herbicide, Agent, usually chemical, for killing or inhibiting the growth of unwanted plants such as weeds or invasive species. Modern herbicides are put into two categories: selective (affecting specific plant species) and nonselective (affecting plants generally). These in turn are classified as foliage-applied and soil herbicides. Contact herbicides (e.g., sulfuric acid, diquat, and paraquat) kill only the plant organs with which they are in contact. Translocated herbicides (e.g., amitrole, picloram, and 2,4-D) are effective against roots or other organs, to which they are transported from aboveground treated surfaces (soil). Herbicides can be synthetic (human-made) or naturally occurring, the latter being more commonly used in organic farming. See also defoliant.
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