Dollar weed, also known as pennywort, is a common and frustrating invader that thrives in moist environments. Many people turn to household items like vinegar as a potentially effective, non-chemical alternative for weed control. Understanding how this common kitchen product interacts with the plant and what concentration is required is necessary before attempting treatment.
How to Identify Dollar Weeds
Dollar weed (Hydrocotyle spp.) is a perennial plant named for its distinct, rounded leaves. These leaves are bright green, fleshy, and typically measure one to two inches in diameter, resembling miniature lily pads or silver dollars. A key identifier is the placement of the leaf stem, or petiole, which attaches directly to the center of the leaf.
The plant has a low-growing habit and spreads aggressively, forming dense mats of foliage. It reproduces primarily through underground stems called rhizomes, which allow it to establish quickly and make removal difficult. Finding dollar weed often indicates excessive moisture or poor drainage in the soil, as it is a water-loving plant.
The Chemistry of Vinegar as a Herbicide
Vinegar’s ability to kill plants is due to its active component, acetic acid, which gives vinegar its characteristic sour taste and odor. Acetic acid acts as a contact herbicide, meaning it only damages the parts of the plant it physically touches. Its mechanism involves rapidly dissolving the integrity of the plant’s cell membranes.
Once the cell walls are compromised, the plant tissues lose structure and begin to leak fluids. This process leads to desiccation, causing the foliage to wilt, turn brown, and die quickly, often within hours of application. This burn-down effect is limited strictly to the above-ground parts of the weed.
Practical Application and Required Concentration
The effectiveness of vinegar as a weed killer depends on the concentration of acetic acid used. Standard household white vinegar is typically only about 5% acetic acid, which may only provide variable control on very small, young weeds. For established dollar weed, a much higher concentration is necessary to achieve a sufficient burn-down effect.
Horticultural-grade vinegar, containing 10% to 20% acetic acid, is required for effective control of mature weeds. Applying this stronger solution on a warm, sunny, and dry day maximizes its efficacy, as the heat and lack of moisture speed up the desiccation process. Due to the corrosive nature of the higher concentrations, safety precautions are mandatory, including wearing eye protection and gloves to prevent skin and severe eye injury.
Spot-treating is the recommended method of application, ensuring thorough coverage of the dollar weed’s leaves while avoiding surrounding desirable plants. The goal is to drench the foliage completely, allowing the acetic acid to penetrate the waxy leaf cuticle and destroy the cell structure.
Environmental Limitations and Non-Selective Effects
A major constraint of using vinegar is its non-selective nature, meaning it will damage or kill any plant it touches, including turfgrass and garden plants. Careless application or spray drift can easily create dead patches in a lawn or harm nearby flowers and shrubs. The rapid-acting acid does not translocate through the plant’s vascular system to the roots.
Since dollar weed is a perennial that spreads through deep rhizomes, killing only the top foliage is often a temporary fix. The extensive root system remains intact and can regenerate new growth quickly, requiring repeat applications to deplete the plant’s energy reserves. While acetic acid breaks down quickly in the environment, regular use of high concentrations can temporarily lower the soil’s pH, making it more acidic. This temporary alteration can impact the health of other plants.