Perennial herb 1 - 3 m tall Stem: erect, tall, possibly rough, branched above. Leaves: alternate, stalked, coarsely toothed, hairless to soft hairy, large (10 - 30 cm long), circular in outline but deeply palmately five- to nine-lobed with each lobe pointed at tip. Inflorescence: terminal, large, with branched clusters of numerous, short-stalked, white, small (1 - 3 cm diameter), radially symmetric, five-petaled, unisexual flowers lacking bractlets under sepals. The flowers on an individual plant are either male (staminate) or female (pistillate), but never both, which means the plant is dioecious. Sepals: five, but fused for over half their length into a cup, then separating into five, short (under 4 mm), triangular to egg-shaped lobes. The calyx as a whole is sparsely soft-hairy or hairless, and 0.5 - 0.8 cm tall. Petals: five, white, non-toothed at apex, and while 0.5 - 0.9 cm long in staminate (all male) flowers, much smaller in pistillate (all female flowers). Stamens: numerous, but filaments fused into a tube. In the staminate flowers this tube is elongate and separates into numerous (fifteen to twenty) stalked anthers at the top, but in the pistillate flowers the tube is very short (only surrounding ovary) and has a single ring of stalkless anthers at its top. Pistil: in female flowers, with eight to ten superior carpels (ovule-bearing structures) enclosed by the short stamen tube, and eight to ten exserted styles with stigmas along their inner side. Fruit: a depressed-globular ring of eight to ten, hardened, 0.5 cm long, kidney-shaped, single-chambered, single-seeded segments (mericarps) with ribs along their backs. The individual mericarps separate from each other and split open to release the seed.
Similar species: Napaea dioica is fairly unique among our Malvaceae species since it has very large, lobed, and toothed leaves, but small, clustered, unisexual flowers. While members of the genera Hibiscus or Iliamna have large leaves which may appear similarly shaped, the flowers are much larger and never unisexual, plus they always have narrow bractlets immediately subtending their sepals. Some leaves of our species of Malva may look similar, but they usually have more rounded lobes or deeply cut lobes, and the flowers are larger and not unisexual.
Flowering: June to July
Habitat and ecology: Local in alluvial soil along streams and rivers.
Occurence in the Chicago region: native
Notes: There are substantial colonies of Napaea dioica along the Fox and DuPage Rivers in the southern sector of the Chicago Region, and further west in north-central Illinois, it is actually frequent. This is the only species in the genus Napaea (monotypic).
Author: The Field Museum
From Flora of Indiana (1940) by Charles C. Deam
Local and very rare. Alluvial banks of the Wabash River and moist roadsides. I have had the staminate form in cultivation since 1918 and the pistillate form since 1931.
Stems 1-2 m, branched above; lvs orbicular in outline, 1-3 dm, deeply 5-9- lobed or parted, the lobes coarsely toothed or incised; fls very many in a large terminal panicle; cal 5-8 mm, sparsely pilosulous with simple hairs or glabrous; pet of staminate fls 5-9 mm, of pistillate fls much smaller; mature carpels 5 mm, ribbed on the back. 2n=ca 30. Moist alluvial woods; Va. to Pa., w. to Wis. and Io., rarely escaped elsewhere. June-Aug.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.