The genus Oxalis, commonly known as wood sorrel, includes many species widely regarded by gardeners as aggressive weeds. Despite its delicate appearance, often resembling a three-leaf clover, the plant’s robust reproductive strategies allow it to rapidly colonize garden beds, lawns, and container plants. Controlling an Oxalis infestation is difficult due to its specialized biological mechanisms for propagation.
Identification and Diversity
The Oxalis genus contains over 500 species. Not all are problematic weeds; some, like the purple-leaved Oxalis triangularis, are cultivated as ornamentals. The most common weedy culprits are low-growing species like Creeping Woodsorrel (Oxalis corniculata). This plant is easily identified by its trifoliolate leaves, which consist of three characteristic heart-shaped leaflets joined at the base. The foliage ranges in color from bright green to deep reddish-purple or bronze. Small, five-petaled flowers, typically yellow, appear throughout the growing season and are followed by distinct seed capsules. While the foliage is superficially similar to clover, the heart-shaped leaflets and five petals distinguish wood sorrel from true clovers.
Mechanisms of Aggressive Spread
The invasive nature of many Oxalis species is due to a two-pronged reproductive strategy that ensures persistence and wide dispersal. Many weedy species, such as Bermuda buttercup (O. pes-caprae), reproduce through tiny, persistent underground storage structures called bulbils. These small, scaly buds detach easily and remain dormant in the soil, ready to sprout when conditions are favorable. Mechanical disturbance, like digging, inadvertently breaks the bulbils away and spreads them, turning a small infestation into a larger one. Even if the visible top growth is removed, these underground structures retain enough stored energy to quickly regrow the plant.
The second method of spread is an explosive, or ballistic, seed dispersal mechanism. Once the small, cylindrical seed capsules mature, a slight touch or natural drying can cause them to rupture violently. This action flings the tiny, dark seeds up to 16 feet away. The seeds are highly viable and germinate almost immediately, allowing rapid colonization.
Targeted Control and Eradication
Eradicating Oxalis requires a systematic approach that targets both the ballistic seeds and the persistent underground bulbils. When manually removing the plant, use a small trowel to dig up the entire root system and all associated bulbils, ensuring no fragments are left behind. Simply pulling the visible top growth leaves the regenerative bulbils in the soil, which quickly send up new shoots. Cultural controls are effective against the seeds, which require light to germinate.
Applying a thick layer of mulch, two to three inches deep, suppresses germination by blocking light exposure. For larger, heavily infested areas, soil solarization—covering the soil with clear plastic during warm, sunny weather—can raise soil temperatures high enough to kill both seeds and dormant bulbils. Chemical control presents a challenge because the waxy coating on the leaves can repel herbicides, and most treatments do not penetrate deeply enough to kill the underground bulbils. Post-emergent broadleaf weed killers are required to kill the top growth, but since the bulbils often survive, repeat applications are necessary.