Guide

When to put down pre-emergent: soil temperature and GDD

Short answer: crabgrass starts sprouting when the top inch of soil holds about 55°F, which tends to line up with about 200 growing degree days (base 50°F). Pre-emergent only works if it's down, and watered in, before that.

The soil-temperature trigger

“Germination begins when soil temperatures reach 55°F in the spring.”— Iowa State Extension “germinates in the spring when soil temperatures exceed 55 F for four consecutive days and nights.”— UT Extension W146 “when soils reach a daily average of 55 deg F for about 5 days at a 1″ soil depth then it is time to put out your preemerge.”— K-State

One warm afternoon doesn't count. What matters is the soil's multi-day average, which is why Fairsky estimates soil temperature from about five days of air temperatures (past and forecast) and labels it as an estimate. Its pre-emergent verdict calls roughly 50–58°F soil prime time and 58–65°F late but not lost.

Growing degree days (GDD)

“research suggests that either 200 GDD need to accumulate with a base of 50 °F”— Purdue Turf

A growing degree day is a running total of warmth: each day adds how far its average temperature ((high + low) ÷ 2) sits above 50°F, and cold days add nothing. Fairsky aims to have pre-emergent down by about 150 GDD, ahead of the 200 GDD emergence mark (the 150 lead is a Fairsky default).

Water it in

“need approximately 0.5 inch of rainfall or overhead irrigation within 24 to 48 hours after application to be activated.”— UT Extension W146 “Try to time the application to receive a ½ inch of rainfall or irrigation within 3-5 days of applying”— Iowa State Extension

Fairsky's verdict looks for a quarter to half an inch of rain in the next three days, and says hold off if more than an inch is due in 24 hours, since a downpour can wash granules off (that runoff line is a Fairsky default).

When each city usually reaches 200 GDD

From NOAA's 1991–2020 monthly normals (base-50°F degree days, summed from January 1): the month in which each metro's average year passes 200 GDD. Real springs run early or late, so use your city's live page for this year.

Sources

Quotes were checked on the linked pages in October 2026. Thresholds marked as Fairsky defaults are our own conservative choices, not citations. Product labels differ; your label wins.

Questions

At what soil temperature should I apply pre-emergent?

Iowa State Extension says crabgrass germination begins when soil temperatures reach 55°F in the spring, and K-State suggests applying when soils reach a daily average of 55°F for about 5 days at a 1-inch depth. Pre-emergent has to be down before that.

What is 200 GDD for crabgrass?

Purdue Turf cites research that crabgrass emergence follows about 200 growing degree days accumulated with a base of 50°F. Each day adds the amount its average temperature is above 50°F.

Does pre-emergent need to be watered in?

Yes. UT Extension says about 0.5 inch of rain or overhead irrigation within 24 to 48 hours activates it; Iowa State says half an inch within 3 to 5 days.

Live verdicts: Apply pre-emergent, by city

Fairsky is not a substitute for official warnings. For warnings, go to weather.gov.

Live forecasts: National Weather Service / NOAA (public domain), fetched by your browser. Climate averages: NOAA NCEI U.S. Climate Normals, 1991–2020 (public domain). Project thresholds: product data sheets, ACI and university extension guidance, linked on each page; your product’s label wins.

When to put down pre-emergent: soil temperature and GDD

The soil-temperature trigger

Growing degree days (GDD)

Water it in

When each city usually reaches 200 GDD

January

February

March

April

May

Sources

Questions