Herbicides are chemicals that retard or kill plant growth.
One of the ways herbicides are categorized is in terms of the time they are applied. These weed killers are applied before the weeds sprout, which prevents weed germination while allowing desirable plant growth to continue normally. Pre-emergent herbicides are usually applied in a granule form, which permeates the soil to about an inch or so. They are an effective weed control when applied beneath a weed block. Pre-emergent herbicides do not kill existing weeds and the product will lose some efficacy if the soil is often disturbed by feeding birds, digging or cultivation or is used as a pathway.