Crab Grass Preventer – When is too Early?

Q: Since it has been so warm lately, should I put out my crabgrass preventer earlier than normal? Several readers

A: To know when to apply a weed preventer you first have to know when the seeds you are trying to prevent are likely to germinate. Crabgrass seeds need several days of soil temperatures higher than 55 degrees in order to germinate. Most years in Atlanta, March 15 is the date experts recommend you apply the pre-emergent. Only the most shallowly buried crabgrass seeds will be sprouting then but many more will germinate in April and May. That is why other weed scientists recommend applying a pre-emergent in March and in May. The March application prevents the early-bird weeds. A follow-up application in May keeps an active layer of chemical right on the soil surface to catch the late-awakeners.

We often have warm periods in January and February but rarely as long as in recent weeks. It’s understandable that you might wonder if warm soil conditions have arrived early this year. The good news is that soil temperatures have not yet been consistently above 55 degrees so it is not time to limber up your spreader. How can a gardener determine how warm the soil is currently? The simplest way to find out is to visit the Automated Environmental Monitoring System.

This Web site monitors dozens of weather sites around the state. You can choose a site near your home to see the current soil temperatures, precipitation, air temperatures, etc. Obviously, conditions are different between Savannah and Suwanee but they are also markedly different between Griffin and Duluth. If you are Internet savvy, you can time your weed prevention activities precisely.

On the other hand, maybe there is no need to be so precise. The weed preventers that are commonly available to homeowners last a long time in the soil when it is cold. If you put out pre-emergent chemicals now, they will still be present when seeds are trying to pop up all over. My recommendation? Wait until the end of February to spread your pre-emergent product then re-apply in May.

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