Early spring is a good time to weed and reseed any weedy or bare patches in your lawn. If the weeds have become well established in these spots, when the soil has dried sufficiently you might want to consider tilling or cultivating to prepare a deep new seedbed that will enable new grass seedlings to develop deep, competitive roots. However, avoid tilling or cultivating areas that are heavily infested with perennial weeds that can reproduce from vegetative buds on the roots, such as Canada thistle and quackgrass. Cutting these plants into smaller pieces can result in even higher populations because a new plant can be produced from a piece of root as small as ½ inch in length. If you till these areas, rake carefully to remove as many root pieces as possible.
In the case of tough, deep-rooted perennials such as Canada thistle, quackgrass, or field bindweed, it may be more effective to remove all vegetation, including the weeds, prior to seeding the area by treating the area with a systemic herbicide such as glyphosate (e.g. Roundup) or dicamba, which will move down into the root system and kill the plant. If you wish to keep existing grasses, make spot applications when using glyphosate, avoiding contact with the grasses because glyphosate will kill any plant with which it comes in contact. A simple method of spot treating individual weeds is to apply the Roundup with a sponge or spray bottle. Be sure to wear rubber gloves when using these methods. If you use a sponge, discard it when you are done. Roundup is of relatively low toxicity to humans and the environment. Wear protective clothing when using any pesticide and follow label directions carefully.
Prepare the new seedbed carefully and apply a fertilizer that will give grass seedlings a boost to make them competitive during establishment. Select and sow high quality, weed seed-free grass seed that is suited to the growing conditions in your yard. The first plants to grow on bare or disturbed ground are weeds; regular mowing will enable the newly seeded grass to soon crowd out most of the annual weeds that will almost surely germinate in the new seeding. Be sure to prevent blooming of these annual weeds, because in many species seeds will form from the blossom, even after the blossom has been cut, and any seeds that are produced can remain viable in the soil for many years. Spot applications of glyphosate on any perennials that appear can help prevent reinfestation by those species.
References:
Danneberger, T.K. 1993. Turfgrass Ecology and Management. G.I.E. Inc., Cleveland, OH. 201 pp.
Handbook of Integrated Pest Management for Turf and Ornamentals. 1994. Leslie, A.R., ed. U.S. Environmental Protection Agency. Lewis Pub., Boca Raton, FL. 660 pp.