Broadleaf weeds, such as dandelions and clover, present a common challenge in turf management. Non-selective herbicides, like glyphosate, eliminate weeds but also destroy the surrounding turfgrass. The solution lies in specialized products known as selective herbicides, which are chemically formulated to target one type of plant while leaving another unharmed. This targeted approach is possible because of fundamental biological differences between the weeds and the desirable grass. This article will explain the mechanism behind this selectivity, detail the chemical agents used, and provide practical guidance on application.
The Biological Basis for Selective Killing
Selective weed control works by exploiting the structural differences between broadleaf weeds and turfgrass. All flowering plants are categorized into two main groups: monocots (grasses) and dicots (broadleaf weeds). This classification begins at the seed, where monocots emerge with a single leaf, while dicots emerge with two seed leaves.
The most significant distinction for herbicide activity is the plant’s anatomy and response to growth hormones. Broadleaf weeds (dicots) have net-like leaf veins, a primary taproot system, and a vascular system arranged in a ring. Grasses (monocots) exhibit parallel leaf veins, a fibrous root system, and vascular bundles scattered throughout the stem.
Selective herbicides that target broadleaf weeds are typically synthetic auxins, which mimic the plant’s natural growth hormones. When absorbed by a broadleaf weed, a synthetic auxin causes catastrophic overstimulation of the plant’s growth mechanisms, leading to uncontrolled cell division. This rapid, unsustainable growth overwhelms the plant’s resources, causing stems to twist and leaves to cup, ultimately resulting in death.
Grasses are largely resistant to this effect because of differences in their cellular structure and metabolism. Monocots rapidly break down and detoxify the synthetic auxin compounds before widespread damage occurs. They also possess structural defenses, such as specialized sclerenchyma cells surrounding their vascular tissue, which prevent the circulatory system from collapsing under the hormone’s influence, allowing the grass to survive.
Key Active Ingredients in Selective Herbicides
The herbicides that selectively eliminate broadleaf weeds are classified as synthetic auxins and are often combined for a wider range of control. These products are frequently sold as “three-way” mixes, typically combining 2,4-D, MCPP, and Dicamba. Each component targets a different spectrum of common turf weeds.
2,4-D, MCPP, and Dicamba
The oldest and most common is 2,4-D (2,4-Dichlorophenoxyacetic acid), which provides effective control against common taprooted weeds like dandelions and broadleaf plantain. MCPP (Mecoprop) is included to broaden the spectrum, offering better control of weeds such as chickweed, clover, and prostrate knotweed. Dicamba (3,6-Dichloro-o-anisic acid) is incorporated for its ability to tackle persistent, deep-rooted weeds.
Dicamba is known for its mobility in the soil, requiring careful application to prevent uptake by the roots of nearby ornamental plants. Combining these three chemicals ensures that a single application can manage a diverse population of broadleaf species simultaneously.
Triclopyr and Quinclorac
For weeds difficult to control with the standard three-way mixture, two other active ingredients are often employed. Triclopyr is highly effective against stubborn weeds like ground ivy and oxalis. This synthetic auxin is available in amine and ester formulations, with the less volatile amine form preferred for use on turfgrass to reduce the risk of vapor drift in warmer conditions.
Quinclorac is primarily recognized for its control of grassy weeds like crabgrass, but also provides effective post-emergent control for certain broadleaf weeds such as clover and speedwells. Using a combination of these ingredients allows for an effective strategy against nearly all broadleaf invaders in a lawn.
Optimal Application Timing and Technique
The efficacy of a selective herbicide application depends on timing and proper technique. These products work best when broadleaf weeds are actively growing, typically in the spring and fall. Fall applications (late September to early November) are particularly effective for perennial weeds because the plant is naturally moving carbohydrates down to its root system, translocating the herbicide directly to the roots.
Temperature Requirements
The ideal air temperature range for applying most post-emergent selective herbicides is between 65°F and 85°F. Application should be avoided when temperatures exceed 85°F, as this increases the risk of the chemical volatilizing into a gas and causing vapor drift damage to non-target plants. Applications below 60°F will result in slower uptake and diminished effectiveness because the weed’s growth processes have slowed.
Post-Treatment Care
Liquid applications require specific post-treatment care to maximize absorption. It is recommended to wait at least 24 hours, and preferably 48 hours, after application before watering or allowing rainfall. This waiting period ensures the foliage has enough time to absorb the chemical before it is washed off. The lawn should also not be mowed for four to five days after treatment, allowing the systemic herbicide to fully translocate from the leaf tissue down to the root system before the target leaves are removed.
Safety and Wind Conditions
Safety precautions are necessary during application to prevent drift and personal exposure. Herbicide application should only occur when wind speeds are between 3 and 10 miles per hour. Applicators should wear appropriate Personal Protective Equipment (PPE), including long pants, long-sleeved shirts, closed-toe shoes, and chemical-resistant gloves made of nitrile or neoprene.