Plant Story--Oxalis, Wood Sorrel

Wood sorrels, Oxalis, with flowers like little yellow stars, are common plants in disturbed areas across much of North America. They are worth a second look. 

common wood sorrel, Oxalis stricta

Wood sorrels, genus Oxalis, are the dominant group of the family named for them, the wood sorrel family, Oxalidaceae. There are at least 700 species of Oxalis, found all over the world. Some are native to Europe, giving us a long European history with them, but most species are New World and Oxalis has especially many species in Mexico and Brazil, and curiously, South Africa. We see them as weeds in lawns and along paths but in the tropics some species grow over 6' tall and others are vines.   

The flowers typically have five petals in an open arrangement. Yellow is a common color but some species have white or pink or hot pink flowers. The leaves are mainly compound with three leaflets. Many species fold their leaves at night. The fruit is often a pod (capsule) with the squarish sides. When the capsule dries out, pressure inside causes it to burst open, scattering the seeds. 

wood sorrel seed capsule

My yard, and much of North America, has two common weedy yellow-flowered species of Oxalis, upright yellow wood sorrel, also called common wood sorrel, Oxalis stricta, with bright green leaves, and creeping wood sorrel Oxalis corniculata with leaves that are often brown or even purple.

creeping yellow wood sorrel, Oxalis corniculata

The common name wood sorrel refers to the similarity in taste between Oxalis species and the sorrels, Rumex species. Both contain significant amounts of oxalic acid, a mild and tangy organic acid, also found in spinach. Oxalic acid was identified from wood sorrel plants, hence the name oxalic acid.  Oxalic acid can be poisonous in large amounts but people like small amounts of it. As a child I frequently nibbled wood sorrel leaves and seed pods. Tasty!

The wood sorrel of western Europe, Oxalis acetosella, was eaten as a vegetable for hundreds of years. Grieve, reviewing medicinal plants in the 1930s, says use of wood sorrel was pushed out by the availablilty of French sorrel, Rumex scutatus (buckwheat family, Polygonaceae) which has the same sharp taste but larger, succulent leaves. A number of plants have sorrel in their common name because they share the tangy oxalic acid: sheep sorrel (Rumex acetosella), common sorrel (Rumex acetosa), and mountain sorrel (Oxyria remiformis), for example. If you haven't munched any of these, fresh spinach (Spinacia oleracea, amaranth family, Amaranthaceae) contains enough oxalic acid to give you a sense of the flavor. It is not clear to me whether American wood sorrels have safe levels of oxalic acid or not;  they do not appear in any of my foraging books. However, all across the continent, Native American tribes reported eating various species of wood sorrels; raw or cooked, with sugar as a dessert, to complement dried fish, to ease thirst on a long walk. 

The Menominee boiled both yellow wood sorrels to make a yellow dye. The Meskwaki got an orange dye from upright yellow wood sorrel. 

Oxalis is based on the Latin word for sour, oxys. The species epithets acetosella and acetosa are old Latin common names for sorrels, from acetum, "vinegar."  The species epithet corniculata means "with horns" and stricta means "upright," both terms trying to help distinguish the species from other Oxalis species. Apparently the upright seed pods of creeping wood sorrel looked like horns (see photo above) and common wood sorrel plants grow as small compact individuals, not creeping along the ground like O. corniculata and others.

Unidentified weedy wood sorrel in San Francisco, California

English sources call wood sorrel sour grass and three-leafed grass, grass in this case being a very old general word for "plant". Wood sorrel of Europe flowers early in the spring, so was called cuckoo's sorrel, because the cuckoo returned at the end of winter about when the wood sorrel flowered. Common names alleluya and hallelujah referred to the singing of Easter hymns at the time it flowered. It was also called poor man's lettuce. 

Wood sorrel is one of the candidates for being the shamrock. Even the Irish are not sure what plant was the one St. Patrick used to explain of the trinity to the Irish. Polls asking the Irish what plant is the shamrock most often (46-51%) pick lesser clover, Trifolium dubium, common in Ireland, but four other plants including wood sorrel have supporters. In the 1830s English botanist James Bicheno championed wood sorrel as the real shamrock. He was highly influential so for generations wood sorrel was argued to be the shamrock and its distinctive leaves drawn when shamrocks were needed. Twentieth century evaluation of the evidence found, actually, no clear link of any shamrock to St. Patrick; the oldest version of the story is from about 1680. The folk tradition is that it was "a young clover." Today, wood sorrel is not believed to be the shamrock. 

The two common American wood sorrels are very diverse, having different characters in different areas across the continent. For example, creeping wood sorrel is a polyploid complex, with different plants having  24, 36, 42, 44 or 48 chromosomes (blog on polyploidy first of series of posts), while upright yellow wood sorrel has flowers that range from homostylus (stigma and styles all at the same level) to strongly heterostylous (two kinds of flowers, one with stigma higher than stamens, the other with stigma lower than stamens (blog on heterostyly). 

Many species of Oxalis are garden flowers, but they mostly have larger leaves and flowers than Oxalis corniculata and O. stricta. 

growing wood sorrels as a ground cover,
under trees and next to a lawn, Hangzhou, China

A very interesting group of plants...and I still find them tasty.

Comments and corrections welcome.

References

Grieve, M. 1932. A Modern Herbal. Dover. New York. online

Mac Coitir, N. 2015. Ireland's Wild Plants. The Collins Press. Wilton, Cork.

Moerman, D. E. 1998. Native American Ethnobotany. Timber Press, Portland, Oregon. online

Nesom, G. L. 2020. Oxalis Linnaeus. Flora of North America online link

Stearn, W. T. 1996. Stearn's Dictionary of Plant Names.  Cassell Publishing. London. 

Kathy Keeler, A Wandering Botanist

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