With all of the recent winter weather, a summer lawn may be the last thing on your mind. However, now is the time to start thinking about controlling summer annual weeds, such as crabgrass.
Crabgrass germinates in the early spring, grows throughout the summer, and dies off in early fall. By the time you see it invading your lawn in early summer it can be difficult to control. The easiest way to control crabgrass in your summer lawn is to apply a pre-emergent (preventative) herbicide in early spring.
Pre-emergent herbicides are chemicals that prevent the germination of weed seeds in your lawn. Most are very easy to apply and come in either granular or liquid forms. There are many different brands of pre-emergent herbicides that can be found at any local garden supply store. Make sure to read the label on the package to make sure the product can be used on your specific grass species.
In the past, March 15 was the date usually given as a good rule of thumb for applying pre-emergent herbicides. However, this date may no longer be viable with our changing weather patterns.
Crabgrass seeds begin to germinate when the soil temperature warms up to around 55 degrees. During a very warm winter, this could mean that soil temperatures could reach 55 degrees before the end of February, and all the money you just spent applying herbicide was wasted.
Because we know crabgrass seed germination is based on soil temperatures, monitoring local soil conditions is the best method for planning pre-emergent herbicide application. Luckily, the University of Georgia runs a statewide network of weather stations, called the Georgia Automated Environmental Monitoring Network. All of these stations collect data on all of the important weather conditions, including soil temperatures.
We have two stations located around us that are very useful for gathering soil temperature data. One is located in Lumpkin County at Three Sisters Vineyards, and the other located by Lake Lanier in Hall County. Both of these stations can be found by visiting www.georgiaweather.net and clicking on the red dots for each of these locations.
Soil temperature measurements at these stations are taken at two, four and eight-inch depths. For the purposes of crabgrass germination, I recommend observing the two-inch readings for pre-emergent herbicide decisions. The Georgia Weather website has a fantastic tool, located under the "Soil Temperature Calculator" tab, which will show you the average soil temperature during whichever date range you choose.
I would recommend applying your pre-emergent herbicides when the averagee soil temperature at two-inches rises above 50 degrees. According to the weather stations, the soil temperature is around 46 degrees in Hall County and 44 degrees in Lumpkin County. We are not quite in the critical temperature range for crabgrass germination yet, but several days of above-average temperatures could put us in that range quickly.
If you are not able to go by soil temperatures, applying pre-emergent herbicides no later than March 1 is recommended by the University of Georgia.
Clark MacAllister is the Dawson County extension agent. For more information, call (706)265-2442.