Yellow Wood Sorrel
Oxalis stricta
Wood Sorrel family (Oxalidaceae)

Description: This herbaceous perennial plant is typically 3-8" tall, although sometimes it becomes up to 1' tall. This plant often branches abundantly, particularly toward its base. The stems are light green and terete; they are covered with short appressed hairs. Alternate leaves occur along the stems; they are trifoliate. In the absence of sunlight, the leaflets of these trifoliate leaves droop downward and fold along their central veins; this process reverses itself when sunlight reappears. Individual leaflets are -" (6-13 mm.) long and similarly across; they are obcordate in shape and smooth (entire) along their margins. Both the upper and lower leaflet surfaces are pale green; the upper surface is glabrous or nearly so, while the lower surface is covered with short appressed hairs. The ascending petioles of the leaves are up to 2" long, light green, and terete; they are covered with short appressed hairs. The petiole bases are slightly swollen as a result of their fused stipules. Small umbels of 2-6 flowers are produced from the axils of leaves. These flowers are about " (6 mm.) or a little more across when they are fully open.

Each flower consists of 5 yellow petals, 5 light green sepals, 10 stamens, and a pistil. The 5 styles of the pistil are joined together, except at their apices. The petals are oblanceolate in shape with either broadly rounded or slightly notched tips; sometimes they are reddish toward their bases. The sepals are oblong-lanceolate in shape, covered with short appressed hairs, and about 3-4 mm. in length; they are shorter than the petals. The pedicels are up to 1" (2.5 cm.) long, light green, and covered with short appressed hairs; they are initially ascending or erect while the flowers are in bloom, but they later become reflexed (bent downward) as the seed capsules develop. At the base of each umbel, there is a pair of tiny linear-lanceolate bractlets. The blooming period can occur from late spring to the mid-autumn; a small colony of plants may remain in bloom for 2-4 months. The flowers are diurnal. Afterwards, the flowers are replaced by seed capsules about 8-20 mm. long that are cylindrical in shape and 5-sided; their apices are beak-shaped. Immature capsules are light green and their sides are covered with either short appressed hairs or a combination of short appressed hairs and long spreading hairs. At this time, the pedicels become sharply reflexed, bending slightly to moderately downward at their bases, and then bending sharply upward near the bases of the seed capsules to hold them erect. As the capsules continue to mature, they split open into 5 parts to eject their seeds up to several feet away from the mother plants. The small seeds are about 1.0-1.5 mm. long, reddish brown to brown, broadly ellipsoid in shape, and somewhat flattened; they have several transverse ridges that are often whitened. The root system consists of a taproot that branches.

Cultivation: The preference partial to full sun, mesic to dry-mesic conditions, and soil containing loam, clay-loam, gravelly loam, or sandy loam. This plant can spread aggressively by reseeding itself, particularly in open areas where the ground surface has become exposed.

Range & Habitat: Yellow Wood Sorrel (Oxalis stricta) is common, occurring in every county of Illinois, where it is native (see Distribution Map). It is widely distributed in central and eastern United States, including adjacent areas of southern Canada. Habitats include open woodlands, grassy meadows, lawns, gardens, edges of driveways, areas along parking lots, vacant lots, roadsides, areas along railroads, construction sites, landfills, and sunny waste areas. Areas with a history of disturbance are preferred.

Faunal Associations:
The nectar and pollen of the flowers attract Halictid bees and other bees, flower flies (Syrphidae), bee flies (Bombyliidae), and a butterfly, the Cabbage White (Pieris rapae). Insects that feed destructively on wood sorrels (Oxalis spp.) include Abstrusomyzus reticulatus (Wood Sorrel Aphid), Rhopalosiphoninus staphyleae (Mangold Aphid), Melanoplus confusus (Little Pasture Grasshopper), Melanoplus femurrubrum (Red-Legged Grasshopper), Melanoplus keeleri luridus (Keeler's Grasshopper), and the caterpillars of a Noctuid moth, Galgula partita (The Wedgling). Some vertebrate animals also feed on wood sorrels. The seeds of these plants are eaten by several bird species, including the Bobwhite, Painted Bunting, Slate-Colored Junco, Horned Lark, Field Sparrow, Grasshopper Sparrow, Savannah Sparrow, and Tree Sparrow. Some mice, including the Deer Mouse and White-Footed Mouse, eat the seeds. The foliage is browsed occasionally by the White-Tailed Deer and Cottontail Rabbit, even though it is mildly toxic from the presence of oxalic acid.

Photographic Location: Along a railroad in Urbana, Illinois.

The Yellow Wood Sorrel (Oxalis stricta) has dainty trifoliate leaves and small yellow flowers. Small amounts of the sour leaves can be added to salads. This wood sorrel can be difficult to distinguish from a similar species that shares the same common name, namely Oxalis fontana. The latter species differs by having long spreading hairs along its stems and petioles, or these parts of the latter plant may be glabrous; as its seed capsules develop, they remain erect or ascending, rather than becoming sharply reflexed like those of Oxalis stricta. Because Oxalis fontana prefers habitats that are less sunny than Oxalis stricta, its leaflets are usually a deeper shade of green, and it is usually a taller plant that is less branched. Other differences include the occasional presence of cymes, rather than umbels, among the flowers of Oxalis fontana, and the seeds of this latter species have non-whitened ridges. There is disagreement regarding the taxonomy of these two species of wood sorrel. The treatment here follows Mohlenbrock (2002) and Yatskievych (1999). Other authorities refer to Oxalis stricta as Oxalis dillenii, and they refer to Oxalis fontana as Oxalis stricta (Mohlenbrock considers Oxalis dillenii to be a junior synonym of Oxalis stricta). This has been a source of confusion regarding the identity of these two species. It is also possible that some plants in the field may display hybrid characteristics between these two species, making them difficult to identify.