A defoliant is a chemical used on trees and plants to make all their leaves fall off. Defoliants are especially used in war to remove protection from an enemy. a chemical substance, used especially in war, that makes all the leaves of plants drop off (defoliare, from folium ). Chemical dust or spray applied to plants to cause their leaves to drop off prematurely. Defoliants sometimes are applied to crop plants such as cotton to facilitate harvesting. They have also been used in warfare to eliminate enemy food crops and potential areas of concealment (as in the Vietnam War). See also Agent Orange