Oxalis (Oxalis sp) should be familiar to California hikers as well as gardeners: native Redwood Sorrel carpet the floors of our redwood forests while Yellow Oxalis heralds the arrival of springtime in Davis, blanketing lawns, gardens, and empty lots. Like Four o'Clocks, oxalis flowers open during the daylight hours and close as evening nears. The plants seems to enjoy cool, moist growing conditions and flourish in deep shade, semi-shade, or sun. Typically, by summer in Davis, these plants die back, only to begin their return when the weather cools in winter.
Oxalis pes-caprae, known locally and simply as yellow oxalis, is considered by many a weed. Elsewhere, you may find this plant referred to as Bermuda Buttercup, Cape Sorrel, African WoodSorrel, SourSob, or Sour grass. Rare and endangered in its native South Africa, it has become as troublesome as escargot and tamarix in California without any native biological controls. Yellow oxalis begins blooming in early spring here and definitely brightens the landscape, but weaker ornamentals and grasses are easily overcome by this invasive plant.
Vibrant oxalis flowers have a simple five-petal structure. This non-invasive purple South American variety is known as Oxalis regnelli 'Triangularis' . Yellow oxalis spreads by rhizomes and bulblets, evident when a mature plant is pulled or dug up. The plant spreads very easily once soil is disturbed, as the bulblets scatter about. Lacking an aerial stem, the plant has many leaf or flower stalks growing randomly and close together from a single unbranched stem usually found underground.