Preplant is directed at removal of all existing vegetation prior to seeding, sodding or planting ornamental landscape plants.
Preemergent control is directed at destroying the weed seedlings as they begin to germinate but before they emerge from the ground. The most common use is for control of annual grasses such as crabgrass before they emerge in the spring.
Postemergent control is directed at visible, growing plants after they have emerged from the ground. This is usually the method of control for many of our common broadleaf plants such as dandelion.
Selective herbicides control specific types or groups of weeds (e.g., broadleaves vs. grasses). Controlling dandelions or crabgrass in a bluegrass lawn without harming the bluegrass are examples of selective control.
Non-selective herbicides control all green vegetation present in the area of application. Non-selective control of all existing weedy vegetation may be done when preparing new areas of ground for establishing a lawn or planting a flower or shrub border.