What is a weed?

A weed is a plant that easily moves in and colonizes new sites, usually following some type of disturbance. A weed is a plant out of place.

The weeds in vernal pool grasslands were brought to California from other parts of the World. The plants and animals which controlled their populations in their homelands do not live here. This allows the weeds to grow out of control and displace plants and animals native to California.