Crabgrass (Digitaria spp) is a major problem in both cool-season and warm-season turfgrasses As a general rule, crabgrass tends to be more of a problem in temperate climates and less of a problem in tropical regions. In temperate regions the most important and likely the first weed control practice of the year is the proper timing of a pre-emergent herbicide application for crabgrass.
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)