When to apply crabgrass preventer

Crabgrass is an annual: every plant grows from a seed that sprouts in spring. Pre-emergent crabgrass preventer stops those seeds from establishing, but only if it's in the soil before they germinate. Put it down too late and it does nothing. Put it down far too early and it breaks down before the main flush of germination.

The short answer

Apply between 250 and 500 growing degree days (base 32F), counted from January 1. That's Michigan State University's GDD Tracker model, and it's the timing Lawn Guardian uses. Follow up with a second application 6 to 8 weeks later.

How to count growing degree days

For each day, average the high and low temperature in Fahrenheit and subtract 32. If the result is positive, add it to your running total, starting January 1. A day with a high of 60F and a low of 40F adds 18. A cold week adds almost nothing, and a warm week adds a lot, which is why the total tracks what your lawn has actually experienced rather than the calendar.

When the window opened in 2026

We replayed this year's actual weather through Lawn Guardian's care engine for five real lawns:

Location	Window opens (250 GDD)	Window closes (500 GDD)	Crabgrass germination began
Oklahoma City, OK	Jan 15	Feb 12	Feb 25
Knoxville, TN	Feb 6	Feb 22	Mar 4
Kings Park, NY (Long Island)	Mar 11	Apr 1	Apr 13
London, ON	Mar 31	Apr 15	Apr 13
Ottawa, ON	Apr 18	May 4	May 16

The window closes a week or two before germination in most places, which is the point: the barrier has to be in place first. It spans three months from Oklahoma to Ottawa, so a single calendar date can't be right everywhere, and even in one place it shifts year to year with the weather.

Why not just use soil temperature?

The familiar rule of thumb is to apply before the soil reaches 55F. That's the right target, but near the surface, soil temperature swings with every warm spell. In the same 2026 replay, a trigger of "soil at 45 to 52F and rising" would have opened the window in the first week of January in Oklahoma City and Knoxville, and four to six weeks early on Long Island and in Ontario. Accumulated warmth doesn't spike on one mild afternoon.

Applying it

Common active ingredients are prodiamine (for example Barricade) and dithiopyr (for example Dimension). Dithiopyr also controls very young crabgrass that has just emerged.
Use a spreader for even coverage, then water it in lightly so it reaches the soil.
Pre-emergents stop grass seed from germinating too. Don't overseed in the same months; check the label for the reseeding interval.
A thick, well-fed lawn is the most durable crabgrass control there is: dense turf leaves nowhere for seeds to take hold.

Get told the week your window opens

Lawn Guardian tracks growing degree days at your address from the actual weather and forecast. It emails you when your crabgrass window opens, and again a few days before it closes. Free this season, no card needed.

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Sources: Michigan State University Extension: About GDD Tracker 4.0 · Purdue University Turfgrass Science: Use Growing Degree Days to Better Time Your Applications · University of Minnesota Extension: Crabgrass · Michigan State University Turfgrass Science: Crabgrass Preventer Timing with Soil Temperature