1. Aruncus dioicus (Walter) Fernald, Rhodora. 41: 423. 1939.
Actaea dioica Walter, Fl. Carol., 152. 1788
Leaves: leaflets green to gray-green, 6–15 cm, base subcordate to attenuate, apex acute to long-acuminate. Flowers: sepals 1 mm; petals 0.5–1.5 mm, apex acute to obtuse; stamens 1.5–2 mm, filaments slender, ± equal, anthers versatile; styles [0.2–]0.3–0.8 mm. Seeds 1.5–2(–2.5) mm.
Varieties 14 (4 in the flora): North America, Eurasia.
Aruncus dioicus is superficially similar to Astilbe biternata (Saxifragaceae), false goat’s-beard, which is endemic to the southern Appalachian Mountains. Both species produce tall, dioecious plants with biternately to bipinnately compound leaves and plumelike terminal panicles with many, relatively small white, unisexual flowers, and may grow side by side. Aruncus dioicus is more widespread and may be readily distinguished by its unlobed rather than 3-lobed terminal leaflets, flowers with 20 rather than 10 stamens, and 3 carpels reflexed in fruit rather than 2 erect carpels. See T. L. Mellichamp (1976) for an analysis of this remarkable example of convergent evolution.
Aruncus dioicus is striking and is grown in gardens throughout North America. It is variable throughout the temperate and subarctic regions of the Northern Hemisphere, especially in China and Japan. H. Hara (1955) recognized the following varieties that are found in the flora area.