Flower:

Single irregular 5-petaled flower at the end of a slender stalk at the top of the stem and arising from the leaf axils. Flowers are yellow, up to ~½ inch (≤ 12 mm) long. The upper two petals are erect or bent back, the two lateral petals have short hairs at the base (bearded), the lower petal is hairless, has purple veins near the base, and forms a short rounded spur at the back.

The 5 sepals around the base of the flower are hairless, sometimes fringed around the edges, the two lower largest, lance-linear to lance-shaped, blunt to pointed at the tip, and have a short extension at the base (auricle) that is straight across to rounded along the end and does not elongate in fruit. Flower stalks are hairless to sparsely hairy.

Leaves and stems:

Typically, there are 4 or 5 alternate leaves on upper half to 4/5 of the stem as well as 1 to 3 basal leaves, though sometimes the lowest 1 or 2 stem nodes are leafless so leaves may appear all clustered on the upper stem. Leaves are heart-shaped, pointed at the tip, often abruptly narrowed at the tip. Largest leaves are up to about 2½ inches (64 mm) long, often somewhat wider than long.

Surfaces are variably hairy, from hairless to densely short-hairy. Edges are toothed and may or may not have a fringe of hairs. Leaf stalks are hairless to sparsely short-hairy. At the base of the leaf stalk is a leafy appendage (stipule) that is lance-oblong to egg-shaped, mostly rounded and asymmetric at the base. Stems are single or a few from the base, ascending to erect or may be prostrate from the base at flowering time. No stolons are produced.

Fruit:

Both petalled (chasmogamous) and petal-less, self-pollinating (cleistogamous) flowers produce fruit, in an ovoid capsule that becomes erect just before splitting into 3 sections and releasing its seed. Chasmogamous flowers bloom in spring. Cleistogamous flowers are produced all summer into fall. All capsules are green drying tan or brown, hairless to densely hairy, up to ½ inch (7 to 13 mm) long.

Seeds are straw-colored maturing to medium orange-brown, up to 2.7 mm long.

Notes:

Smooth Yellow Violet is one of the most common violets in Minnesota, usually in mesic to moist soil, almost always in wooded areas. It's a great addition to a shade garden. In Minnesota, this was previously lumped in Viola pubescens (Downy Yellow Violet), but a major overhaul of Viola accounts published in 2023 has split up V. pubescens, making V. eriocarpa the dominant yellow woodland violet here. V. pubescens is still present, but its numbers are few and is more common in Wisconsin.

V. eriocarpa is recognized by: stems up to 14 inches tall usually with 4 or 5 alternate leaves and 1 to 3 basal leaves, sometimes the lower 1 or 2 nodes are leafless; heart-shaped leaves often a bit wider than long, pointed at the tip, hairless to densely hairy; stipules mostly rounded and asymmetric at the base; yellow flowers, lateral petals bearded; green capsules, orange-brown seeds. Plants commonly have more than one stem from the base.

While V. eriocarpa is pretty variable in many characteristics, V. pubescens is much more consistent: stems are single from the base, only 2 or 3 stem leaves all clustered near the tip, almost always lacks basal leaves, leaf bases are less heart-shaped (more straight across to wedge-shaped), and stipules are more wedge-shaped at the base. In the case where a plant has a single stem with leaves all near the tip, look on the lower stem for a stipule without a leaf. That would be a sign of V. eriocarpa rather than V. pubescens.

The third yellow violet in Minnesota is Viola nuttallii (Yellow Prairie Violet), which is a rare species only present in a few locations in our western counties. Its leaves are narrowly lance-shaped and toothless, and it lives in open prairies, not woodlands.