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Bermuda Buttercup (not native)

Oxalis pes-caprae

single yellow flower
Writer's yard | Feb. 2020

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 butterfly 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.

Other Common Names:

Sour grass

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.

Distribution 2,7,89,522

Bermuda buttercup is native to the Cape region of South Africa, where it is rare and endangered.522 It has invaded most of the Mediterranean climates of the world and some subtropical and semi-arid areas.46

In California it is a weed of disturbed habitats below 2500 feet (800m), mostly along the coast and occasionally in the Central Valley.7 It was introduced into the state around 1925 by the horticultural trade and has since become a naturalized weed in gardens, agricultural fields and other disturbed areas. Recently it has been reported invading natural dune areas in the northern part of the state.

In the Reserve, Bermuda buttercup is frequent along the boundaries and at trailheads; only a few, small clumps have been reported within the Reserve.

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.

Alternate Scientific Names:

Oxalis cernua, Bolboxalis cernua

Jepson eFlora Taxon Page

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 roots487,492 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 up to a yard (3 m) 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.

patches of yellow flowers beside the water

West Basin | Feb. 2020

patch of yellow flowers

West Basin | Feb. 2020

tiny white bulb

Bulblet on thread | Author's yard | March 2021

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.

field of yellow flowers beside the water

West Basin | Feb. 2020

opening yellw bud

Writer's yard | March 2020

plant with clover-like leaves and yellow flowers

Writer's yard | Feb. 2020

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

patch of yellow flowers beside the water

West Basin | Feb. 2020

single yellow flower

Writer's yard | Feb. 2020

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