A dense, vigorous lawn is your best defense.
Weeds are opportunists. They will find bare spots or places where your grass is weak and exploit those problems to their advantage.
Perennial weeds (those that regrow from their roots every year) can spread and make a lawn unsightly. Annual weeds (those that die at the end of the season and come back from seed the next year) can leave bare spots that are vulnerable to runoff.
No matter what weeds you have, the first line of defense is preventive practices. Try these options to get at the root of the problem first, before resorting to herbicides.
Mow high. Do not mow grass shorter than recommended for the species you grow. Mowing at 3 inches or higher helps grass shade out weeds and encourages a thicker, more competitive turf. See other sections of this site to make sure that you are using the right grass species, fertilizing and watering correctly, and generally doing all you can to encourage healthy grass.
Reduce compaction. Pay special attention to heavily used areas and sections next to pavement. Weeds can gain a foothold in these spots and spread to the rest of the lawn if it is weak.
Repair bare spots by raking in seed in early spring so that the new grass can compete with the weeds that are sure to come up.
If lawn is thin, fertilize to improve density.
Hand weed. This is easiest when the soil is slightly moist. Check your garden center or catalogs for tools that help get tough perennials out by their roots.
Let the weeds be your guide. If weeds dominate an area, it's likely that there is something wrong with either the growing conditions or your lawn practices. Dense stands of prostrate knotweed (Polygonum aviculare) are a good sign of soil compaction. Don't just pull out the weeds. Relieve the compaction. Violets (Viola spp.) are a good sign of low light levels. One solution might be to seed shade-tolerant fine fescues.
If you use herbicides...
Use the right product at the right time. Follow label directions and keep accurate records.
Choose products that have the least potential for leaching into groundwater. More highly water-soluble materials have the highest potential (e.g. 2,4-D, dicamba, MCPP).
Use extreme caution when handling materials close to wells and impervious surfaces where runoff may enter storm sewers.
To avoid volatilization and drift, which release pesticides into the air, do not spray when temperatures are high or it is windy.
To help prevent polluted runoff, do not apply pesticides when heavy rains are expected or the ground is already saturated or frozen.
Empty containers should be triple rinsed and disposed of properly. Unused materials should be returned in the original container to authorized hazardous materials collection sites.
The types of herbicides include:
Most common for crabgrass and goosegrass.
Applied to soil before weeds are expected.
Have low solubility and bind to organic matter. Have high runoff potential if not watered in properly.
Most common for perennial broadleaf weeds.
Applied after weeds have emerged and are actively growing.
Have high solubility and do not bind to organic matter. Have high leaching potential. Avoid application before irrigation or rain.
Kill or injure all plants they come in contact with.
Used to kill vegetation before reseeding.