Chemical control of weeds is requires less time, costs less, and is more effective. For all of these reasons the least expensive, most effective weed control programs include herbicide applications. Preemergence herbicides control weeds at the seed germination stage, or as they are emerging through the soil surface. Postemergence herbicides control existing weeds. Contact vs Translocated.
Contact herbicides kill or injure only that part of the plant with which they come in contact.
Annual weeds may be killed, but regrowth of perennial weeds from below-ground parts usually occurs following the application of a contact herbicide. Translocated herbicides are absorbed by the leaves or roots of plants and move within the plant. They are needed to kill underground parts of perennial weeds. Selective vs Nonselective. Nonselective herbicides kill or injure almost all plants.