A herbicide is a chemical substance specifically formulated to control or eliminate unwanted plants, commonly known as weeds. The primary purpose of these compounds is to manage vegetation that competes with desired crops, turfgrass, or landscapes for resources. The question of what a herbicide kills cannot be answered with a single plant group because the chemical’s effect is entirely dependent on its precise formulation. Different herbicides are designed to exploit subtle biological differences between plant species, determining whether they target a single type of weed or destroy all vegetation they contact.
Differentiating Targets: Selective and Non-Selective Herbicides
The fundamental difference in what herbicides kill is defined by their classification as either selective or non-selective. Selective herbicides are precision tools engineered to eliminate certain plant types while leaving others, such as desirable crops or turf, relatively unharmed. This capability is widely used in agriculture and lawn care, allowing farmers and homeowners to remove weeds without damaging the main plant species.
Non-selective herbicides, conversely, are designed for total vegetation control, meaning they kill virtually every plant they touch. These compounds are often used to clear ground completely before planting a new garden bed or for managing weeds in areas where no plant growth is desired, such as along fence lines, driveways, or railroad tracks. The active ingredient glyphosate is a well-known example of a systemic, non-selective chemical that is absorbed by the foliage and transported throughout the entire plant, ultimately leading to its death. Choosing the correct type is paramount, as misapplication of a non-selective product will indiscriminately destroy both weeds and any surrounding desirable vegetation.
Primary Targets of Selective Herbicides: Broadleaf Plants and Grasses
Selective herbicides primarily target two massive groups of plants: broadleaf weeds and unwanted grasses, relying on the structural differences between them. Broadleaf plants, which are dicots, are characterized by their net-like leaf veins and the presence of two seed leaves upon germination. Common broadleaf targets include familiar lawn invaders such as dandelions, clover, thistles, and wild violet.
Herbicides designed to kill these plants often contain compounds like 2,4-D or Dicamba, which function as synthetic auxins, or plant growth hormones. These chemicals overwhelm the broadleaf plants’ systems, causing their leaves and stems to curl, twist, and grow uncontrollably, leading to the disruption of nutrient flow and eventual death. Desirable grasses, which are monocots, possess a different physiology that prevents them from absorbing or reacting to these synthetic hormones in the same destructive way.
The second major target of selective compounds is unwanted grasses. These grassy weeds, such as crabgrass, annual rye, and foxtail, are monocots just like desirable turfgrasses (e.g., Kentucky bluegrass or fescue) or cereal crops (e.g., corn and wheat). Herbicides targeting these weeds are formulated to exploit more subtle metabolic differences, often inhibiting a specific enzyme present in the weed grass but not in the crop or turf.
For instance, certain selective herbicides used in corn or soybean fields kill grassy weeds by inhibiting the plant’s ability to synthesize specific amino acids necessary for growth. Similarly, pre-emergent herbicides target the seeds of grassy weeds like crabgrass, inhibiting cell division in the root as the seed attempts to germinate, preventing the plant from ever emerging.
Specialized Targets: Woody Brush, Vines, and Aquatic Vegetation
Beyond common lawn and field weeds, specialized herbicide formulations are designed to kill plants with unique structural defenses. Woody plants and brush, including invasive shrubs, small trees, and tough vines like poison ivy or honeysuckle, require more concentrated or specialized application methods. These plants have robust root systems and thick bark that resist typical foliar spray applications.
For effective control of woody brush, herbicides containing active ingredients like triclopyr or concentrated glyphosate are often applied directly to the plant’s bark or a freshly cut stump. This basal bark treatment uses an oil-based carrier to penetrate the bark, or the cut-stump method applies the chemical to the cambium layer to ensure the systemic compound reaches the root system. These methods ensure a high dose reaches the vascular system, preventing the robust plant from resprouting.
Another distinct class of targets is aquatic vegetation, which includes submerged, floating, and emergent plants like water hyacinth and hydrilla, as well as algae. Controlling these plants requires the use of aquatic-approved herbicides, often containing compounds like diquat or fluridone, that are specifically registered with environmental protection agencies. These specialized formulations are necessary to minimize harm to non-target aquatic organisms, such as fish and amphibians, while effectively managing the nuisance vegetation.