Once your grass is well-established, you can encourage its growth and discourage weeds by applying a combination of fertilizers and herbicides. When you should apply these substances and how much you should apply depends on several factors, like where you live and the extent of your weed problem.
Fertilizer can help your lawn grow thick and lush, but if it's not used properly, it can actually damage the grass. A slow-release nitrogen fertilizer is best, and no more than 1 pound (0.45 kilograms) of nitrogen should be spread per 1,000 square feet (93 square meters). It should be applied early in the season when the turf begins actively growing, so the timing varies among regions. Fertilizer should not be applied too early or late, however, as lingering cold or early heat can stress the grass. Check the packaging to see when and how much you should water after applying the fertilizer.
Herbicides must also be used with care, as their effectiveness often depends on when they're used. If you have a widespread weed infestation, it's best to apply a pre-emergent herbicide to your lawn before the seeds germinate in the spring. Be aware, however, that you can't use this treatment if you plan to plant new grass, as the herbicide will also prevent those seeds from germinating. For more isolated problems, spot treating with a non-selective herbicide should be enough to do the trick. Ultimately, the best way to discourage weeds is to have a thick, healthy lawn.