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 petiole-like 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.

Flowering Aug-Sep. Damp, interdunal hollows, limestone pavements (alvars); 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.

Slender perennial from a caudex, 2-5 dm, rough-puberulent in the infl, otherwise glabrous except for the scabro-ciliolate lf-margins; lvs basally disposed, tending to be weakly triple-nerved and often conduplicate, the larger ones linear-oblanceolate to linear, to 20 נ2 cm, the cauline few, 7-15 below the infl; infl corymbiform, its heads crowded, relatively few, mostly less than 50; invol 5.5-8 mm, its bracts firm, blunt, scarcely striate; rays ca 6 and disk-fls ca 13; achenes glabrous, 3-5-angled, the angles weakly nerved; many of the pappus-bristles slightly clavate and subplumose; 2n=54. Swamps and moist beaches, often in moist sandy swales behind the dunes; Genesee Co., N.Y., and along the shores of the n. parts of Lake Michigan and Huron, rarely inland in ne. Mich.

Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.

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