Define Herbicides


Pesticides work to eliminate pests from an area. The division of chemicals used to control plant pests, or weeds, are called herbicides. Plants are considered weeds if they are unwanted in an area for any reason. Herbicides function in many ways to benefit agricultural production, turf control and ornamental landscaping.


One of the earliest recorded utilization of herbicides occurred in 146 B.C. when Romans used salt on the agricultural fields of Carthage so that no crops would grow. Early herbicides worked to kill all plants in an area. In 1945, the British introduced the first selective herbicide, 2,4-D. Manufacturers, horticulturists and botanists continue to introduce new herbicides that function in numerous ways to both prevent and control weed growth in turf, pastures and agricultural production.

Purpose of Weed Elimination

Weeds aren't simply an eyesore in a flowerbed. Weeds compete with other plants for water, light, minerals, space and nutrients. Many types of weeds release allelopathic chemicals, toxic to cereal crops. When weeds are included in the harvest of a crop, they reduce the quality of the crop. Weeds in forage crops and pasture grasses reduce grass production. Herbicides remove the weeds and improve the quality of the grazing environment. This results in better beef and meat production for ranchers and producers. Several types of herbicides are labeled for use in waterways, including ponds, lakes and rivers. Invasive aquatic weeds often negatively impact water quality and wetland environments. Too much aquatic vegetation can destroy habitats, clog hydroelectric screens and cause a reduction in property values, according to, an industry task force for herbicide research.

Herbicide Selectivity

Non-selective herbicides kill all plants upon application. Selective herbicides target only specific types of weeds, while leaving other plants unharmed. For example, a beef producer applies a selective herbicide for broadleaf weed control to a Bermuda grass pasture. Only the broadleaf weeds, such as henbit and nettles, are killed, leaving the Bermuda grass to thrive.

Application Considerations

Different types of herbicides are applied at different points in the life cycle of a weed. Preemergent herbicides disrupt the germination of weed seeds in the soil. Application occurs before seeds begin to germinate, usually in early spring through fall. Postemergent herbicides kill plants already sprouted from the soil. The majority of postemergents are applied when the targeted weed is actively growing.

Activity Within Plants

Different types of herbicides affect plants in different ways. Classifying herbicides by activity places them in two categories: contact or systemic. Contact herbicides directly kill the plant tissues first touched during application. For example, when contact herbicides affect a plant, the leaves will often be the first parts to wither and die. Contact herbicides effectively control annual weeds when applied before seed production. Systemic herbicides work from the inside out on a cellular level to kill a plant. They are translocated throughout the entire structure of the plant. The entire plant is affected, including the root systems. A plant treated with a systemic herbicide often features a stem that begins to curve downward and leaves that curl under. The process is slow, but provides quality weed control on perennial and biennial weeds.

Keywords: definition of herbicide, types of herbicides, use of herbicides

About this Author

Lillian Teague is a professional writer and editor with more than 10 years' experience in taking hard-to-understand subjects and making them easily understood. She's written thousands of articles for newspaper, periodicals and the Internet. Published work includes VA publications, MMS publications, USAF's The Mobility Forum,,, and many others.