Spring · barrier goes down
April 29
April 15 (warmest parts) to May 14 (coolest)
When to Apply Pre-Emergent in New Hampshire
In New Hampshire, spring pre-emergent typically goes down around April 29, when 2-inch soil first holds 50°F, and must be in place before soil reaches 55°F, typically by May 14. Fall pre-emergent for poa annua follows as soil cools, typically from July 27.
Typical Spring Pre-Emergent Window in New Hampshire
Spring · must be down before
May 14
May 7 (warmest parts) to June 5 (coolest)
The window opens when 2-inch soil first holds 50°F and the product needs to be watered in before soil reaches 55°F, the point where crabgrass seed germinates. Across New Hampshire that first crossing runs from April 15 in the warmest parts to May 14 in the coolest.
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 New Hampshire 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. In New Hampshire the crossing typically arrives around May 14, earlier in the warmest parts of the state and later in the coolest. 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 New Hampshire
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 July 27 in New Hampshire, and ends as it passes 65°F, typically around September 9. 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 New Hampshire 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 New Hampshire: each city page tracks the live 2-inch reading from its nearest station against both windows. See when to apply pre-emergent in Manchester, when to apply pre-emergent in Nashua or when to apply pre-emergent in Concord, or find your city on the New Hampshire soil temperature page.
Typical dates are computed from historical 2-inch soil temperature averages at 3 monitoring stations in New Hampshire; your own window depends on your nearest station's readings.