Two types of herbicides are commonly used: pre-emergent and post-emergent. The names describe when the herbicide kills weeds. Pre-emergents kill weeds before sprouts emerge from soil. Post-emergents decimate actively growing weeds that have already emerged from soil. Post-emergent herbicides are weed killers that are applied to weeds that are actively growing above ground. These products work by being absorbed into the leaves and taken down into the roots, much like the way water moves through the plant. Post-emergent herbicides are some of the most popular substances for weed control. While pre-emergent herbicides work to prevent weeds from ever growing, post-emergent herbicides work on weeds that have already grown. They utilize a mixture of chemicals to kill the weed and ensure that it does not grow back.