Postemergence Herbicides

Foliar-applied, contact herbicides

(e.g., diquat, glufosinate and pelargonic acid) kill green plant tissue that is contacted by the herbicide spray. Thus, good coverage of weedy foliage is essential for good control and spray contact with desirable plants should be avoided or minimized. Typically, contact herbicides rapidly kill green plant tissue in a matter of hours or a day. Contact herbicides work best on annual weeds but will also kill the shoots or top growth of perennial weeds. Since contact herbicides don't affect the underground portions of perennial weeds, these weeds will regrow new shoots and repeated applications will be required to eradicate the perennial (analogous to mechanical control methods). Better perennial weed control can be obtained with translocated herbicides.

Foliar-applied, translocated herbicides

(e.g., clethodim, fluazifop, glyphosate, and sethoxydim) are applied to the foliage of susceptible weeds. They are absorbed into leaves and green stems and translocated to meristematic tissue (growth zones) in shoots and stolons and in underground roots and tubers. Translocated or systemic herbicides move with food materials in the plant. Thus, systemic herbicides are most effective when weeds are actively growing and using food materials produced in leaves. Perennial weeds are much more easily killed using systemic herbicides that are translocated to roots, rhizomes and tubers underground than with contact herbicides. Systemic herbicides often work slowly with complete weed kill requiring 7 to 21 days.

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