When to Apply Pre-Emergent in Hawaii

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 Hawaii

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 September 4 in Hawaii, and ends as it passes 65°F, typically around November 10. 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 Hawaii 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.

Check Your City

Timing varies within Hawaii: each city page tracks the live 2-inch reading from its nearest station against both windows. See when to apply pre-emergent in Honolulu, when to apply pre-emergent in Kapolei or when to apply pre-emergent in Hilo, or find your city on the Hawaii soil temperature page.