Figure 1. Forsythia blooming
The ideal situation is to apply a preemergent herbicide just prior to crabgrass germination. At this point in time your herbicide will prevent germination and last longer through the growing season. Besides the traditional calendar date for application, soil temperatures and flowering ornamentals can be used to predict crabgrass germination. Below are some of those keys. It should be noted factors like turf cover, and soil moisture for example can influence crabgrass germination and emergence.
1) Soil Temperature (pre-emergent herbicide applications should be made prior to these occurrences (~ 3 C below the threshold))
Minimum soil temperatures of ~13 - 15 C) at the 2.5 cm depth at daybreak for 4-5 days are required for germination.
Mean soil temperature of ~16 - 19 C at the 2.5 cm soil depth are required for germination.
Soil temperatures greater than 23 C are required for significant crabgrass emergence
2) Phenotypic Keys for crabgrass germination (pre-emergent herbicide applications should be made 14-days prior to these events: Forsythia bloom withering (more applicable in northern temperate regions)
Daffodil (Narcissus spp.) bloom withering
Dogwood (Cornus spp.) bloom withering (more applicable in warmer regions)