Herbicides are chemical and organic materials used for the purpose of killing weeds in the garden, lawn and agricultural crops. Herbicides kill weeds in a variety of ways using different modes of action, or the way in which the herbicide affects the weed plant's ability to survive.
Selective Versus Nonselective
Herbicides are first grouped into whether they are selective herbicides or nonselective herbicides. Selective herbicides only affect a certain variety of weed, such as broadleaf weeds or grassy type weeds. Nonselective herbicides kill any vegetation that they touch.
Contact Versus Systemic
Herbicides kill plants according to a mode of action: contact or systemic. Contact herbicides kill whatever plant that they touch when sprayed or spread. When contact is thorough, the plant will die. Systemic herbicides are absorbed by the plant and move through the plant's vascular system. According to the University of Rhode Island, contact herbicides are mainly effective against annual weeds, while systemic herbicides work on both annual and perennial weeds.
Soil applied herbicides inhibit certain functions of the weed. Root mitotic inhibitors block the formation of new cells, says the University of Illinois, which ruins the roots of the plant and slowly kills it. Pigment inhibitors destroy chlorophyll. Shoot inhibitors also affect cell growth and division.
Foliar applied herbicides also affect the growth of plants, but are applied to the leaves of the plant as opposed to the soil. Hormone herbicides reduce growth of the newest stems by disrupting protein synthesis within the plant. ALS enzyme inhibitors reduce the plant metabolism, making it hard for the plant to feed properly. Lipid inhibitors reduce the development of shoots and leaves, stunting the growth of the plant.
Herbicides are applied either in spray or granular form. Sprays are used for application to weed foliage due to the control. Soil-based herbicides are also sprayed, but may be spread using small granules that are activated by water. Herbicides are applied according to the instructions on the packaging.