How do you get rid of lawn weeds? | Ask MetaFilter: "Now modify your lawn care. You're probably cutting the grass the wrong height and frequency. If you cut the grass too short, it is weakened and unable to resist weeds (which get more light than they should). If you wait too long between trims, the amount you cut off is too much and weakens the plant, the long grass trimmings can cover up growing grass, and you get more plant growth at the expense of root growth. Try to cut to a slightly higher height (2.5-3' is generally good), more often, and your lawn will begin to get its legs back. The best defense against weeds is healthy turf.

Then, this fall, give it a winterizer appropriate for your area. Grass continues to grow over the winter, and a winterizer helps root growth. In the spring, give it a fertilizer with crabgrass preventer, before it begins to come up. You might end up with some bare areas; when it's warm enough, properly till and seed them."