A post-emergent herbicide is a chemical product used to control weeds that have already sprouted and are visible above the soil line. Unlike pre-emergent herbicides, which stop weed seeds from germinating, post-emergents target existing, actively growing vegetation. The optimal choice depends entirely on the specific weed species present and whether you want to kill only the weeds or all plant life in the treated area.
Selective Versus Non-Selective Herbicides
Selective herbicides are formulated to eliminate specific types of plants while leaving others unharmed, making them ideal for lawn applications. These products operate by interfering with metabolic processes unique to certain plant groups, such as broadleaf weeds, sparing turfgrass. For example, a selective product can kill dandelions and clover in a lawn without damaging the desirable grass.
Non-selective herbicides are designed to kill virtually all plant life they contact. These products are the appropriate tool for total vegetation control, destroying both weeds and any surrounding grass or desirable plants. Typical use cases include clearing weeds from sidewalks, driveways, fence lines, or preparing a garden bed for new planting. Using a non-selective herbicide in a lawn will result in dead patches of grass alongside the dead weeds.
Common Active Ingredients and Their Target Weeds
The effectiveness of any post-emergent product is determined by its active ingredients, each targeting weeds through a specific mechanism of action. A common broadleaf killer is 2,4-D (2,4-Dichlorophenoxyacetic acid), a synthetic auxin that mimics natural plant growth hormones. When absorbed, 2,4-D causes uncontrolled growth in susceptible plants, primarily broadleaf weeds, leading to stem curling and eventual death. Products containing 2,4-D are effective against common lawn weeds like dandelions, plantain, and clover, but have little effect on grasses.
Many consumer selective herbicides use a combination of 2,4-D with other active ingredients like Dicamba and Mecoprop (MCPP). Dicamba controls tough broadleaf weeds, and Mecoprop is especially useful against chickweed and clover, creating a three-way blend that expands the range of controllable broadleaf species. This combination is important because no single ingredient is effective against every broadleaf weed. For grassy weeds that are structurally similar to turfgrass, such as crabgrass, a specialized product is necessary.
Quinclorac is a selective post-emergent herbicide effective against grassy weeds like crabgrass, even in its more mature stages. This ingredient is absorbed through the foliage and roots, translocating throughout the plant to disrupt normal growth processes. This often results in the weed’s leaves twisting and changing color before dying. Quinclorac is safe for use on many established turfgrasses, though a methylated seed oil (MSO) surfactant is often recommended to enhance its effectiveness.
For non-selective, total vegetation control, Glyphosate is the most widely used active ingredient. Glyphosate is a systemic herbicide that is absorbed through the leaves and then moves throughout the plant to the roots. It works by disrupting the shikimic acid pathway, a metabolic process essential for the production of aromatic amino acids that is present in most plants, thus killing both broadleaf and grassy weeds. Because of its non-selective nature, Glyphosate is used for clearing areas where no plant growth is desired, and it is most effective when applied to young, actively growing weeds.
Application Timing and Techniques for Maximum Efficacy
The timing of a post-emergent application affects its success, as weeds must be actively growing to absorb and transport the herbicide effectively. Application is most successful when daytime temperatures are consistently between 65 and 85 degrees Fahrenheit. Applying in temperatures above this range can stress the target weeds, reducing absorption, and can also lead to volatilization of the chemical, causing it to drift and damage non-target plants.
Weeds are most vulnerable when they are young, typically in the two-to-four-leaf stage, because they have thinner cuticles and more active growth. Mature weeds with thick, waxy cuticles are much harder to control and may require a higher application rate or multiple treatments. It is important to apply the herbicide when the foliage is dry, allowing the product to stick and be fully absorbed without being diluted.
For systemic herbicides, which need time to translocate throughout the plant, you should avoid applying if rain is forecast within 6 to 24 hours to prevent the product from washing off the leaves. You must also apply on a calm day with low wind speeds to prevent herbicide drift, which can carry the chemical to sensitive plants. Whether you use a spot treatment for individual weeds or a broadcast application for a widespread infestation, always follow the label instructions for proper mixing and application rates to avoid damaging desirable plants. You should wear appropriate personal protective equipment, such as gloves and eye protection, when handling and applying concentrated herbicides.