cannot wait the full interval, aerifying first may help allow some seed-to-soil contact despite the herbicide barrier. Irrigate immediately after seeding to stimulate germination before barrier re-forms.

Alternatives for New Seedings

Since crabgrass preventers inhibit establishment of new grass seedlings, a new lawn cannot be treated immediately with pre-emergent herbicide without risking lawn establishment issues.

There are two alternative strategies for new lawns:

Delay Preventer Application

Wait until the new lawn has gone through at least 2-3 mowings and has filled in and thickened up, typically 6-8 weeks after seeding. This indicates the grass is mature enough to tolerate application of crabgrass preventer.

Use a post-emergent herbicide like quinclorac or hand weeding to control any early crabgrass.

Use Siduron (Tupersan)

This alternative crabgrass preventer has a different mode of action than traditional options like prodiamine and dithiopyr. Siduron only prevents root growth of newly germinated seeds but does not inhibit shoot growth.

This allows new grass seedling shoots to establish while still providing pre-emergent crabgrass control.

Siduron can be applied immediately after seeding or at time of seeding new lawns. It provides 2-3 months of weed control.

Takeaways on Timing Crabgrass Preventers

  • For maximum effectiveness, apply crabgrass preventers in spring about 4-6 weeks before the average last frost date for your area.
  • Base timing on an indicator like forsythia blooms to target soil temperatures of 55°F.
  • Split applications may be needed in warmer climates for season-long control through summer.
  • Avoid applying too early; reapply if it has been over 12-16 weeks.
  • If applied too late, combine with post-emergent control options.
  • Allow 2-4 months between application and overseeding lawns.
  • On new lawns, delay application or use siduron until grass is established.

Properly timed crabgrass preventer in spring is the most effective strategy for killing this weed before it takes root in lawns. Consistent early application will reduce crabgrass populations over time, making control much easier. Pay close attention each spring to optimal timing for your climate. A small investment of time in early spring will pay off all summer long with a lush, crabgrass-free lawn!

Frequently Asked Questions

When is the best time to put down crabgrass preventer in Ohio?

In Ohio, target applying crabgrass preventer around mid-April when forsythia is in full bloom. This timing coincides with soil temperatures reaching 55-60°F, right before the average last spring frost date. Split applications 8 weeks apart in early April and early May can provide season-long control.

How early can I put down crabgrass control in Connecticut?

In southern Connecticut, apply crabgrass preventer around early April when forsythia blooms. In northern areas of the state, hold off until mid-to-late April to avoid too early of an application. Apply before the average last frost date in your specific area.

Can you put down crabgrass preventer too early?

Yes, applying too far in advance of the optimal spring window can lead to failure. Many products have a 12-16 week residual, so applying in late winter can leave lawns unprotected in early summer when crabgrass pressure is highest. Stick within 4-6 weeks of your last average frost date.

When should I put down crabgrass control in Florida?

In northern Florida, apply crabgrass preventer around late February to early March. In southern areas, apply January through mid-February. The warm-season grasses grown in Florida green up very early, making earlier prevention important.

What month is best to apply crabgrass preventer?

In northern states, April is generally the best month for crabgrass preventer application. In the transition zone, target March through April. Farther south, February and early March are ideal. Crabgrass begins germinating much earlier in warmer climates.

What happens if you put down crabgrass preventer too late?

Applying too late after soil temperatures exceed 55°F allows some seeds to germinate before the preventer is down. This escaped crabgrass must then be controlled post-emergence with other products like quinclorac or manual removal. But late application can still prevent further germination.


Crabgrass preventers provide highly effective control of this stubborn lawn weed, but proper timing in spring is paramount. Use visual indicators like forsythia blooms and average frost dates, along with soil temperature thresholds to determine optimal application windows based on your climate. Avoid applying too early or too late. Follow label instructions for product rate and watering in. Timely spring treatment followed by observant spot-treatment of any escaped plants is the best IPM approach to keep lawns crabgrass-free.