Walking the yard today, I noticed little buds on the Lilacs and Forsythias. From a phenological perspective, crabgrass will start germinating when the Forsythias start blooming. Our box turtles should emerge out of hibernation once the blooming begins.
Here's some other indicators:
When the yellow forsythia and crocus are blooming, it's time to prune roses and fertilize the lawn.
Crabgrass is an indicator of soil temperature. When the seeds germinate, the soil temperature will be at 55°F, at a depth of 4". This also coincides with the time Forsythias bloom.