When to Apply Pre-Emergent in Louisiana
Pre-emergent in Louisiana is timed by 2-inch soil temperature: spring crabgrass preventer goes down as soil warms from 50°F toward 55°F, and fall poa annua preventer as soil falls back through 70°F. Your city page tracks the live reading against both windows.
Typical Spring Pre-Emergent Window in Louisiana
Spring pre-emergent goes down when 2-inch soil first holds 50°F and must be in place before soil reaches 55°F, where crabgrass seed germinates. Louisiana does not yet have enough qualified monitoring stations for a typical statewide date; your city page tracks the crossing from its nearest station.
Spring pre-emergent and spring seeding are an either/or on the same turf: a standard pre-emergent blocks grass seed for the length of its reseeding restriction, which covers the spring seeding window. Mesotrione is the one cool-season exception. For a lawn being seeded this spring, see when to plant grass seed in Louisiana and the spring overseeding guide.
The 55°F Rule
Crabgrass seed germinates once 2-inch soil holds 55°F for several consecutive days. A pre-emergent works by forming a barrier the seedling cannot grow through, so it must be down and watered in before that crossing; applied after it, the product does nothing for seed that has already sprouted. The spring pre-emergent guide covers the research behind the threshold and the growing-degree-day cross-check.
Fall Pre-Emergent Timing for Poa Annua in Louisiana
Annual bluegrass (poa annua) germinates as soil cools in late summer and fall. The ideal application band opens as 2-inch soil falls back through 70°F, typically around October 30 in Louisiana, and ends as it passes 65°F, typically around November 14. Applications stay worthwhile until soil drops to about 55°F, after which most of the season's poa annua has already germinated.
Fall pre-emergent and fall seeding are an either/or on the same turf: the barrier that stops poa annua also stops grass seed. A lawn being overseeded this fall skips the fall pre-emergent; see when to plant grass seed in Louisiana and the fall pre-emergent timing guide.
Product Selection and Application Tips
Look for products containing prodiamine, dithiopyr, or pendimethalin. Dithiopyr has mild post-emergent properties and can suppress very young crabgrass even if applied slightly late. Weed-and-feed products that pair fertilizer with a pre-emergent follow the same soil-temperature window: the preventer half only works if it is down before the 55°F crossing. Water lightly within 24 hours of applying to move the product into the top layer of soil where weed seeds germinate, and keep to the label's annual maximum.
Common products
- Liquid, needs a pump sprayer: Prodiamine (Barricade)
- Granules, for a spreader: Scotts Halts (pendimethalin)
Check Your City
Timing varies within Louisiana: each city page tracks the live 2-inch reading from its nearest station against both windows. See when to apply pre-emergent in New Orleans, when to apply pre-emergent in Baton Rouge or when to apply pre-emergent in Shreveport, or find your city on the Louisiana soil temperature page.
Typical dates are computed from historical 2-inch soil temperature averages at 2 monitoring stations in Louisiana; your own window depends on your nearest station's readings.