Bermuda Buttercup (not native)

Native to South Africa, where it is endangered, Bermuda buttercup (Oxalis pes-caprae) is a small plant with bright yellow flowers and leaves like shamrocks.  Its alternate name, sour grass, comes from the presence of oxalic acid in the sap, which gives the plant a refreshingly tart flavor.

In someone else’s yard, Bermuda buttercup is beautiful; in your own, it becomes an intractable enemy. Bermuda buttercup has become naturalized throughout the Mediterranean climates world-wide. It has the unlikely distinction of having achieved noxious weed status in much of California without having produced a single seed.

Description ^2,4,23,34,59

Bermuda buttercup is a rounded perennial, less than 20 inches (50 cm) high, that grows from an underground bulb. A short stem supports a clump of clover-like leaves; clumps can form dense carpets. Leaves are usually 1½ inch (4 cm) or less across, and consist of three heart-shaped leaflets from a single point. Leaflets are often marked with red-brown splotches. Leaves and stems contains oxalic acid, which gives them a tart flavor, and results in another common name: sour grass.

Showy, symmetrical, bright yellow flowers open only during the day. Twenty of fewer flowers occur in clusters held above leaves on long peduncles and opening a few at a time. Each has five sepals and five petals. The light green sepals are lanceolate, and overlapping, forming a tube around each flower base. There may be tiny orange tubercles at the sepal tips. Yellow petals form a narrow tube at their bases and flare outward to 1½  inch (4 cm) across. Flowers may have tiny purple lines radiating outward from throat. There are ten bright yellow stamens, alternating short and long, and one pistil with five styles with capitate, papillose stigmas that are shorter than the anthers. In our area, blooms occur during the wet season, usually Nov. – April.

Fruits are not produced in California. Plants reproduce vegetatively by means of numerous tiny bulblets.

Three ploidy “types” exist in this species. In addition to diploid plants, tetraploid and pentaploid plants have been found. These are morphologically distinguishable by the length of the stamens and the relative position of the styles. So far as is known, plants in California are all pentaploid.

Classification ^11,23,143

Bermuda buttercup (Oxalis pes-caprae) is a dicot herb in the wood sorrel family (Oxalidaceae), a family of mostly herbs with compound leaves and five sepals and petals, five styles and ten stamens. It may be best known for oxalic acid in the cell sap that gives the species a tart taste.

Of the 16 species of Oxalis found in California, only five are native.⁷ The only other genus member reported from the Reserve is the weedy Oxalis corniculata, also a common garden weed, with small yellow flowers from a creeping, rooting stem.

Ecology

Bermuda buttercup is an aggressive invader but doesn’t produce seeds. That seems wildly improbable. In spite of research, the explanation is still incomplete, but seems to hinge on two independent adaptations.

The first is presumably a mechanism to inhibit self-fertilization in the native range.⁵²³ There are three ploidy levels in this species and they are primarily self-sterile. Thus a diploid plant can be pollinated by tetraploid or a pentaploid plant, but not by another diploid plant, etc. It is thought that the independent introductions of Bermuda buttercup into non-native habitats have each been based on a single ploidy level. Since the original plants, and their vegetative progeny are all of the same ploidy type, they can not cross-fertilize and no seeds are produced. (This explanation is, of necessity, oversimplified. For details visit the scientific literature)

The second adaptation is a unique system for vegetative reproduction and dispersal.⁵²⁴ This involves fleshy contractile roots^487,492 and a thin underground stem often called a thread that bears numerous small bulblets.  While contractile roots usually serve to lower the parent bulb into the ground, the contractile root of Bermuda buttercup served to direct the thread and the bulblets, out and away from the parent plant. As many as 40 renewal bulbs have been found distributed more than a foot (30 cm) away from the parent plant. The threads are easily broken and any additional earth movement (anything from gopher digging to cultivation to major construction) further distributes the renewal bulbs.

Human Uses ^34,525,526

In its native range in South Africa, Bermuda buttercup is used in traditional dishes. In California, it’s most common use may be by small children who chew on the stems for the sour tang. It has also been used to make summer drinks and hot teas, and in desserts.

Interesting Facts

The genus name, Oxalis comes from the Greek word oxys meaning  “sharp” or “ sour.”²¹ The organic acid, oxalic acid was named for the genus, which used to be its primary source.⁴¹