Preemergence herbicides prevent emergence of crabgrass plants. These products must be applied prior to crabgrass germination which on an average year could occur as early as April 1 in southern Indiana and three or more weeks later in northern Indiana. It is essential to apply these products early in spring prior to crabgrass germination.
Last year we had a warm spring and crabgrass germinated earlier than normal throughout the state. This year seems much cooler compared to last year. Our average temperatures in West Lafayette are 8 °F cooler than last year. In the past month, our air temperatures have averaged 1-3 degrees cooler than normal (30-year average) in Northern Indiana and 1-2 degree warmer in Southern Indiana.
There are several different ways to try and anticipate when crabgrass might germinate.
Soil Temperature: Most will try and gather data on soil temperature. The 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.
Air Temperature Using Growing Degree Days: However, it is often inconvenient to obtain soil temperature data and often easier to track air temperature data. Using a growing degree day (GDD) model based upon air temperatures, research suggests that either 200 GDD need to accumulate with a base of 50 °F (https://gddtracker.msu.edu/?model=10&offset=0&zip=49001 ) or that 4 consecutive days with an average daily temperature of 64 °F. Currently, the models show that crabgrass still has not germinated throughout the northern-half of Indiana which is consistent with our field observations. This still allows for the application of a preemergence herbicide in these areas if not yet treated. If you live in southern or central Indiana and you still have not treated for crabgrass with a preemergence herbicide, it is still possible to control crabgrass if you choose a product that contains dithiopyr – the only preemergence crabgrass herbicide that also has some early postemergence activity.
Plant Phenological Indicators: The flowering of other 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 typically be in full bloom prior to crabgrass germination and flowers will wither near crabgrass germination. Other plants common in the landscape that bloom before crabgrass germinates include saucer and star magnolia, and Bradford Callery pear. The initiation of redbud blooms are also a good indication of when crabgrass may germinate. 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.
Aaron Patton, Assistant Professor/Turfgrass Extension Specialist