Herbicides are weed / plant killers designed to kill unwanted plants, such as dandelions in your lawn. Although herbicides are chemicals designed to kill certain unwanted plants, using them introduces another chemical into the storm water system through runoff from your yard and therefore into our watersheds.
These chemicals, although helpful in getting rid of weeds in your lawn, may also kill beneficial water plant species called phytoplankton. These small plants provide food source for many fish and birds. Killing them could affect the entire food chain and disrupt the ecological balance needed to maintain healthy streams and lakes.
Many weeds grow early in the season before the grass has a chance to grow in and crowd them out. If you give the grass proper treatment and mow occasionally, these weeds will eventually die out. Also, most herbicides such as the Weed and Feed fertilizers serve to control only the short lived weeds such as dandelions and will not control crabgrass or goose grass. The best tactics against weeds are to hoe or pull them up by the root and to keep your grass in great shape so it won't let the weeds have a chance.