A herbicide is a chemical substance designed to kill unwanted plants, especially weeds. Farmers spray herbicides on their fields to eliminate weeds that would otherwise compete with crops for water, nutrients, and sunlight. A cornfield treated with herbicide can produce a much larger harvest because the corn plants don't have to fight weeds for resources.

Gardeners might use herbicides to keep driveways clear of dandelions or prevent crabgrass from invading their lawns. Cities use herbicides to manage vegetation along roads and railroad tracks.

Some herbicides kill everything green they touch, while others target specific types of plants. Scientists have even developed crops that resist certain herbicides, allowing farmers to spray entire fields without harming their crops. While herbicides help produce food efficiently, people debate their environmental effects, since chemicals that kill weeds can sometimes affect other plants, insects, or water sources. Many farmers and gardeners try to use herbicides carefully and only when necessary.