Perennials, 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.

Heads radiate, the rays pistillate and fertile, yellow or in 2 spp. white; invol bracts usually ±imbricate in several series, chartaceous at base, commonly with ±herbaceous green tip; receptacle small, flat or a little convex, alveolate, naked or seldom with a few phyllary-like bracts near the margin; disk-fls perfect and fertile, yellow, seldom more than 25(-60); style-branches flattened, with lanceolate, externally hairy appendage; achenes several-nerved, subterete or angled; pappus of numerous equal or sometimes unequal, capillary (in one sp. short and firm) bristles, usually white; fibrous-rooted perennial herbs with simple, alternate, entire or variously toothed lvs and few to many, mostly rather small, campanulate to subcylindric heads; x=9. Nearly 100, mainly N. Amer. (Oligoneuron, Unamia) Hybrids are often found. All our spp. bloom in mid- or late summer and fall, S. juncea being one of the earliest, and S. speciosa one of the latest. Several spp. may have one or two B-chromosomes in addition to the number given. Goldenrods can be divided into several groups on the basis of three sets of characters that are independently distributed with respect to each other: the nature of the underground parts, the nature and distribution of the lvs, and the nature of the infl. Several terms explained below are used without further comment in the key and descriptions. In many spp. the rhizome is very short, stout, and densely rooting, sometimes being more nearly a caudex than a proper rhizome. Such spp. often have several stems clustered together, although the stems may also be solitary. In other spp. the rhizome is elongate and generally more slender, and the stems are usually scattered. Some few spp. have both a short, stout rhizome or caudex and more elongate, slender, sometimes stoloniform rhizomes. In many spp. the lvs are basally disposed. The radical and lowermost cauline lvs are relatively large and usually ±persistent, with the blade either gradually or abruptly contracted to a definite petiole. The cauline lvs (either numerous or often few) are progressively reduced and less petiolate upward, those near and above the middle of the stem usually being sessile or nearly so and of different shape from those below, often relatively as well as actually narrower. In other spp. the lvs are chiefly cauline. The radical lvs are small and relatively inconspicuous, or more often wanting. The lowermost cauline lvs are reduced and generally soon deciduous, so that the stem appears to be naked toward the base at flowering time. The largest leaves are somewhat above the base but evidently below the middle of the stem. The middle and upper lvs are gradually reduced but essentially similar in shape to the larger ones. Most spp. of this group have numerous, sessile or sub sessile lvs. A few spp. fall between the two habit types, or range from one to the other. Measurements of lf-length in the descriptions include the blade and petiole, unless only the blade is specified. Infls are mostly of 3 general types. In one group the heads are in axillary clusters or in a terminal, ±elongate thyrse that is straight, cylindrical, and not at all secund, or the infl consists of several such thyrsoid branches. In another group the infl is paniculiform, with at least the lower branches recurved-secund, or is slender and elongate (sometimes racemiform) and ±one-sided, or at least nodding at the tip. In a third group the infl is short and broad, flat or round-topped, but not at all secund, and is said to be corymbiform.

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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