A pre-emergent herbicide is a weed killer applied prior to the emergence of the weed from the soil. Chemicals like dithopyr (Dimension) and pendimethalin (Pre-M, Scotts' Halts) prevent all seeds from germinating, including grass seed. pre-emergent herbicide is a weed killer applied prior to the emergence of the weed from the soil. Chemicals like dithopyr (Dimension) and pendimethalin (Pre-M, Scotts' Halts) prevent all seeds from germinating, including grass seed.