Answer: Lesco contains the broadleaf weed killer 2,4-D which can cause problems with your Floratam if you apply it in the summer heat. Wait until this winter when temperatures are cooler or you could damage your lawn. (The label should contain this caution.) For future reference, Scott's Bonus S Weed & Feed can be applied when temperatures are over 80F.
Crabgrass is an annual weed and it will die out at the end of the season, but it will leave lots and lots of seeds behind. The best approach to controlling this pesky weed is to apply a pre emergence in February or March to keep the weed seeds from germinating. Chickweed is really difficult to control so Bonus S might be your best approach. Be sure to read and follow label directions so make sure the timing is right on your applications.
Best wishes with your turfgrass.
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