Soil Temperature
The temperature of the soil at a 4-inch depth — the key timing indicator for pre-emergent applications. Crabgrass germinates when soil temperatures consistently reach 55°F. Pre-emergents should be applied before this threshold, ideally when soil temps are 50–53°F.
Calendar dates alone are unreliable guides for pre-emergent timing — soil temperatures vary by up to several weeks depending on the year. A soil thermometer (available at garden centers) takes the guesswork out of timing.
How to check: Insert a probe thermometer 4 inches into the soil in a sunny area of your lawn. Check in the morning for several consecutive days.
Target: Apply pre-emergent when readings are consistently 50–53°F. Crabgrass germinates at 55°F, so you want the barrier in place before that threshold.
Online resources: Many extension services and weather websites publish soil temperature maps — searching "[your state] soil temperature map" will usually find a local resource.
More on Pre-Emergent
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