The Crabgrass War: Pre-Emergent Timing and Strategy

Know Your Enemy

Crabgrass (Digitaria sanguinalis and Digitaria ischaemum) is an annual grassy weed — meaning it germinates from seed each spring, grows aggressively through summer, and dies at first frost. A single crabgrass plant can produce 150,000+ seeds before it dies, and those seeds sit in the soil waiting to germinate the following spring.

This is why prevention is everything. Once crabgrass is growing, you've already lost the first battle.

How Pre-Emergent Herbicides Work

Pre-emergent herbicides don't kill seeds. They create a chemical barrier in the top 1-2 inches of soil that prevents root development in germinating seedlings. When a crabgrass seed sends out its initial radicle (root), the herbicide disrupts cell division, killing the seedling before it ever breaks the soil surface.

This means:

• ✅ The product must be in the soil before germination starts
• ✅ It must be watered in within 24-48 hours of application
• ❌ It will NOT kill crabgrass that's already emerged and growing

Soil Temperature: The Only Timing Method That Works

Forget calendar dates. Forget the "when forsythia blooms" folklore. The only reliable predictor of crabgrass germination is soil temperature.

Crabgrass begins to germinate when soil temperature at 2-inch depth reaches 55°F for 3-5 consecutive days.

How to Track Soil Temps

1. A meat thermometer — Push it 2 inches into the soil in a sunny area. Check at the same time each day for consistency (mid-morning works best).
2. GreenCast by Syngenta — Free online tool that provides real-time soil temperature data by ZIP code: greencastonline.com/tools/soil-temperature
3. Your state's university extension — Many extension offices publish local soil temperature reports during spring

General Timing by Zone

💡 Apply when soil hits 50°F — This gives you a 5°F buffer before germination actually starts. Better early than late.

The Two Best Pre-Emergent Active Ingredients

Prodiamine (Barricade)

• Longest residual control of any pre-emergent — up to 6 months at full rate
• Available as granular (0-0-7 with Barricade) or liquid (Prodiamine 65 WDG)
• Yellow staining on concrete — use caution near driveways and sidewalks
• Best for split applications (half rate in early spring, half rate 6-8 weeks later)

Dithiopyr (Dimension)

• Unique advantage: Has limited POST-emergent activity on very young crabgrass (1-3 tillers)
• Available as granular or liquid
• Shorter residual than prodiamine (~3-4 months)
• Better choice if you missed your timing window by a couple weeks

Split Applications: The Pro Strategy

Instead of one heavy application, split your pre-emergent into two half-rate applications separated by 6-8 weeks. Here's why this is superior:

A split application ensures the chemical barrier is refreshed right when the first application is starting to break down — closing the gap that allows late-season crabgrass escapes.

Split Application Example (Prodiamine 65 WDG)

What If You Missed the Window?

If crabgrass has already emerged (you can see light green, crab-legged plants popping up), you need a post-emergent strategy:

Post-Emergent Options

1. Quinclorac — The most effective post-emergent crabgrass killer for cool-season lawns. Apply with a surfactant when crabgrass has 1-4 tillers.
2. Dithiopyr (Dimension) — Has limited post-emergent activity on very young crabgrass.
3. Mesotrione (Tenacity) — Works as both pre and post-emergent. Turns crabgrass white (bleaching mode of action) before killing it. Safe on most cool-season grasses.

⚠️ Do NOT use Quinclorac on St. Augustine, Bahia, Centipede, or Floratam varieties. It will damage these warm-season grasses. Always read the label.

The Pre-Emergent + Overseeding Conflict

Here's the catch-22 that frustrates every lawn enthusiast: pre-emergent herbicides prevent ALL seeds from germinating — including grass seed.

If you plan to overseed in fall, you need to plan your pre-emergent timing carefully:

Solutions

1. Use Tenacity as your pre-emergent — Mesotrione is one of the few pre-emergents that's safe to use at seeding. It provides 30 days of pre-emergent control while allowing grass seed to germinate.
2. Time your last pre-emergent to wear off by September — If you apply Prodiamine in early March, it should be broken down by fall overseeding time.
3. Accept the tradeoff — Some years you prioritize crabgrass prevention; other years you prioritize overseeding. Rarely can you maximize both.

Full-Season Crabgrass Prevention Calendar

The Bottom Line

Crabgrass prevention is 90% timing, 10% product choice. Get your pre-emergent down before soil hits 55°F and you've won the war before it starts. Miss that window, and you'll spend all summer playing defense.

Split applications with Prodiamine give you the longest protection. Track soil temps, not calendar dates. And if a few escape — Quinclorac is your cleanup hitter.