Once the first warm days arrive, the topic of discussion becomes crabgrass. Crabgrass emergence is well correlated with soil temperatures and degree day accumulations. Mike Fidanza and Peter Dernoeden at the University of Maryland proposed the following from their research (Fidanza, M.A., P.H. Dernoeden, and M. Zhang. 1996. Degree-days for predicting smooth crabgrass emergence in cool-season turfgrasses. Crop Science 36:990-996.)
Ave. Min. soil temp. at which initial appearance is observed =54 F
Ave. mean soil temp. at wich initial appearance is observed = 60 to 64F
Ave. Min. soil temp. at which major emergence occurs = 60 to 70F
Ave. mean soil temp. at which major emergence occurs = 73 to 75 F
Soil temperature at a 1 inch depth is used for these measurements. Measuring soil temperatures first thing every morning is the best way to get this number.
Now regarding crabgrass germination and preemergent herbicide applications, in Southern Delaware the window is generally March 1 to April 1, northern Delaware, March 15 to April 15,
Regarding soil temperatures, a rule of thumb is 3 consecutive night soil temperatures above 50 F. When soil temperatures consistantly stay above 50 F, pre-emergent herbercides, if they are to be applied, should be down or going down.
Degree days can also be used. Degree days for crabgrass germination using a base of 53.6 F are as follows:
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