The genus Oxalis, commonly known as wood sorrel or sourgrass, encompasses over 550 distinct species globally. This widespread botanical family is often found in woodlands, grasslands, and disturbed sites across nearly every continent. The confusion surrounding Oxalis stems from its dual identity: some varieties are intentionally cultivated for their ornamental appeal, while others are notorious, highly aggressive garden invaders. Understanding the biology of this plant is the first step in managing its presence.
Identification and Dual Nature
The physical appearance of Oxalis is characterized by its distinctive foliage, which typically consists of three heart-shaped leaflets joined at a central point, giving it a clover-like look. A notable feature is their habit of folding downward at night or during intense heat. The delicate flowers usually possess five petals, appearing in colors ranging from bright yellow and white to pink and purple.
The common weedy species are typically low-growing and prolific, such as Oxalis stricta (upright yellow sorrel) and Oxalis corniculata (creeping wood sorrel). Another problematic species is Oxalis pes-caprae, often called Bermuda Buttercup, recognized by its bright yellow flowers and ability to form underground bulbs. Conversely, species like Oxalis triangularis are prized for their dramatic dark purple foliage and are intentionally grown as decorative houseplants or garden accents.
Understanding Aggressive Spread Mechanisms
The notoriety of Oxalis as a difficult-to-control plant is linked to its sophisticated reproductive strategy. One challenging aspect is the production of specialized underground storage organs, which include rhizomes or tiny, fragile structures called bulbils. These bulbils, which are essentially small bulbs, can number over 20 per plant in species like O. pes-caprae. When a gardener attempts to pull the plant, the foliage may detach, but the bulbils remain in the soil, ready to sprout into a new plant soon after.
Beyond vegetative reproduction, many Oxalis species employ a highly effective ballistic dispersal mechanism for their seeds. The seeds develop within small capsules that are under tension. Upon reaching maturity or when disturbed, the capsule abruptly ruptures, explosively launching tiny seeds up to several feet away. This rapid projection mechanism ensures that a single plant can quickly colonize a large area with new seedlings. Furthermore, species like O. corniculata spread laterally via above-ground stems, called stolons, which root at the nodes, creating dense, spreading mats.
Comprehensive Control and Management Strategies
Successful management of Oxalis requires a persistent and integrated approach that targets both the plant’s aggressive reproductive structures and its seeds. When opting for manual removal, it is ineffective to simply pull the top growth, as the underground bulbils or taproots will remain and regenerate the plant. Instead, gardeners must use a trowel or similar tool to meticulously dig deep enough to extract the entire root system and any associated bulbils, making repeated efforts necessary, particularly during the plant’s active growth period.
Cultural control methods focus on modifying the environment to suppress the plant’s growth and prevent seed germination. Applying a thick layer of mulch, ideally 2 to 3 inches deep, can effectively block the light necessary for Oxalis seeds to germinate. Another option is soil solarization, which involves covering the infested area with clear plastic sheeting during warm, sunny weather for a period of four to six weeks to superheat the soil, killing bulbs and seeds within. Maintaining a dense, healthy lawn through consistent watering and fertilization also helps, as a thick turf canopy can successfully outcompete the low-growing Oxalis for resources.
Chemical control offers a supplementary strategy, utilizing both pre-emergent and post-emergent herbicides. Pre-emergent products create a chemical barrier in the soil to prevent new seeds from sprouting, though they are ineffective against plants that sprout from existing bulbs or rhizomes. For established plants, post-emergent herbicides designed for broadleaf weeds should be applied when the plant is actively growing but before it has produced mature seed capsules. Non-selective herbicides like glyphosate are also effective, but they must be applied with extreme caution to avoid damaging surrounding desirable plants.