If you plan to use Round-Up to remove weeds from an area and reseed it immediately after the weeds are dead, use a variety of Round-Up that contains only glyphosate. Do not use a variety that contains pre-emergent herbicides, such as oxyflurofen, or it will prevent your seeds from germinating. And do not use extended control formulations of Round-Up that contain dithiopyr, or you'll have to wait six to eight weeks after applying Round-Up before reseeding.
Wait for Vegetation to Die
After applying Roundup following the manufacturer's application instructions, wait seven to 10 days for the vegetation to die down completely before beginning the reseeding process. The top growth of the treated weeds should be completely brown, wilted and obviously dead to ensure that they will not regrow.
Use an electric or gas-powered rototiller to till the soil in the treated area. Set the tines of the rototiller to their greatest depth, and make two passes at right angles to each other with the tiller. This will turn and loosen up the soil, loosen the roots of the dead vegetation and help to prepare the seedbed.
Rake and Remove Dead Plant Debris
Use a garden or crab-grass rake to remove the dead plant debris. If the tines of the tiller could not be set deep enough to loosen the roots of some weeds, pull them manually or dig them out with a shovel. It is important to remove the dead vegetation in case small amounts of the roots were unaffected by the Round-Up; weeds can regrow from a small piece of root. After all the dead plant materials are removed, rake the seedbed smooth, and plant seeds as usual.