5. Sorbus americana Marshall, Arbust. Amer. 145. 1785.
American mountain ash, sorbier d’Amérique
Pyrus americana (Marshall) Sprengel
Shrubs or trees, 40–100 dm. Stems 1–4+; bark gray to bronze; winter buds green to purple, shiny, ovoid to conic, 7–20 mm, glutinous, glabrous or hairy along scale margins and at apex, hairs primarily rufous. Leaves pinnately compound; stipules early deciduous, rufous-ciliate, ˂margins sometimes glandular˃; blade paler abaxially, dull green to yellowish green adaxially, leaflets 11–17, opposite, lanceolate to oblong-lanceolate, 5–10 × 1–2.5 cm, l/w ratio 3.4–5, margins serrulate to serrate, at least in apical 1/2 and often almost to base, apex acuminate to long acuminate, surfaces glabrous or glabrate at flowering, some hairs occasionally persisting abaxially along midvein; leaflet axils and petiole bases glabrous or with rufous and/or whitish hairs adaxially. Panicles 125–400+-flowered, flat-topped or rounded, 6–15 cm diam.; peduncles glabrous or sparsely villous. Pedicels glabrous or sparsely villous. Flowers 5–7.5(–8.5) mm diam.; hypanthium glabrous, hypanthium plus sepals 2–2.5 mm; sepals 0.5–1 mm, margins entire, often with a few thick glands; petals white, orbiculate to obovate, (2–)2.5–3.5(–4) mm; stamens 14–20; carpels 1/2 adnate to hypanthium, apex conic, styles 3 or 4, 1.5–2 mm. Infructescences glabrous or sparsely villous. Pomes bright red to orange-red, globose to subglobose, 4–7 mm diam., shiny, not glaucous; sepals inconspicuous, incurved. Seeds brown, yellowish when fresh or immature, ovoid to ovoid-lanceoloid, 2.5–3.3 × 1.5–2 mm, asymmetric, slightly flattened. 2n = 34.
Flowering spring; fruiting fall. Cool, moist woods, lake and stream shores, rocky hillsides, thickets; 0–1300 m; St. Pierre and Miquelon; N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; Conn., Ga., Ill., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.C., Pa., R.I., S.C., Tenn., Vt., Va., W.Va., Wis.
Sorbus americana reputedly flowers about one week earlier than S. decora. Some authors have described S. americana as having generally thinner leaflets relative to thicker and firmer ones for S. decora; examination of herbarium specimens appears to support this, though the authors made no systematic study of blade thickness. No confirmed putative hybrids with S. decora are reported, although hybridization may occur. The hybrid ×Sorbaronia jackii Rehder [Aronia ×prunifolia (Marshall) Rehder × S. americana; synonym Pyrus ×jackii (Rehder) Fernald] has been reported from New Brunswick as a fastigiate shrub 20 dm tall with leaves intermediate in appearance between the putative parents. Garden hybrids involving S. americana are occasionally cultivated in North America; none has been reported as escaped. These include: Sorbus ×splendida Hedlund (a hybrid with S. aucuparia), S. ×plantierensis Simon-Louis ex C. K. Schneider (with S. aria), ×Sorbaronia monstrosa (Zabel) C. K. Schneider (with A. arbutifolia), and ×Sorbaronia sorbifolia (Poiret) C. K. Schneider (with A. melanocarpa); synonyms of ×Sorbaronia sorbifolia include A. sargentii (Dippel) Zabel, A. watsoniana M. Roemer, and Pyrus ×mixta Fernald.