Flower:

Flowers are typically borne in twos, each on a ¾ to 1 inch stalk at the tip of a long, slender 2 to 4 inch stem arising from the upper leaf axils. Flowers are yellow, about 3/8 inch across, with 5 rounded, oblong petals. The sepals behind the flower are lance-elliptic, about 2/3 the length of the petals and covered with short hairs. Stalks are densely covered in very fine hairs pressed close to the stem (appressed).

Leaves and stems:

Leaves are alternate or whorled with long slender stalks, palmately compound in 3s, ½ to 1 inch across. The 3 heart-shaped leaflets look clover-like and fold up at night and on cloudy days. A small lance-like stipule less than 1/8 inch long may be present at the base of the leaf node. Leaflets are toothless, the surfaces smooth except for scattered appressed hairs on the lower surface and along the leaflet edge.

Stems and branches have fairly dense, short, appressed hairs, becoming less hairy with age. Multiple stems from the base are erect to spreading and much branched. Spreading stems may root at the nodes but only the first few nodes near the base.

Fruit:

The fruit is a green, banana shaped capsule ½ to 1 inch inch long, with 5 angled sides and the 5 sepals spreading at the base. A typical plant has only 1 or 2 capsules in a cluster, on a stalk that is bent down or spreading horizontally. Like the stem it has short, dense appressed hairs that give it a somewhat silvery appearance and may have rows of spreading hairs along the capsule's angles. The capsule stays green when ripe and the seeds are forcefully ejected when the capsule splits open.

The seeds are just over 1 millimeter long, brown, oval and flattened, with rows of transverse ridges that are white capped like snow on mountain ridges.

Notes:

Largely considered a native weed, this and Yellow Wood Sorrel (Oxalis stricta) can be very difficult to discern without a hand lens, or better yet a microscope using mature specimens. The characteristics to look for in Oxalis dillenii are: the short, dense appressed hairs, the unbranched flower clusters with typically two flower buds, and fruits of 1 or 2 erect capsules on bent or spreading stalks. The ridges on the seed are white. O. stricta plants have spreading hairs, up to 7 flowers in a branching cluster, fruits on erect or ascending stalks, and uniformly brown seeds. Another very similar species is O. corniculata, a non-native that is low-growing and creeping, rooting at most nodes, with all brown seeds. O. corniculata has not yet been recorded in Minnesota but is present in neighboring states. Also as an observation, O. dillenii is the predominant species in mowed areas in full sun and branches profusely at the base from repeated mowings. O. stricta is more likely to be found along lawn, garden and woodland edges where they do not get frequent mowing and are often in part to full shade.