The last few years, spreading patches of grassy weeds have become a big problem. There are several types, but they usually start out as a small patch and get bigger each year. Some spread by seed, but most have above ground spreading structures called stolons that are easily pulled up. Lawn mowers tend to spread the stolons increasing the number of these weedy patches. Depending on the type of weed there may be a chemical that can treat it. Applying a Roundup type herbicide to these green and actively growing weeds to kill them completely is the first step. You may have to wait a little while in the spring for these weeds to turn green before you apply the Roundup.  Be sure to read and follow the directions on the herbicide label.