Answer: Obviously, you'll want to keep the weeds from developing flowers and seeds, so hoeing or mowing them down on a consistent schedule will keep future generations from popping up in your lawn. I don't understand why Round-Up applications did not kill the weeds. It is one of the most efficient non-selective herbicides on the market. You might try another application. Your other option is to cover the area with plastic to solarize the soil. Do this now, while the summer sunshine is still high in the sky. This should kill the weeds and their seeds. In the fall you can rototill the area, rake out the old weeds and their roots, and then level and either seed or sod your new lawn. A thick, lush lawn will help crowd out weeds and spot treatment or using a weed and feed product should keep your new lawn relatively weed free.
Good luck with your new lawn!
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