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What is the best soil temperature and at what depth is it measured for the application of preemergence crabgrass killer?

Last Updated: February 13, 2008

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Soil temperature for lawn weed control is measured at a depth of 4 inches. Crabgrass begins germinating after several days of 55-degree soil temps. Forsythia blooms at the same temperature, giving a good plant indicator for applying a crabgrass preemergent. The standard rule of thumb is to apply preemergent in the time period between when the forsythia and the lilacs bloom.

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