With the unseasonably warm weather, we’ve been getting this question a lot lately. Crabgrass is an annual weed and must come back from seed each year. Since, crabgrass must germinate from seed each year, we can control crabgrass by applying a pre-emergent herbicide to the soil that will kill young germinating crabgrass seedlings. This is the step 1 of most 4 or 5 step lawncare programs. For the pre-emergent herbicide to be effective, it MUST be applied BEFORE the crabgrass seed germinates. (with one exception….Dimension, dithiopyr, has a little post-emergent activity, so it can be put down, right as the crabgrass is germinating and it will kill young, 1-2 leaf, crabgrass plants and prevent new ones from germinating.)
We often talk about using environmental cues to determine when is the best time to apply our pre-emergent herbicide. Typically, the time to apply pre-emergent herbicides is when the forsythia bush flowers or when the redbud tree blooms. During most years, I think the forsythia bush blooms fairly early in the spring. So I usually tell people when they see the forsythia blooming it is not too late, crabgrass has not germinated yet….but it is time to go get your preventer and get it applied soon.
Another option is to use soil temperature. Crabgrass typically germinates when the soil temperature is between 57-64F at a one inch depth. K-State has a weather data library and many of the locations have soil temperature probes. You can check out the weather station at the Olathe Horticulture Research Center and you can see the soil temperature has been climbing over the last week from 57F to a high of just over 64F on Friday. To find the soil temperature at a station near you, go to the Weather Data Library. There is a report of soil temperatures for the last week in the left hand column, but it is at the 4″ depth. If you click on the first link, Kansas Weather & ET Data a map will appear. Click on the station on the map that is closest to you (or use the drop down menu, top-right). Choose your date range, and place a check mark in the 2″ soil temperature check box. Hit the Submit Query and your report will pop up.
So What To Do This Year?
I haven’t looked at all the weather stations around KS, but I’m gonna guess that now is the time that everyone should be getting their pre-emergent down. Yes it is about 2 weeks earlier than the average year. And I’m not a weather man, it could get really cold soon and stay cold, but it might not. So I would get your Step 1, your pre-emergent down on your lawn now.
Remember, crabgrass preventers also kill turfgrass seedlings. So if you are going to seed, recently seeded, or if you dormant seeded DO NOT apply crabgrass preventers.
There are a variety of pre-emergent chemicals on the market and most of them work. But in our tests at KSU and most of the research I’ve read, Dimension (dithiopyr) and Barricade (prodiamine) consistently provide the best control of crabgrass.
Like I said above, I’m not a meteorologist, but according to NOAA it looks like the next 6-10 days and the next 30 days are going to be above the average temperature for this time of year. So I still stand by my recommendation to get you Pre-emergent on now.