Target Pests / Scope
Crabgrass (the #1 reason homeowners use pre-emergents), annual bluegrass (Poa annua), goosegrass, foxtail, spurge, chickweed, henbit, annual sedges. Pre-emergents do NOT control established/perennial weeds - for those, you need post-emergent herbicides like 2,4-D or triclopyr.
Products and Recommendations
For crabgrass: Prodiamine (Barricade - longest residual), Dithiopyr (Dimension - some post-emergent activity on young crabgrass), Pendimethalin (Pendulum, Scotts Halts). Consumer products: Scotts Halts Crabgrass Preventer, The Andersons Barricade, Bonide Crabgrass and Weed Preventer, Jonathan Green Crabgrass Preventer plus Fertilizer.
Safety
Low mammalian toxicity for most pre-emergent herbicides. Water in after application (most products require 0.5 inches of water to activate). Keep pets off treated areas until watered in and dry. Pre-emergents do not kill existing plants or pose significant risk to mature turfgrass.
Detailed Guide
TIMING IS EVERYTHING:
The number one question: When do I apply pre-emergent for crabgrass?
The soil temperature rule: Apply when soil temperature reaches 55F consistently at 4-inch depth. Crabgrass germinates at 55-60F soil temp. You want the barrier in place BEFORE germination begins.
The forsythia trick: When forsythia bushes bloom (bright yellow flowers in early spring), soil temperature is approaching 55F. This is your visual cue to apply pre-emergent within the next 1-2 weeks. This folk wisdom aligns remarkably well with soil temperature data.
Regional timing guide:
| Region | Typical Application Window | Soil Temp Timing |
|---|---|---|
| Deep South (FL, Gulf Coast) | Late January - February | Soil rarely drops below 55F |
| Southeast / Mid-Atlantic | Late February - March | When forsythia blooms |
| Transition Zone (TN, NC, VA) | March - early April | When forsythia blooms |
| Northeast / Midwest | April - early May | When forsythia blooms |
| Northern states | Late April - May | When lilacs start budding |
Split application: For maximum crabgrass control, split your pre-emergent into two applications: full rate at the timing above, then half rate 6-8 weeks later. This extends the barrier through the entire germination window. Particularly important in the South where crabgrass season is longer.
Which product?
Prodiamine (Barricade): longest residual, best for single-application programs, yellow-tinted granules. Dithiopyr (Dimension): unique ability to control young crabgrass even after germination (up to 1-tiller stage), making it the most forgiving if your timing is slightly late. Pendimethalin (Scotts Halts): widely available, good efficacy, moderate residual. For most homeowners, Dimension (dithiopyr) is the safest choice because of its post-emergent rescue activity.
Fall pre-emergent: A lesser-known strategy - applying pre-emergent in early fall (September in the North, October in the South) prevents winter annual weeds like annual bluegrass (Poa annua), henbit, and chickweed. These weeds germinate in fall, overwinter, and take over lawns in early spring.