When I first saw large clover-like leaves emerging on one of our eastern slopes shortly after we moved here, I was fearful that we may have the moderately invasive Oxalis pes-caprae on the property. However, much to my surprise, and delight, this clover-like plant turned out to be our native Redwood Sorrel (Oxalis oregana).
Oxalis oregana is native to the west coast of North America, from British Columbia to California. Redwood Sorrel, as its common name implies, is often found in close association with coast redwood trees (Sequoia sempervirens). Ours is growing through redwood leaf litter in moderate to dense shade, primarily in the company of western sword ferns. Other common wild companion plants include giant chain fern, wild ginger and trillium.
The leaves of redwood sorrel are composed of three heart-shaped leaflets borne on 5-20cm stalks.
The stems and underside of the leaves are tinted burgundy to purple.
The conserve water the leaflets fold along a central axis during periods of drought, or when exposed to strong sunlight. The leaflets may also fold during periods of heavy rain which reduces the impact of the raindrops on the leaves.
Flowers are borne on solitary stalks and may reach up to 2 cm in diameter.
Flowers have five petals and sepals. Petal color can range from white to pink, often with darker lavender veins visible.
West coast Native American tribes have a long history of using Oxalis oregana. The Cowlitz are known to have used the juice as a medication for sore eyes. Pomo and Kashaya tribes utilized a decoction of the plant for rheumatism. Some tribes cooked and ate the leaves. Others, like the Pomo, Cowlitz, Makah and Kashaya ate the leaves raw for their tangy sour taste. The sour taste of Oxalis oregana is due to oxalic acid, a compound present in all plants of this genus (in Greek oxys means sour).
Although the leaves are edible, in large quantities raw leaves may be harmful. Oxalic acid can bind calcium, forming calcium oxalate, a major component in kidney stone formation in humans.
In cultivated gardens, Oxalis oregana is used as a ground cover in shade gardens. Dense clay soil, and heavy root competition can slow the establishment of Redwood Sorrel in the garden, but once established it can spread rapidly. Without supplemental irrigation, plants will become dormant in the warm late summer months, but will re-emerge with the winter rains.
 Daniel E. Moerman’s Native American Ethnobotanical Database