If you’re worried about crabgrass taking over your lawn, now is the perfect time to apply a crabgrass pre-emergent herbicide. Pre-emergent herbicides work by preventing the germination of crabgrass seed, and must be applied within a specific window of time to be effective. Because crabgrass has an annual growth pattern where it lives and dies within the same year, the best way to deal with crabgrass is by preventing it from germinating.
We know the perfect time to apply pre-emergent herbicides because crabgrass seed germinates within a specific Growing Degree Day accumulation range. Growing Degree Day (GDD) is a phenological measurement determined by calculating the average of the daily maximum and minimum temperatures compared to a base temperature. While this type of math may be complicated, research and weather stations across the country keep track of GDDs year round. GDDs not only help us know when to apply a crabgrass preventative, but can help us know when trees will bloom, insects wake up, and seeds germinate.
If you’re interested in GDDs where you live, then take a look at the following links:
- Michigan State University GDD Tracker, with links to popular predictable events such as Japanese Beetle emergence
- UW-Extension Agriculture Weather Station, which focuses on growing degree days related to woody plant pests
And finally, what’s the deal with crabgrass anyway? This pesky weed is hard to control with herbicides during the growing season, and can be quite a nuisance for perfectionist lawn owners. Before treating your lawn, verify your weeds are crabgrass and not quackgrass, which is similar in appearance but different in growth habit. Pre-emergent herbicides will not control quackgrass.