5-100(-200) cm; woody caudices or rhizomes. Stems
decumbent to ascending or erect, sometimes branched distally, glabrous or strigose, strigillose, hispid, or short-villous. Leaves
basal (persistent or not by flowering) and cauline; petiolate (proximal) or sessile (proximal and distal, latter sometimes subpetiolate); proximal blades sometimes 3-nerved, ovate-oblanceolate, margins often serrate, faces glabrous or densely hairy; distal sometimes 3-nerved, glabrous or sparsely to densely scabrous, strigillose, or villous, sometimes stipitate-glandular, sometimes resinous. Heads
usually radiate, sometimes discoid, (1-)2-1500+ in racemiform (club-shaped or pyramidal), paniculiform or corymbo-paniculiform, sometimes secund arrays. Involucres
campanulate to cylindric (often spreading upon drying), 3-12 × 1.7-10 mm. Phyllaries
10-35 in 3-5 series, midnerves usually ± swollen and translucent, sometimes plus 2-5 secondary nerves (striate, flat), linear-lanceolate to oblong or ovate, unequal to rarely subequal, margins scarious, (apices rounded to acute or attenuate), faces glabrous or sparsely pilose or puberulent, sometimes minutely stipitate-glandular, sometimes resinous. Receptacles
slightly convex, pitted, epaleate . Ray florets
(0-)2-15(-24), pistillate, fertile; corollas yellow, rarely white (usually glabrous). Disc florets
2-35(-60), bisexual, fertile; corollas yellow, ± ampliate, tubes shorter than throats (usually glabrous), lobes 5, erect to spreading, triangular to narrowly lanceolate; style-branch appendages broadly to narrowly triangular (lengths 0.7-1 times stigmatic lines). Cypselae
narrowly obconic to cylindric, sometimes somewhat compressed, ribs usually 8-10 (either darker and sometimes translucent or lighter than bodies), glabrous or moderately strigillose; pappi
persistent, outer setiform scales (0.25-0.5 mm; rarely present) plus 2 series of 25-45 longer, barbellate bristles, mid apically attenuate, 90-95% length of inner, inner apically weakly to strongly clavate [(0.5-in S. sphacelata
)1.5-5(-7.3) mm]. x
= 9. Solidago
is found primarily in North America with some South American and Eurasian species (8 in Mexico, 4 in South America, 6-10 in Europe and Asia).
Recent studies on the phylogeny of Astereae and Solidago have done much to resolve relationships of the goldenrods to each other and to other genera within the tribe (J. J. Zhang 1996; J. C. Semple et al. 1999; R. D. Noyes and L. H. Rieseberg 1999; J. B. Beck et al. 2004). Traditionally the grass-leaved goldenrods, Euthamia, have been included in Solidago; they are well separated within the North American clade. L. C. Anderson and J. B. Creech (1975) presented anatomic reasons for separating the two genera and also Chrysoma, Gundlachia, and Petradoria. The segregate genera Oligoneuron and Brachychaeta fit within Solidago based on anatomy and DNA studies. Brintonia lies outside Solidago based on DNA and morphology. The position of the monophyletic Oreochrysum also appears to lie outside and phylogenetically close to Solidago. G. L. Nesom (1993) placed the flat-topped goldenrods in Oligoneuron. Results from Zhang´s study indicate that the flat-topped goldenrods are nested within other goldenrod groups that Nesom included in Solidago. Beck et al. (2004) indicated that further work is needed before a definitive molecular-based phylogeny is available. While the flat-topped goldenrods are readily distinct from other goldenrods on the basis of arrangements of heads, additional features such as double pappus with clavate inner bristles and oligoneurate phyllaries occur in other species of Solidago as well. The flat-topped goldenrods are treated here as Solidago sect. Ptarmicoidei following Semple and K. N. Gandhi (2004). Nesom provided much of the formal subgeneric nomenclature that by and large is followed here; there are some differences in assignment of species to subsections. Semple (2003, 2004) proposed a number of additional new names and combinations used here. J. L. A. Hood and Semple (2003) presented evidence that the pappus of Solidago is not simple, as previously assumed, but is like that of related genera: it is in two series, inner series somewhat clavate and slightly longer, or sometimes in three series with an additional, shorter outer series of few setiform scales.