Perennial herb up to 40 cm tall Stem: slender, branched, decumbent at base, and covered with short, down-curved hairs. Leaves: opposite, stalked, palmately three- or five-lobed, each lobe narrow and coarsely toothed. The leaf stalks are covered with down-curved, short hairs. Flowers: violet, stalked, small (up to 2 cm diameter), radially symmetric, and in pairs atop long (up to 8 cm), hairy, axillary stalks. The two flower stalks at the tip of each main stalk are relatively long (more than twice the length of the sepals), and covered with long, soft, spreading hairs. Sepals: five, alternate with petals, green, spreading-hairy, lance-shaped with an abrupt about 1 mm long point at the tip. Petals: five, violet, about same length as sepals, broadly inversely egg-shaped with narrowed base, and wider, flattened to shallowly rounded tip. Next to the base of each petal there is a gland, thus making a ring of five glands alternate the petals. Stamens: ten in two series, all fertile, with filaments widened at base, and anthers at tips. The ring of five longer stamens are aligned with the petals, while the five shorter stamens are alternate with the petals. Pistil: with a single, deeply five-lobed, superior ovary; one elongated style column; and five linear stigmas. Fruit: five, erect, hairy, non-glandular, about 1.7 cm long, single-seeded, rounded base, beaked, nutlike segments surrounding remnant elongated style column of each flower. Each nutlike segment has a narrowed up to 1.5 mm long beak at its tip, which is attached to the lower part of the style column, and then is pulled upwards by the coiling outer wall of the style column, yet stays attached to the entire fruiting structure and ejects the seed from the main body of the nutlike segment. Root system: of a stout, angled rhizome.
Similar species: Geranium thunbergii is fairly similar to the native G. bicknellii and the non-native G. columbinum, but both those species lack a stout rhizome system, they have longer and bristled tips on the sepals, and also a longer beak section on the fruit (over 1.5 mm long). Like those two species, G. thunbergii differs from our other species of Geranium since its individual flower stalks are more than twice the length of the sepals. Geranium sibiricum is also very similar, but it has a short inconspicuous rhizome, longer reflexed hairs on the stem and branches, often only single flowers per stalk, shorter petals which are marked with dark veins, and fruit with a shorter apical beak (about 1 mm). A final non-native species that may appear similar is G. dissectum, but that species has short bristles on the sepal tips and a much shorter, if any, apical beak on the fruit.
Habitat and ecology: Incredibly rare in the Chicago Region, this species has only been documented with certainty once from a collection in a shaded, wet meadow along a stream near a roadside.
Occurence in the Chicago region: non-native
Notes: This species is native to southeast Asia and is used as a garden plant in England and was subsequently introduced to North America, where it sometimes escapes cultivation. Most material cultivated under the name Geranium wilfordii Maxim. is actually G. thunbergii.