Common Yellow Oxalis is considered a native forb, either an annual or short-lived perennial that grows from 5 inches to 24 inches high but is usually less than 12 inches high. Stems, which are usually single, may be erect or later drooping along the ground, have fine hair, and have pale green to reddish coloring.
The leaves are alternate, palmately 3-parted, each leaflet up to 1/2 inch long, not stalked, bright green in color with a smooth surface and a deep notch at the rounded tip creating a heart shape. The leaflet has a distinct center vein along which the leaflet folds at night or when touched - as they are sensitive to touch, wait a few seconds and the leaf begins to fold. The entire leaf is on a long slender stalk that has fine hair and the margins of the leaflets, while not toothed, can have fine hair also.
The inflorescence is an open branched cluster (a cyme) of 2 to 5 long-stalked flowers, the cluster stalk hairy with the flowers held just above the leaves or at the level of the leaves. At the base of the cluster stem there are a few short linear bracts. Clusters branch with age.
The flowers are 5-parted, from 1/3 to 3/4 inch wide with a lemon yellow corolla that splits into 5 bluntly rounded petals. The green calyx has 5 pointed green sepals, much shorter than the petals, slightly hairy. The flowers are perfect with 10 stamens, five long and five short, all united at their base and all with yellow anthers; there are 5 styles that are separate at the top with terminal stigmas, but fused together at the base to a 5-celled ovary.
Seed: The yellow petals whither quickly as the flower matures to an erect seed capsule, from 1/3 to 1+ inches long that is cylindric in shape, somewhat angled, and sharp-pointed. The capsule is 5-valved and when mature, splits into the 5 sections, ejecting very small brown flattened ovoid shaped seeds that are covered with transverse wrinkles. The stalk of the capsule ascends, usually at an angle of less than 45 degrees, but is not horizontal or first bending downward like that of O. dillenii.
Habitat: Common Yellow Oxalis grows from a rhizomatous root system, without a taproot, that allows it to spread vegetatively. It grows primarily in disturbed sites, roadsides, woodland edges, untended lawns, etc. Dry conditions are tolerated, full to partial sun required.
Names: The genus Oxalis is derived from the Greek work oxys, meaning 'acrid, sharp, sour' which reflects the sour taste of stems and leaves. The species stricta means 'erect' or 'upright', referring to the erect seed capsule stalks. The author name for the plant classification - 'L.' is for Carl Linnaeus (1707-1778), Swedish botanist and the developer of the binomial nomenclature of modern taxonomy.
Comparisons: There are a number of yellow flowered Oxalis. The closest most confusing species in our area will be O. dillenii, Slender Yellow Woodsorrel. Some differences are that it has a taproot usually, the flower cluster is usually unbranched with only one or two flowers and the seed capsule stalk has a horizontal bend before the capsule turns erect. Botanist Guy L. Nesom in 2009, published a taxonomy of the yellow flowered Oxalis in Eastern North America where he provides a key and descriptive text: PDF of document.
Above: The contrasting colors of the plant are quite nice. Drawing from Britton N.L., and A. Brown. 1913. An illustrated flora of the northern United States, Canada and the British Possessions. 3 vols. Charles Scribner's Sons, New York.
Below: Front and back views of the flower. The green sepals are slightly hairy. Note that 5 of the 10 stamens are shorter. The 5 styles are fused at the base but separated at the stigmas.
Below: 1st photo - The shamrock like leaf has heart-shaped leaflets, each with a center fold line where the leaflet closes at night or when bruised. 2nd photo - The upright seed capsules do not have a horizontal bend in the stalk which is an identifying characteristic to separate this species from O. dillenii.
Below: 1st photo - Stems can have reddish coloring and fine hair. 2nd photo - Upper stem parts are hairy. 3rd photo - The inflorescence has 2 to 5 stalked flowers. Here one is open, another is forming. Only one flower in a cluster is open at one time. Note the small pointed bracts at the base of the cluster.
Notes: Common Yellow Wood Oxalis is considered indigenous to the Garden area. Eloise Butler noted it in her Garden Log on May 31, 1907. In 1938 Curator Martha Crone made note of it in bloom and it was one of 4 Wood Sorrels she listed on her 1951 Garden census. It is one of four Wood Sorrels found in Minnesota and is the most widespread - very common in the southern part of the state. It is found throughout the U.S. except in 4 western states and is found in most of the lower Canadian Provinces. The 4 species of Oxalis found in Minnesota are O. montana (formerly O. acetosella), Mountain Woodsorrel; O. dillenii, Slender(or Southern) Woodsorrel; O. stricta, Common Yellow Oxalis; and O. viloacea, Violet Woodsorrel.
References: Plant characteristics are generally from sources 1A, 32, W2, W3, W7 & W8 plus others as specifically applied. Distribution principally from W1, W2 and 28C. Planting history generally from 1, 4 & 4a. Other sources by specific reference. See Reference List for details.
Identification booklet for most of the flowering forbs and small flowering shrubs of the Eloise Butler Wildflower Garden. Details Here.
Friends of the Wild Flower Garden, Inc. Text and photos are by G. D. Bebeau unless otherwise credited. "www.friendsofthewildflowergarden.org"