in A. Gray, Manual, 211. 1848.
Plants 30–60+ cm; caudices branching; vascular-bundles and petiole bases marcescent (attached to old stems for more than a season). Stems 1–5+, erect, slender, glabrous. Leaves: basal tapering to long, winged petioles, blades ovate, to 7–22 mm wide, margins entire, apices acute, faces glabrous; proximal cauline tapering to winged petiolelike bases partially clasping stems, blades sometimes 3-nerved (2 prominent lateral nerves arising proximally and running alongside midnerve for some distance before diverging), linear-oblanceolate, 158–177 × 7–22 mm, margins entire, ciliate, apices acute to obtuse, glabrous; mid to distal sessile, blades linear-lanceolate, mid often 3-nerved (obscurely sometimes), distal 1-nerved, 47–100 × 4–10 mm, reduced distally. Heads (2–) 5–50 (–100+), not secund, in corymbiform arrays, branches and peduncles glabrous or moderately strigillose. Peduncles 5–6.2 mm, sparsely to moderately strigillose; bracteoles 0–2, linear. Involucres campanulate, (5.5–) 6–8 (–9) mm. Phyllaries in 3–4 series, unequal, linear to oblanceolate, margins ciliate, weakly striate, obtuse to rounded. Ray-florets 6–12 (pale to bright-yellow); laminae 7.4–7.9 × 0.5–0.6 mm. Disc-florets 8–15; corollas 4–4.3 mm, lobes 0.9–1.5 mm. Cypselae obconic, 1.4–1.8 mm, glabrous, ribs sometimes dark; pappi of bristles 4–5.5 mm, clavate. 2n = 54.
Phenology: Flowering Aug–Sep.
Habitat: Damp, interdunal hollows, limestone pavements (alvars)
Elevation: 100–400 m
Ont., Mich., N.Y.
The large involucres and the presence of the unique 3-nerved proximal and mid cauline leaves distinguish Solidago houghtonii from other taxa in sect. Ptarmicoidei. The 3-nerved venation pattern may be morphologic evidence of a hybrid origin for S. houghtoni between a parallel-nerved member of sect. Ptarmicoidei and a 3-nerved member of subsect. Triplinerviae. Molecular data indicate that S. gigantea may have been one of the parental species (P. Laureto, pers. comm.).
Reports of Solidago houghtonii from New York have been rejected as being based on misidentified specimens (J. K. Morton 1979). Some collections have the large involucres typical of S. houghtonii; molecular data indicate that the species is present in Genessee County, New York (P. Laureto, pers. comm.).
Solidago houghtonii can be difficult to distinguish from S. ohioensis × S. ptarmicoides hybrids because both have hairy peduncles in arrays. The hybrids have pale yellow rays even when fresh; post-flowering rays of S. houghtonii can be pale yellow. J. K. Morton (1979) hypothesized an allopolyploid origin for the hexaploid S. houghtonii via an unreduced gamete of S. ohioensis × S. ptarmicoides backcrossed to S. ohioensis with subsequent chromosome doubling. Additional work is needed to confirm this hypothesis. An aberrant collection (M. R. Penskar 1067, MICH) had about 400 heads (mostly in bud) due to axillary branches developing along nearly the entire stem; stems usually have fewer than 50 heads. Some plants from Michigan are more robust than those found in Ontario.
Solidago houghtonii is listed by the U.S. Fish and Wildlife Service as threatened in Michigan. It is in the Center for Plant Conservation’s National Collection of Endangered Plants.