Herbicides are used to control unwanted plants in many different locations. Postemergence herbicides are sprayed on actively growing weeds. In turfgrass, several herbicides are used to control weeds before they germinate and begin to grow. These are called "preemergence herbicides" and are commonly sold as "weed preventers." They control germinating weed seeds and subsequent growth. Therefore, to be effective, preemergence herbicides must be present in the upper soil surface before weed seeds germinate.