For homeowners seeking to maintain a lush, uniform lawn, the presence of weeds presents a common challenge. Fortunately, chemicals designed to remove unwanted plants without harming the surrounding turfgrass do exist. These products are known as selective herbicides, representing a specialized category of pest control agents. This selective action is the result of specific chemical formulations and a deeper understanding of plant biology.
The Science of Selective Killing
The ability of certain herbicides to kill broadleaf weeds while leaving grass unharmed stems from fundamental biological differences. Turfgrasses are monocots, possessing a distinct anatomy and metabolism compared to broadleaf weeds, which are dicots. Many selective weed killers function by mimicking a plant’s natural growth hormone, auxin, which regulates cell division and elongation.
When a broadleaf plant absorbs these synthetic auxins, its growth systems are overwhelmed, leading to uncontrolled, abnormal growth. This results in twisted stems, cupped leaves, and ultimately, the destruction of the plant’s vascular tissue, causing death. Grass plants are able to tolerate the exposure to these chemicals due to a combination of factors.
Grass leaves are narrow and vertical, which limits the surface area available for the herbicide to be absorbed compared to the wide, horizontal leaves of broadleaf weeds. Grass plants also have specialized schlerenchyma cells that protect the flow of nutrients and water from the chemical’s disruptive effects. Furthermore, grass species often possess a greater ability to metabolize and break down the herbicide into inactive forms, a process known as detoxification.
Broadleaf Weed Control Ingredients
The most common and effective chemicals used to eliminate broadleaf weeds in a lawn setting are synthetic auxin herbicides. These are often formulated as a “three-way” mix to ensure a broad spectrum of weed control, as different weeds respond better to specific compounds. The active ingredients include 2,4-D, MCPP (Mecoprop-p), and Dicamba.
The chemical 2,4-D is one of the oldest and most widely used selective herbicides, highly effective against common weeds like dandelions and plantain. MCPP adds control for weeds such as clover and chickweed that may be less susceptible to 2,4-D alone. Dicamba, a benzoic acid herbicide, targets perennial and more resistant broadleaf weeds like knotweed and wild buckwheat.
This combination of synthetic auxins ensures that the vast majority of common broadleaf weeds are targeted in a single application. These compounds are readily absorbed by the plant’s leaves and roots, where they translocate throughout the system to disrupt growth patterns. The combined action causes the susceptible dicot plants to essentially grow themselves to death.
Managing Grassy Weeds in Turf
Controlling grassy weeds, which are also monocots like turfgrass, is a more challenging task that requires a different approach than broadleaf control. Since the chemical cannot rely on the fundamental biological differences, control is often split into preventative and curative methods. Pre-emergent herbicides are designed to stop weed seeds from sprouting and establishing.
Chemicals such as prodiamine and pendimethalin are applied to the soil before the weed seeds germinate, typically in early spring for summer annuals like crabgrass and foxtail. These compounds work by inhibiting cell division in the young root system, causing the seedling to die shortly after it sprouts. Pre-emergent products require a thorough watering immediately after application to move the chemical into the root zone.
For grassy weeds that have already emerged, post-emergent selective chemicals are necessary, and these are highly specialized to target the weed species. Quinclorac is a common active ingredient specifically used to control visible crabgrass, even in its more mature stages. For sedges, which are often mistaken for grasses, sulfentrazone is a preferred chemical for post-emergent control. Applying these specialized products requires careful adherence to the label to avoid injury to the desirable turf species.
Application Best Practices
To ensure selective herbicides work effectively without harming the lawn, strict adherence to application methods and timing is necessary. Always read the product label completely, as it contains instructions specific to the active ingredients, turfgrass type, and target weeds. Post-emergent herbicides are most effective when applied to actively growing weeds, so application should be avoided during drought or extreme heat.
Applying liquid herbicides when temperatures are above 85 degrees Fahrenheit (30 degrees Celsius) can increase the risk of damaging the lawn. Ideal application temperatures for many post-emergent products fall within the range of 65 to 85 degrees Fahrenheit. Avoid application if rain is expected within 24 hours, as this can wash the product off the weed foliage before it is fully absorbed.
Wind speed must be low to prevent spray drift, which can carry the chemical onto desirable ornamental plants or garden beds. For post-emergent liquids, applying enough spray to wet the leaf surface is sufficient; excessive application will not improve control. Conversely, pre-emergent granular products must be watered into the soil to activate the chemical barrier that prevents weed germination.