Soil Temperature: Research says that crabgrass begins to germinate when the average daily soil temperatures reach 57 to 64 °F at a one-inch depth although large quantities of crabgrass seedlings will not start germinating until soil temperatures increase to 73 °F or above at a one-inch depth (Fidanza et al., 1996). Soil growing degree days (in contrast to the air temperature model highlighted below) have also been sued by others to predict large crabgrass emergence (Myers et al., 2004). The problem with using soil temperatures to predict germination is that most turf managers do not have access to daily soil temperature averages or are not collecting data with their own stations for their location. Going outside and collecting a soil temperature measurement with a thermometer provides only an instantaneous measurement and not an average over time.
Air Temperature Using Growing Degree Days: Because it is often inconvenient to obtain soil temperature data and often easier to track air temperature; consider using a growing degree day (GDD) model based upon air temperatures. Research suggests that 200 GDD need to accumulate with a base of 50 °F (http://www.gddtracker.net/?model=10&offset=0&zip=47905) before crabgrass germinates (source: Dr. Ron Calhoun). Currently (March 16, 2012), the models show that crabgrass still has not germinated in southern Indiana but that it is about to emerge. This still allows for the application of a preemergence herbicide if not yet treated.
Plant Phenological Indicators: The flowering of landscape plants can also be used as a good estimate of when crabgrass might be germinating. Many are aware that forsythia is traditionally considered a good plant to indicate that crabgrass will soon start germinating. Forsythia will be in full bloom prior to crabgrass germination and forsythia flowers will wither near crabgrass germination (Masin et al., 2005; Cardina et al., 2011). Most think that crabgrass germinates when forsythia blooms, but this is false. Instead, turf managers should use forsythia blooms as an encouragement to hurry up and get their preemergence application made before crabgrass begins to germinate. Other plants common in the landscape that bloom before crabgrass germinates include saucer and star magnolia and Bradford Callery pear (Cardina et al., 2011). The initiation of redbud blooms and sometimes crabapples are also a good indication of when crabgrass may germinate (Cardina et al., 2011). However, having said all this, researchers have also documented that ornamental plant flowering is not always a consistent predictor of crabgrass germination, especially with forsythia (Fry et al., 2001).
- Cardina, J., C.P. Herms, and D.A. Herms. 2011. Phenological indicators for emergence of large and smooth crabgrass (Digitaria sanguinalis and D. ischaemum). Weed Technol. 25:141-150.
- Fidanza, M.A., P.H. Dernoeden, and M. Zhang. 1996. Degree-days for predicting smooth crabgrass emergence in cool-season turfgrass. Crop Sci. 36:990-996.
- Fry, J., S. Rodie, R. Gaussoin, S. Wiest, W. Upham, and A.Zuk. 2001. Using flowering ornamentals to guide application of preemergence herbicides in the Midwestern U.S. International Turfgrass Soc. Res. J. 9:1009-1012.
- Masin, R., M.C. Zuin, and G. Zanin. 2005. Phenological observations on shrubs to predict weed emergence. Int. J. Biometeorl. 50:23-32.
- Myers, M.W., W.S. Curran, M.J. VanGessel, D.D. Calvin, D.A. Mortensen, B.A. Majek, H.D. Karsten, and G.W. Roth. Predicting weed emergence for either annual species in the northeastern United States. Weed Sci. 52:913-919.