The specific type of pesticide used to control or eliminate unwanted plants is known as a herbicide. Herbicides, often called weed killers, function by disrupting fundamental biological processes within plant tissues, leading to injury and eventual death. These chemicals are categorized based on which plants they affect and the specific way they interfere with a plant’s growth and survival mechanisms. Understanding these classifications is necessary to select the appropriate chemical for weed management.
Defining Targets: Selective Versus Non-Selective Herbicides
The most practical distinction among herbicides is based on their target specificity, separating them into selective and non-selective types. Non-selective herbicides are formulated to kill virtually any plant they contact, effectively clearing all vegetation from an area. Glyphosate is a widely known example used for total weed control in areas like driveways or before planting.
In contrast, selective herbicides are engineered to eliminate specific weed species while leaving desirable plants unharmed. This is achieved through differences in plant physiology, such as leaf shape, waxiness, or metabolic pathways. Selective broadleaf herbicides, often containing active ingredients like 2,4-D or dicamba, are commonly used to remove weeds such as dandelions and clover from turfgrass lawns.
Selective herbicides exploit the physiological differences between monocots (grasses) and dicots (broadleaf weeds) to target one group without damaging the other. For example, a selective herbicide might control grassy weeds within a broadleaf crop, or remove broadleaf weeds from a grass lawn. The choice between a selective or non-selective product depends entirely on whether the user intends to preserve surrounding vegetation.
Biological Mechanisms of Action
Herbicides are also grouped by their mechanism of action (MOA), which describes the exact biological pathway they disrupt within the plant cell. One major group is the amino acid synthesis inhibitors, which prevent the plant from building proteins necessary for growth. Glyphosate functions by blocking an enzyme called EPSP synthase in the shikimate pathway, preventing the production of essential aromatic amino acids.
Another significant group includes the growth regulators, often termed synthetic auxins, which mimic and overwhelm the plant’s natural hormones. Chemicals like 2,4-D cause abnormal, uncontrolled cell division and growth, leading to warped leaves, twisted stems, and a fatal disruption of the plant’s internal transport system. This overstimulation is highly effective against broadleaf plants because their hormone systems are more susceptible to the synthetic auxin than those in grasses.
A third category is the photosynthesis inhibitors, which interfere with the process plants use to convert light energy into chemical energy. These chemicals bind to a protein within the chloroplasts, blocking the transfer of electrons in Photosystem II. This interruption prevents the production of sugars and leads to the accumulation of toxic byproducts, causing rapid destruction of the plant’s cell membranes.
Lipid synthesis inhibitors represent a fourth major MOA, primarily targeting grassy weeds. These herbicides block the production of lipids, which are fundamental components of cell membranes and necessary for new cell growth. By preventing the plant from synthesizing these essential building blocks, the herbicide stops the growth of new shoots and roots, causing the plant to starve and desiccate.
Systemic and Contact Application
The final classification for herbicides is based on how they move within the plant body after application, dividing them into systemic or contact types. Systemic herbicides are absorbed through the leaves, stems, or roots and are then translocated throughout the plant via its vascular system. Since they are transported to all parts, including underground root structures and rhizomes, they are highly effective for killing perennial weeds with extensive root systems.
This internal movement means that systemic products, such as glyphosate, do not require complete coverage of the plant foliage to be effective. They often act slowly, with visible effects appearing over several days or weeks. This slow action is due to the time required for the chemical to travel to the growing points and roots and disrupt the plant’s metabolism completely. The total kill, including the root, makes this method preferred for persistent weeds.
In contrast, contact herbicides only kill the plant tissue they directly touch. These chemicals are not translocated significantly within the plant, meaning they must achieve thorough coverage of the foliage to be successful. Contact herbicides work much faster, often causing visible damage like browning and wilting within hours or a day. Because contact products do not move to the roots, they are best suited for controlling annual weeds or for a rapid “knockdown” of above-ground perennial weeds. Deep-rooted perennial weeds often regrow after a contact application because the underground structures remain viable.