It is critical to know when the first crabgrass germination will occur so that preemergence herbicides can be applied just prior to crabgrass germination. The following is an article on the subject from Virginia Tech.
It is likely that higher than average winter temperatures will move our PRE herbicide application dates up a week or two as compared to average years, but the only way to really know this is by charting growing degree days. Growing degree days are calculated as the average of the maximum and minimum temperature for a given day minus the base temperature for crabgrass germination (50 F). Thus, a maximum of 75 and minimum of 27 gives us one growing degree day. If the value is negative, zero growing degree days are added to the running total. Crabgrass starts emerging when the running total reaches 70 to 140 units. You can also keep an eye on soil temperatures and expect emergence after 4 to 5 days of temps reaching 55 to 58 F at the 4" depth. Forsythia bloom is usually a good indicator in northern climates with crabgrass emergence occurring at 50% bloom drop but plants like Forsythia, dogwood, and daffodil will probably be less useful this year due to the sporadic warming trends in January and February.
Extracted and modified for 2008 from "Whacky Winter Weather Worries" in the Crop and Soil Environmental News, January 2007, by Mike Goatley, Shawn Askew, Brandon Horvath, Rod Youngman, and Erik Ervin http://www.ext.vt.edu/news/periodicals/cses/2007-01/winter_worries.html
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