The Common Blue Violet (Viola sororia) is a native, low-growing perennial plant often regarded as a troublesome weed in lawns and gardens across North America. While its small, heart-shaped leaves and delicate blue-purple flowers are attractive, its aggressive nature allows it to quickly colonize turf, creating a dense mat that chokes out desirable grass. Eradicating this plant is a difficult task because its biology provides multiple mechanisms for survival and rapid spread. Understanding these survival tactics is the necessary first step toward implementing effective control strategies.
The Biology Driving Persistence
The violet’s tenacity stems from a combination of underground growth and a highly efficient reproductive cycle. The plant spreads not just by seed, but also through thick, branching underground stems called rhizomes. This rhizomatous growth allows the violet to form dense colonies as new shoots emerge from these horizontal structures. Removing only the visible top growth or applying a non-systemic herbicide will not kill the entire plant because the rhizomes remain intact beneath the soil, ready to sprout again.
Beyond its root structure, the blue violet employs a dual-flower strategy to ensure prolific seed production. In spring, it produces the familiar, showy, open flowers (chasmogamous flowers), which rely on insect pollination to set seed. Later in the season, the plant develops small, closed flowers (cleistogamous flowers) that remain hidden near the soil line. These cleistogamous flowers self-pollinate without opening, guaranteeing a high yield of fertile seeds even without pollinators. The seeds are then forcefully ejected from a capsule and dispersed by ants, ensuring the plant returns year after year.
Non-Chemical and Cultural Control Strategies
For small infestations or in garden beds where chemical use is a concern, manual removal offers a non-chemical solution, though it requires diligence. Success depends entirely on removing the entire plant, including the fleshy taproot and the extensive network of rhizomes. Using a small trowel or digging tool to loosen the soil and excavate the entire root mass is necessary, as any small piece of rhizome left behind can generate a new plant. The task is easier when the soil is moist, typically after a rain, which allows the root system to slide out more cleanly.
In areas heavily infested, such as flower beds or small zones of a lawn, smothering can effectively starve the plants of light. This method involves covering the affected area with heavy materials like black plastic sheeting or thick layers of cardboard, which are then covered with a layer of mulch. Blocking sunlight prevents the violets from photosynthesizing, eventually killing the plants and exhausting the energy stored in their rhizomes. This approach can take several weeks or months to be effective but avoids the use of synthetic chemicals.
The most effective long-term defense against blue violet invasion is maintaining a dense, healthy lawn. Violets thrive in thin, weak turf, often preferring moist, shaded conditions. Adjusting the mower height to three to four inches encourages the grass to grow taller and thicker, which shades the soil and physically crowds out the low-growing violet. Proper fertilization and deep, infrequent watering promote a vigorous turf that better competes with the weed for resources, helping to prevent new violet colonies from establishing.
Effective Herbicide Application
When blue violets have spread significantly throughout a lawn, chemical control is often the only practical path to eradication. The plant’s waxy leaf coating and robust rhizome system make it highly resistant to many common broadleaf weed killers, such as those relying solely on 2,4-D. Effective control requires a systemic broadleaf herbicide containing active ingredients like triclopyr, dicamba, or a combination of both. These ingredients are often formulated into “three-way” or “four-way” weed control products, and triclopyr provides the best activity against the violet.
The most effective time for herbicide application is in the fall, typically from late September through November, when daytime temperatures are consistently above 55°F. During this period, the blue violet actively translocates nutrients down to its root system in preparation for winter. Applying a systemic herbicide at this time allows the chemical to be carried deep into the rhizomes and roots, maximizing the chance of a complete kill. Multiple applications are often necessary, with re-treatment recommended 10 to 14 days later, or in the following spring or fall, to control new growth.
To overcome the violet’s waxy leaves, which repel the herbicide spray, a non-ionic surfactant should be added to the mixture, if the product label allows. The surfactant acts as a wetting agent, helping the herbicide droplets adhere to and penetrate the leaf surface, improving chemical uptake. When spraying, follow all label directions precisely, especially regarding application rates and avoiding drift, which could damage desirable plants nearby. Products containing triclopyr should be used with caution on certain warm-season grasses like bermudagrass, as they can cause injury.