Oxalis is a large and diverse genus of flowering plants, commonly known as Wood Sorrel or Sourgrass. With hundreds of species found across the globe, this plant takes on many forms, sometimes cultivated as an ornamental and other times acting as a persistent garden weed. Identifying whether the plant is a welcome addition or a bothersome invader requires careful visual identification. This guide focuses on the physical traits necessary to distinguish Oxalis from other plants it often mimics.
The Characteristic Leaf Structure
The most reliable feature for identifying Oxalis is its unique foliage, which consists of leaflets arranged in a pattern that often resembles a clover. These leaflets are typically trifoliate, meaning they appear in groups of three, though some species may exhibit four leaflets. Unlike true clover (Trifolium), each individual leaflet is distinctly heart-shaped, giving the overall leaf structure a delicate appearance.
The surface of the leaves can vary widely in color depending on the species, ranging from a vibrant light green, common in weedy varieties like O. corniculata, to deep shades of burgundy or purple, seen in ornamental types like O. triangularis. A useful distinguishing mark from true clover is the absence of the pale or white crescent-shaped pattern often found on the leaflets of Trifolium.
A behavior exhibited by many Oxalis species is nyctinasty, where the leaves fold down along the central vein. This folding occurs in response to environmental cues, such as the onset of night or during periods of intense sunlight or heat. The movement protects the leaf surface from excessive water loss, making the plant appear temporarily wilted or closed.
The stems supporting these characteristic leaves are often thin and delicate, sometimes exhibiting fine hairs, particularly in creeping varieties. The arrangement of the leaves along the stem is alternate, though they may appear clustered at the nodes or near the base in low-growing plants. Examining the stems near the soil line offers additional clues about the specific species present.
Identifying the Flowers and Seed Pods
The reproductive structures of Oxalis offer defining characteristics that confirm identification. The flowers are generally small, typically measuring less than one centimeter across, and display five distinct petals. These petals are often fused at the base, giving the bloom a funnel-shaped or star-like appearance when fully open.
The color palette of the blossoms is diverse, commonly featuring bright yellow, prevalent in the most widespread weedy species, but also including shades of white, pink, or even lavender in cultivated varieties. The flowers are typically produced in small clusters or cymes, rising on thin stalks that elevate them slightly above the dense mat of foliage.
Following successful pollination, the plant develops small, cylindrical fruit capsules that are noticeably ribbed along their length. These capsules are the seed pods, and their mechanism for seed dispersal is a unique and conclusive identification feature of the genus.
When the seeds inside the capsule mature, the pod is triggered to dehisce, or burst open, often with a slight audible pop. This explosive action forcibly ejects the seeds outward, sometimes propelling them several feet away from the parent plant. Observing these small, ridged capsules and their sudden action is a definitive way to confirm the presence of Oxalis.
Growth Patterns and Underground Systems
The overall appearance of Oxalis depends heavily on its specific growth habit, which varies from low, creeping groundcovers to small, upright clumps. Weedy species often spread aggressively, using slender runners or stolons that root at the nodes to establish new plantlets across a wide area. Other species form tight, dense tufts that make the plant appear more contained.
The underground structures are fundamental to the plant’s persistence and are often the reason for its difficulty to manage. Many perennial Oxalis species develop tough, fleshy taproots that anchor the plant deeply and store energy for regrowth. Others rely on thin, brittle rhizomes that spread laterally just beneath the soil surface.
A particularly problematic feature of many weedy varieties is the production of small, easily detached bulbils or bulblets, which are miniature storage organs. These small, scale-covered structures are easily broken off during any attempt to pull or disturb the plant, with each detached piece having the potential to sprout into a new, independent plant.