If your yard looks like this, you have a big problem. All of the seeds on each of the Poa annua plants is a potential new one next spring. Immediately cut this grass and bag it. Keep cutting and bagging it until it dies out, and that should be in late May or early June. Then, in mid October, go to a good landscape supply company like Ewing or John Deere, and buy a pre-emergence especially for Poa annua control. If your lawn looks as bad as this one, you will probably need two applications, two to three weeks apart. That should really reduce the amount of weeds next spring, but it will probably take several years for total control.