Herbicides work in a variety of ways, but are usually divided in to soil-applied herbicides and foliar-applied herbicides, says the University of Illinois Extension. Herbicides are either selective, meaning they work on only certain plant types, or non-selective, which work on all weeds. Herbicides are further broken down into classes that outline their chemical structure and activity.
Phenoxy Acid Type
Phenoxy acid types, says the University of Rhode Island, are generally foliar sprayed applications. Phenoxy is a systemic herbicide, selectively used for broad leafed weeds in grass. It only attacks broad leaf plants and does not damage grass. As it attacks the weed it inhibits growth, twisting and curving the stems and the leaves. Mecoprop and 2,4-D are included in this class.
Benzoic Acid Type
Like phenoxy-type herbicides, benzoic acid types are applied as a foliar spray. Benzoic is systemic, selective and used to control broad leafed weeds. The danger of benzoic herbicides is its soil mobility; it moves quickly and readily through the soil and has the potential to destroy nearby ornamental plants and trees. Dicamba herbicide is in this class.
Dinitroaniline-type herbicides are used as a pre-emergence control of weeds. Pre-emergence herbicides attack weed seeds before they are able to germinate by inhibiting the cell division within the seed. Dinitroaniline types are selective. A disadvantage of dinitroaniline-type herbicides is that the chemical may evaporate through volatilization before it is able to take effect. Pendimethalin, trifluralin and benefin are examples of the dinitroaniline type.
Bipyridylium type is a nonselective herbicide that requires contact with the foliage of the plant and has no activity in the soil. Bipyridylium types destroy the membrane of the plant causing wilting and eventually death.
Substituted Urea Type
Substituted urea types are selective, soil-applied herbicides that absorb through the roots of the plant. Unlike other soil-applied herbicides, substituted urea types are growth inhibitors and do not affect photosynthesis.
Primarily contact herbicides, they are foliar applied. Arsenical types cause a yellowing and withering of the leaves. There are several name brand arsenical varieties.