These preemergence herbicides must be applied and watered in (through rainfall or irrigation) prior to crabgrass germination to be effective. Two growing degree models are available to help you time your preemergence application.
The first GDD model is for application timing. The link below helps you determine what stage you should be in of your applications. These stages are designed for turf professionals. Homeowners should target the early or optimum timings.At the website for this model you will see the map with areas of the state colored into five different stages. Here is a key to what these stages mean.
- Done = this means that you should be done making your preemergence herbicide application for crabgrass. If you are at this stage but you haven't finished applying your spring preemergence herbicide, you should consider using dithiopyr (Dimension).
- Late = this means that crabgrass might be germinating now. You should finish your applications or consider switching to a preemergence herbicide containing dithiopyr.
- Optimum = optimum timing for a preemergence herbicide for crabgrass control.
- Early = denotes the start of preemergence herbicide applications for most lawn care companies. It is not too early to apply though as preemergence herbicides do not breakdown in the soil until late spring when soils warm and microbial activity increases.
- Under = denotes that it is very early in the growing season. While this may be called "under", research shows that these very early spring preemergence herbicide applications provide good crabgrass control.
See the application timing GDD model here: http://www.gddtracker.net/?model=7&offset=0&zip=47905
The second growing degree model estimates crabgrass germination. Research suggests that 200 GDD need to accumulate with a base of 50 °F before crabgrass germinates (source: Dr. Ron Calhoun). See this model here: http://www.gddtracker.net/?model=10&offset=0&zip=47905. More details on how to predict crabgrass germination, including other theories and approaches, can be found at last year's blog posting: When Will Crabgrass Germinate?
For more information on controlling crabgrass, see these two Purdue publications.
Midwest Regional Turf Foundation for their financial support of this GDD Tracker website for use by all Indiana turf professionals.
Dr. Aaron Patton, Turfgrass Extension Specialist