Annual herb 10 to 60 cm tall Leaves: opposite, stalked, 3 - 7 cm wide, rounded or kidney-shaped in outline but deeply, palmately five- to nine-lobed, with lobes oblong and further deeply lobed or toothed. Flowers: pale pink, short-stalked, 1.5 - 2.5 cm diameter, radially symmetric, and though paired atop short axillary stalks, often appearing as a many-flowered, rounded, umbel-like cluster due to crowded arrangement of leaves. The individual shorter flower stalks are about the same length as the sepals, and they are covered with long, spreading, normally non-glandular hairs. Sepals: five, alternate with petals, green, three-nerved, soft-hairy (hairs short except longer on nerves), up to 1 cm long, narrowly egg-shaped with tip narrowed to a short stiff bristle. Petals: five, pale pink, equal in length to sepals, inversely egg-shaped with narrowed base, and notch at center of 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 up to 1 mm long anthers. 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, long-hairy (hairs about 1 mm, ascending, and dark), 2 - 5 cm long, single-seeded, rounded base, beaked, nutlike segments surrounding remnant elongated style column of each flower. Each nutlike segment has a 1 - 2 mm long, narrowed 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. Fruit stalks may elongate up to one and a half times the length of the sepals when mature. Stems: several, freely branched, and covered with long, straight, soft, spreading hairs, which may recurve further upward.
Similar species: Geranium carolinianum is similar to several non-native species that have occasionally escaped in our area. Geranium sibiricum differs since the apical beak of the fruit is only about 1 mm long. Geranium molle differs by not having bristle-tips on the sepals. Geranium dissectum differs by having the flower stalks covered with both long, gland-tipped hairs and intermixed short hairs; the petals are deeper pink; and the nutlike portions of the fruit are covered with short (about 0.5 mm long), spreading, white hairs. A more common non-native species that is similar to G. carolinianum is G. pusillum, which lacks bristle-tipped sepals and only has five fertile stamens. Other species in our area have looser, less densely clustered inflorescences with the individual flower stalks over twice the length of the sepals.
Flowering: late May to July
Habitat and ecology: An occasional species of disturbed sandy fields, also appearing after fire, and often in wooded areas around the edges of old campfires. In Cook County, Illinois, this plant also occurs in calcareous meadows where limestone is near the surface.
Occurence in the Chicago region: native
Notes: Most of the plants in our area could be designated as G. carolinianum var. confertiflorum as described above (Gleason and Cronquist 1991). This variety has tight, compact inflorescences with mostly soft and long (about 1 mm) hairs. In the typical variety, the inflorescence is relatively loose and open and is covered with shorter hairs (only about 0.5 mm) that are often mixed with short glands.
Author: The Field Museum
From Flora of Indiana (1940) by Charles C. Deam
This species prefers sandy to very sandy soils and is found as a weed in fallow fields, hayfields, pastures, and open, pastured woods and along roadsides and railroads. On account of its weedy nature it is debatable whether this species is a native of the state. Some of our oldest floras do not list it and others record it as found in waste places and fields and along roadsides and railroads. [Deam provides the following treatment for var. confertiflorum, a form with spreading rather than retrorsely appressed pubsecence on the stem:] In addition to this variety intermediate forms occur. This form is not very distinct in Indiana. The habitats are similar to those of the [full] species.
Annual; stems several, freely branched, eventually to 6 dm, villous with spreading or somewhat retrorse hairs, and becoming glandular above, lvs rotund- reniform, 3-7 cm wide, deeply 5-9-cleft with oblong to obovate, deeply toothed or lobed segments; peduncles mostly 2-fld, the pedicels to twice as long as the cal; sep to 1 cm, short-awned, equaling the retuse pet; fr 2-5 cm, the stylar beak 1-2 mm, the carpel-bodies hirsute with long antrorse hairs ca 1 mm; seeds very obscurely reticulate; 2n=52. Dry, barren or sandy soil and waste places; Me. to B.C., s. to Fla., Tex., and Calif. May-Aug. Var. carolinianum, chiefly southern, but extending n. to Mass. and Mich., has a relatively loose and open infl; var. confertiflorum Fernald, the common form in our range, has very short upper internodes, the fls thus in compact, many-fld, umbel-like terminal clusters. (G. sphaerospermum Fernald, plants with seeds of maximum plumpness and sep of maximum width an unusual combination)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.