I often say that I can tell when spring is here by when I get my first call about snakes. Well, I haven’t had that call... yet! The second sign of spring is when I get calls from homeowners wondering if it is too early to put on a crabgrass preventer treatment on their lawn. (It is!) That has happened, probably in response to advertisements on TV and radio encouraging people to make those applications now.
Let’s have a quick review of crabgrass biology to help understand how to use preemergence herbicides (crabgrass preventers) most effectively. The most important factor to control crabgrass is knowing the soil temperature.
If you take it yourself, you are getting a soil temperature at that moment. Some websites give you a more useful, average daily temperature or even the average temperature over several days. One of the easiest that’s updated daily is found at CropWatch: Soil Temperature Update, just select your closest location.
Now, back to the crabgrass biology class... and I promise we won’t dissect any earthworms or frogs or pigs like we had to in high school biology class!
Crabgrass is a warm season, summer annual grass. A summer annual plant comes up from seeds each spring and dies during the growing season or, at the latest, with a killing frost in the fall. A warm season plant won’t germinate until temperatures get warmer. In the case of crabgrass, it won’t germinate until the soil temperature is above 50°F and some of the seed won’t germinate until soil temps near 70°F.
How Preemergence Herbicides Work
Now that we know a little about crabgrass, let’s talk about how a preemergence herbicide works. As the name suggests, these products kill the target plant before they emerge, so the timing when they are applied is extremely important. They only kill very young plants as the seeds germinate, they will not control weeds that already have green growth above the soil surface.
Exception: Products containing dithiopyr (Dimension) or mesotrione (Tenacity) will provide control of newly emerged crabgrass.
The most important thing to remember is these herbicides are at full strength the day they are applied and lose their effectiveness over time. How fast they lose their effectiveness depends on several factors. The other thing you need to remember is the herbicide needs to move into the soil where the crabgrass seeds are germinating, usually in the top inch of soil.
Water It In
When applying a preemergence herbicide in the spring, regardless of whether it is combined with a granular fertilizer or if it is just the herbicide alone, it is important that it moves into the soil so it doesn’t sit on the surface. Herbicides left on the surface for 72 hours (3 days) or more start to break down because of the exposure to sunlight. If you don’t get rain in this period after application, it is important to apply a minimum of a half inch of irrigation to dissolve the product and move it into the soil.
Once in the soil, the active ingredient bonds to the soil particles and organic matter near the surface and kills the germinating seed. Microbes in the soil will also start to break down the herbicide, reducing its effectiveness. That is why it may be necessary to apply a second treatment in mid-summer to have season-long control. Remember, some crabgrass seeds will not germinate until soil temperatures get above 70°F.
So When Should I Apply a Preemergence Herbicide?
How do you put all of this together? Since crabgrass doesn’t germinate until soil temperatures are above 50°F, it doesn’t make sense to apply a crabgrass preventer until close to that time. This usually occurs in late April or early May. Unless the weather changes drastically, the soil probably won’t warm that much until May this year.
The first of May is a great time to make your FIRST fertilizer application in the spring. Applying fertilizer too early causes a rapid flush of growth in the spring, but can actually weaken your turf during stress periods in the summer. So you can apply a granular fertilizer that also contains a crabgrass preventer around the first of May and accomplish two chores at once.
A final thought... most preemergence herbicides cannot distinguish between a crabgrass seed and a turfgrass seed. DO NOT apply these products to areas you reseeded or overseeded until the new turf is up and has been mowed a time or two. Applying a preemergent herbicide too early could result in it killing the new turf as those seeds germinate.
Exception: Products containing the active ingredients siduron (Tupersan) or mesotrione (Tenacity) are safe to use on new bluegrass or fescue seedings.