A pre-emergent herbicide is a chemical that stops weeds from growing before they appear. It is often used as a preventative measure. For example, if a certain type of weed (such as crabgrass) is likely to take root in a lawn, a pre-emergent herbicide keeps it from growing.
Consumers should carefully read the manufacturer's guidelines before spraying a pre-emergent herbicide. Some of these herbicides might damage existing plants or lawns. It is also important to spray at the right time and keep in mind that most of these herbicides only last for six to eight weeks before the protection wears off.