Postemergence herbicides are very useful tools for site preparation and selective applications to existing weeds in ornamental plantings. Postemergence herbicides may be contact or translocated in their mode of action. The contact herbicides cause a rapid phytotoxic response at the cellular level. Cells lose their integrity and die within hours or minutes. We speak of a 'burndown' with contact herbicides because of the rapidity and appearance of the symptoms.
Adequate coverage is important when using a contact herbicide.
Obviously, protection of desirable plants is also essential. Translocated herbicides are absorbed through leaves or roots and move systemically. Absorption through the leaves is the primary entry for postemergence herbicides with ornamental labels. Allowing adequate time between application and rainfall or irrigation is critical for success with these chemicals. These chemicals kill plants by targeting and interrupting one or more physiological processes within the plant.