Herbicides are chemical agents used to control or destroy unwanted vegetation. Whether a specific herbicide will kill grass depends entirely on its chemical formulation and classification. These products are designed to either target specific plant types, often leaving grasses unharmed, or to eliminate nearly all plant life indiscriminately. Success hinges on the active ingredients, precise application techniques, and timing.
Understanding Selective and Non-Selective Types
The primary factor determining a product’s effect on grass is its classification as either selective or non-selective. Non-selective herbicides are designed to kill almost all plant life they contact, including desirable grass species. They are commonly used to clear areas completely, such as along fence lines, in driveway cracks, or when preparing a plot for new landscaping. The active ingredients do not discriminate between grass, broadleaf weeds, or ornamental plants.
Selective herbicides, by contrast, are formulated to target specific plant types while leaving others, like turfgrass, undamaged. These are the products most homeowners use to control weeds like dandelions and clover within an established lawn. The chemical selectivity works by exploiting physiological differences between broadleaf weeds and the grass itself. Broadleaf weeds are typically more susceptible to the chemical action than the grasses.
It is important to note that selective products are not universally safe for all grass types. The product label must be checked against the specific lawn variety. Furthermore, some selective herbicides are specifically designed to eliminate grassy weeds, such as crabgrass, from a lawn of different grass species. Using the wrong selective product can still harm the desired turfgrass, though not as universally as a non-selective formula.
How Herbicides Target Plant Physiology
Herbicides interfere with the plant’s essential processes for growth and survival. One common mechanism involves synthetic auxins, which are growth regulators that mimic natural plant hormones. When absorbed, these chemicals cause uncontrolled, excessive growth in susceptible plants, leading to the collapse of cell walls and disruption of structural integrity. This action often makes products selective, as broadleaf plants are generally more affected than grasses.
Other herbicides, such as the widely used non-selective product glyphosate, work by inhibiting specific enzyme pathways. Glyphosate, for example, blocks the enzyme EPSPS, which is necessary for the plant to synthesize critical aromatic amino acids. Without these essential building blocks, the plant cannot create the proteins it needs to sustain cellular processes, resulting in death.
Some herbicides are designed to disrupt photosynthesis, the process by which plants convert light energy into food. These formulas either reduce the flow of electrons or cause the formation of reactive molecules that destroy the plant’s cell membranes and chlorophyll. The speed of the chemical action depends on whether the herbicide is systemic, moving throughout the plant’s vascular system, or contact, only killing the tissue it directly touches.
Timing and Application Errors to Avoid
Proper application technique is crucial, as errors lead to poor results or damage to desirable grass. A common mistake is failing to follow the label’s dilution rate. Overapplication causes turf injury, while under-application results in ineffective weed control. The concentration must be measured precisely, as the difference between a lethal dose for a weed and a damaging dose for grass is often small.
Environmental conditions significantly affect herbicide performance. Applying herbicides during high heat, typically above 90°F, increases the risk of the chemical volatilizing and injuring non-target plants. Weeds stressed by drought or cold are also less susceptible because their physiological activities have slowed down.
Timing the application based on the weed’s life cycle is essential.
Pre-Emergent Application
Pre-emergent herbicides must be applied before weed seeds germinate, typically when soil temperatures reach a certain threshold in the spring or fall. These products form a chemical barrier in the soil to prevent sprouting. Using a pre-emergent product after weeds have already sprouted is ineffective.
Post-Emergent Application
Post-emergent products are applied only after the weed has visibly emerged and is actively growing. For these products, a rain-free period is needed after application to allow for sufficient absorption, which can range from one to six hours depending on the product.
Windy conditions must be avoided, as this increases the likelihood of chemical drift. Drift carries the herbicide onto non-target plants, causing non-selective damage to nearby desirable grass or ornamental plants, even if a selective product was used. Calibrating the spray equipment ensures an even and consistent application rate across the entire area.