Biennial herb 30 - 60 cm tall Stem: erect, densely hairy with short, slender hairs up to 0.5 mm long, and also with longer, 2 - 3 mm long, stiff, spreading hairs. Leaves: alternate, stalked (though stalks mostly shorter than the inflorescence branches), 6 - 15 cm wide, circular in outline, but very shallowly, palmately five- to seven-lobed, with wide bases between the lobes, and the lobe tips pointed. The lowest leaves are sometimes pinnately divided. All leaf bases inversely, broadly v-shaped, or cut abruptly straight across. Inflorescence: of loosely-flowered, rounded, branching clusters on long (mostly taller than leaves), densely hairy stalks arising from the upper leaf axils. The inflorescence branches, as well as the individual flower stalks, have a dense covering of intermixed short and long hairs. Flowers: slender-stalked, lavender or pinkish purple, 0.9 - 1.3 cm long, radially symmetric, and tubular. Sepals: five, but fused at the very base, then separating into narrow, densely bristly-hairy lobes. Alternating the base of each lobe is a conspicuous, 0.5 - 1.5 mm long, reflexed, earlobe-shaped appendage, which enlarges in fruit. Petals: five, but fused for about half their length, then separating into erect, oblong, flat-tipped lobes. Five linear nectaries line the inside of the petal tube opposite each petal lobe. Stamens: five, filaments usually hairy, attached near the base of the petal tube (between the linear nectaries), alternating with the petal lobes, and extending only a few millimeters beyond the petals. Pistil: with a single-chambered, superior, bristly ovary; and a single style ending in two, short, rounded stigmas, which extend a few millimeters beyond the petals. Fruit: a spherical, single-chambered, two-valved capsule with one to three seeds. The earlobe-shaped appendages at the base of the sepal lobes enlarge greatly in fruit. Root: a taproot.
Similar species: Hydrophyllum appendiculatum is similar to H. virginianum but that species has short, appressed, and ascending hairs on the inflorescence branches; the main inflorescence stalk forks into two; there are no appendages between the sepal lobes; and the stem hairs (if present) are appressed and under 0.5 mm long. Also somewhat similar is H. canadense, but that species usually has the leaves over-topping the inflorescence, the leaf bases are indented like the top of an upside heart, there are no large reflexed appendages between the sepal lobes (sometimes only a small upright tooth), the stem hairs are up to 1 mm long and all of one kind, and the inflorescence branches are mostly hairless, though if sparsely hairy the hairs are all of one length.
Flowering: early May to July
Habitat and ecology: Locally frequent in good quality mesic woods.
Taprooted biennial; stems 3-6 dm, the upper part and infl densely pubescent with short slender hairs 0.3-0.5 mm and conspicuously hirsute with spreading hairs usually 2-3 mm; cauline lvs mostly overtopped by the cymes, orbicular in general outline, 6-15 cm wide at anthesis, truncate or with a broadly V-shaped base, shallowly 5-7-lobed, with obtuse or rounded sinuses; sep separate nearly to the base, densely hirsute, alternating with conspicuous small reflexed appendages; cor lavender or pink-purple, 9-13 mm, the lobes about equaling the tube; stamens and style exsert 1-3 mm; 2n=18. Rich moist woods; s. Ont. to s. Minn., s. to Pa., Tenn., Mo., and e. Kans. May, June. (Decemium a.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.