Weed killers, also known as herbicides, are chemicals designed to control or destroy unwanted plants. They work by disrupting the biological processes plants rely on for growth and survival. The effectiveness of any herbicide depends on its chemical composition, the target plant species, and how it is applied. Understanding these factors is necessary to successfully manage vegetation without harming surrounding desired plants. Herbicides are a proven tool for agriculture and home gardening when used correctly.
The Biological Mechanisms of Destruction
Herbicides function by targeting specific pathways inside plant cells unique to plant life.
One common mechanism involves inhibiting the synthesis of essential amino acids, the building blocks for proteins. For instance, chemicals like glyphosate prevent the plant from creating aromatic amino acids by blocking the EPSP synthase enzyme. Without these proteins, the plant starves and cannot sustain new growth, leading to a slow decline.
Other herbicides interfere directly with the plant’s ability to generate energy through photosynthesis. These chemicals bind to proteins within the chloroplasts, stopping the transfer of electrons required to convert light into usable food. When this process is blocked, the plant cannot produce the sugars it needs for fuel, resulting in death from a lack of energy.
A third major group of herbicides acts by mimicking natural plant hormones called auxins, which regulate growth. These synthetic auxins flood the plant’s system, causing a hormonal imbalance. This leads to disorganized, uncontrolled cell growth, characterized by twisted stems and malformed leaves. The plant essentially grows itself to death because its internal communication system is overwhelmed.
Categorizing Herbicides
Herbicides are categorized based on their scope of action.
Selective herbicides are formulated to kill only certain types of plants while leaving others unharmed. This works by exploiting biological differences, such as targeting broadleaf weeds (dicots) without harming grasses (monocots). This allows a homeowner to treat a lawn for dandelions without killing the turfgrass.
In contrast, non-selective herbicides are designed to kill virtually any plant they contact by disrupting core functions common to all vegetation. These products are used for clearing an area completely, such as along fence lines or driveways.
Herbicides are also classified by how they move inside the plant: systemic or contact action. Systemic herbicides are absorbed by the leaves or roots and transported throughout the entire plant via its vascular system. This movement allows the chemical to reach and destroy the deep root systems of perennial weeds.
Contact herbicides only destroy the plant tissue they physically touch and do not move significantly within the plant. These chemicals are fast-acting, causing visible damage within hours or days. Because they do not translocate to the roots, contact products are most effective against annual weeds with shallow root structures.
Why Application Success Varies
When an herbicide does not work, the cause is often related to external factors rather than a failure of the chemical itself.
One common issue is applying the chemical at the wrong time in the plant’s life cycle. Weeds are most susceptible when they are young and actively growing, meaning their metabolic machinery is readily absorbing the herbicide. Once a weed is mature or dormant, its reduced activity significantly lowers the uptake of the chemical.
Environmental conditions at the time of application play a significant role in efficacy. Systemic herbicides must remain on the plant surface long enough to be absorbed; rainfall shortly after spraying can wash the product away. Temperature is also a factor: cold weather slows metabolism, while high temperatures cause the plant to conserve moisture and thicken its leaf cuticle. Both extremes reduce the plant’s ability to absorb the product.
Inaccurate mixing is another frequent issue. Using too little product will only injure the plant rather than kill it, while using too much can cause rapid scorching. This quick surface damage prevents the herbicide from moving down to the roots, allowing the weed to recover.
The repeated use of the same herbicide or products with the same mechanism of action can lead to the evolution of herbicide resistance in weed populations. Resistance occurs when naturally tolerant plants survive and reproduce, creating a population that the chemical can no longer control.