Answer: A healthy lawn will crowd out weeds but new grass won't be able to compete with established weeds. So to get your lawn off to the best start possible, I'd kill off the weeds first and then overseed. You can start over completely by rototilling the area and raking out the weeds or you can use a weed killer on the lawn, rake out the old weeds, and overseed the entire lawn. Then in the early spring use a pre emergent herbicide to keep weed seeds from germinating. By this time next year you should have the greenest lawn on the block.
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