The answer to whether weed killer will kill grass depends entirely on the chemical composition of the product. Not all herbicides affect all plants equally, and understanding this distinction is crucial for effective lawn care. These chemicals are deliberately designed to target unwanted vegetation while keeping desirable turfgrass healthy. Getting the right results requires knowing the difference between the two primary categories of weed control products.
The Critical Difference: Selective Versus Non-Selective
Herbicides are categorized into two major groups: non-selective and selective. Non-selective herbicides are designed to kill nearly all plant material they contact by acting on fundamental biological processes common across the plant kingdom. The active ingredient glyphosate, for example, inhibits an enzyme pathway necessary for the production of amino acids required for plant survival. These products are systemic, meaning the chemical is absorbed by the leaves and moves throughout the entire plant, resulting in total vegetation control. They are typically used for clearing paved areas, preparing ground for new planting, or maintaining fence lines where no plant life is desired.
Selective herbicides are engineered to eliminate specific types of weeds without harming surrounding plants, such as turfgrass. These formulations target the anatomical and metabolic differences between grasses and broadleaf weeds. The most common selective products are effective against dicots (broadleaf weeds like clover and dandelions) while leaving monocots (which include nearly all turfgrasses) unharmed. These chemicals are specifically intended for use in an established lawn to manage weed problems.
How Selective Herbicides Protect Turfgrass
The ability of a selective herbicide to spare grass is rooted in the distinct biological makeup of monocots and dicots. Turfgrasses are monocots, characterized by single seed leaves, parallel leaf veins, and scattered vascular bundles. Broadleaf weeds are dicots, featuring two seed leaves, net-like leaf veins, and vascular bundles arranged in a ring. This structural difference provides the first line of defense for the grass.
Many common selective lawn herbicides, such as 2,4-D, are synthetic auxins that mimic natural plant growth hormones. When a dicot absorbs this chemical, it causes an uncontrolled, fatal acceleration of cell division and growth, overwhelming the plant’s system. Grasses are protected because their monocot structure and metabolism allow them to either limit the chemical’s movement or rapidly break down the synthetic auxin. This metabolic resistance prevents the poison from causing widespread cellular damage.
Preventing Accidental Lawn Damage
Even when using a selective product, accidental damage can occur due to misapplication, which overwhelms the grass’s natural defenses. Applying an excessive concentration (an overdose) floods the turfgrass with the chemical, surpassing its metabolic capacity to inactivate it. The resulting chemical burden can cause discoloration, leaf curling, or even death to patches of the lawn. Carefully following the label instructions for dilution and application rate is a fundamental preventive step.
Environmental conditions at the time of application are another major factor that can turn a selective product into a damaging one. Applying herbicides during periods of extreme heat (typically above 85°F) or when the turfgrass is suffering from drought stress makes the lawn highly susceptible to injury. Stressed grass is less capable of metabolizing the chemical. High temperatures can also increase the volatility of certain herbicide formulations, causing them to vaporize and drift onto non-target plants. Users must check the forecast and ensure the lawn is adequately hydrated before treatment.
Damage can also occur when non-selective products are used too close to the lawn, leading to wind drift or volatilization onto the turf. Even a fine mist carried by a light breeze can deliver a lethal dose of a non-selective chemical to the edges of a healthy lawn. Improper cleaning of application equipment, such as a sprayer used for a non-selective product, can contaminate a subsequent selective treatment. Thoroughly rinsing all equipment before switching products prevents this type of cross-contamination.