Answer: I'm afraid you'll have to decide which is more important - overseeding your thin lawn or applying a crabgrass preventer. If you use crabgrass control now, you should not overseed until this fall. If you overseed now, you shouldn't use crabgrass contol now.
Crabgrass starts germinating when soil temperatures are 50 to 55 degrees F for ten or more days and germination can take place over 6 weeks. Pre-emergence herbicides provide excellent weed control for crabgrass. The compound doesn't prevent the weed seed from germinating, but kills the newly sprouted seedling. Late April to early May is an ideal time to apply pre-emergence control. Proper lawn maintenance practices limit crabgrass invasions. A dense stand of turfgrass prevents the weed from germinating and establishing. Fertilize at the end of May and remember that fall fertilizing, overseeding, aeration and thatch control can limit spring problems.
Best wishes with your lawn.
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