Weeds in your lawn are usually the result of poor grass growth. They invade lawns that are not thick enough to choke them out, so the first step in keeping weeds out of your lawn is correct maintenance of the grass. First, determine why your lawn hasn't been growing well. Your lawn may also need core aeration if the soil has become too compacted.

When the growing conditions for your grass have been improved, it's best to follow up with a broadleaf weed killer. The best time to apply it is in early May because the weeds will be growing rapidly then and will absorb it quickly. The next best time is in September. Temperatures should be in the mid 70's. Do not apply weed killers at temperatures higher than 70 degrees. Make sure that you choose a weed killer designed specifically for lawn weeds. Others may kill the grass as well.

Apply granular weed killers or weed and feed fertilizers with a spreader to wet grass and weeds. The granules must stick to the leaves of the weeds in order to work properly. If you have recently re-seeded your lawn, and you wish to apply a granular pre-emergent weed killer in very early spring, make sure to choose one labeled specifically for newly seeded lawns.

If you use a liquid weed killer, follow the directions exactly when mixing it and apply only the amount specified with a sprayer. It's also a good idea to have a set of measuring spoons that are only used for weed killers and insecticides. Never use the same measuring spoons that you use in the kitchen. Apply liquid weed killers on calm days to avoid the spray blowing onto desirable plants. Broadleaf weed killers cannot tell the difference between a dandelion and your prize-winning tomatoes.

Some pre-mixed weed killers contain the active ingredients Banvel and Dicamba. While very effective on tough weeds, they are also harmful to trees and shrubs with shallow root systems. Keep herbicides with these ingredients away from trees and shrubs.