The best way to control crabgrass is to kill it before it grows with a pre-emergent herbicide, but when is it too late to treat? Unfortunately, there’s no single definitive answer to this question. The short answer is to apply it before the crabgrass has a chance to germinate. The longer answer is that it depends on the weather and which pre-emergent is being used. Pre-emergent herbicides work by creating a chemical barrier that kills the crabgrass when it germinates. Some pre-emergent products provide some post-emergent control as well. Determining when a seed germinates depends on many variables in the environment. For crabgrass, the biggest variable is temperature. Crabgrass only germinates at sustained soil temperatures above 57°F at a one-inch depth. The best way to track the soil temperature is with growing degree days (GDD). Because it is tough to track soil temperatures without specialized equipment, we can use air temperature. Air temperatures will usually need to be at around 50°F for 200 degree days for germination.
200 days! That sure seems like a long time! That’s a common misconception that sometimes even confuses our office staff, but GDD isn’t just based on a calendar. GDD is based on a formula calculated by monitoring the median temperature. When the median temperature (High plus low divided by 2) for a day is above the range needed for germination, the number of degrees above that point is added to the GDD. For example, if the air temperature is at 52°F one day, 53°F the next day, and 49°F the day after that we have 5 GDD for that time period (there’s no such thing as negative degree days because plants can’t un-grow). It’s important to remember that areas in full sun or next to pavement will be warmer, and areas in the shade will be cooler than the average, so you have to take that into account with your own lawn.
(GDD Formula1)
All this math is a major pain just to find out if it’s too late to apply your pre-emergent crabgrass control. The rule of thumb that many landscape professionals used to go by was when the forsythia starts blooming. These days we have meteorologists help us out. To track the GDD yourself go to https://www.wunderground.com/history/ and follow these steps2:
- Enter your zip code (not all locations are included, check nearest weather station to your site) and hit enter
- Click the “custom” tab/button below the date
- Enter the start date below the word “from” (ex. Jan. 1) and the end date below the word “to” (current date)
- Hit the get “history” button
- Read your growing degree days (base 50) in the ‘Sum’ column (=Cummulatlive DD to date for the year)
Of course, the easiest way to make sure your pre-emergent crabgrass control gets applied on time is to call MRW Lawns at 301-870-3411 and let us take the work and worry out of having a real wonderful lawn.
Sources:
1. http://agron-www.agron.iastate.edu/courses/Agron541/classes/541/lesson02b/2b.1.1.html
2. https://extension.umd.edu/ipm/landscape-and-nursery-ipm-alerts
P.S. If the growing degree days are over 200 in your area, don’t panic! There are post-emergent products that can help control crabgrass, even when fully grown; and in the worst-case scenario, crabgrass is an annual weed, so you’ll get another crack at it next year!