20. Euthamia (Nutt.) Cass. (flat-topped goldenrod)

(Sieren, 1970, 1981)

Plants perennial herbs, often colonial from relatively long-creeping, branched rhizomes, often somewhat aromatic, the stem bases sometimes somewhat woody. Stems usually solitary, erect or ascending, with few to many ascending branches above the midpoint, with often relatively coarse, longitudinal ridges, glabrous or less commonly sparsely pubescent with minute, spreading hairs. Basal and lower stem leaves absent at flowering. Stem leaves gradually or relatively abruptly reduced toward the tip of the stem, sessile or nearly so, the blade linear to narrowly lanceolate, sharply pointed at the tip, more or less tapered to a slender or slightly expanded but not clasping base, the margins entire but sometimes roughened with minute, stout, ascending, stiff hairs, the surfaces glabrous or less commonly sparsely to moderately pubescent with minute, spreading hairs mostly along the veins, slightly to strongly resinous with small, impressed or somewhat pustular glandular dots. Inflorescences appearing as relatively dense, flat-topped panicles, the heads solitary or more commonly in small clusters at the branch tips, the stalks mostly short, with relatively few leaflike, linear to elliptic bracts 0.2–1.0 cm long. Heads radiate, slightly to strongly resinous-sticky. Involucre 3–6 mm long, cup-shaped to nearly cylindrical. Involucral bracts in 3–5 unequal, overlapping series, linear to narrowly oblong-lanceolate or lanceolate, the ascending tips rounded to less commonly bluntly or sharply pointed (sometimes in the same head), often somewhat concave or thickened along the midvein, entirely straw-colored to light yellow or some of the bracts with a short, elliptic to obovate green area toward the tip, glabrous, slightly to strongly resinous. Receptacle flat or slightly convex, with low, toothlike ridges around the attachment points of the florets, sometimes also with short, fine hairs. Ray florets 7–35, pistillate, the corolla 1–3 mm long, ascending to somewhat spreading, yellow, not persistent at fruiting (but sometimes trapped irregularly by the pappus and resinous exudate). Disc florets 3–13, perfect, the corolla 1.5–4.0 mm long, yellow, not persistent at fruiting (but sometimes trapped irregularly by the pappus and resinous exudate). Pappus of the ray and disc florets similar, of numerous slender, finely barbed bristles about as long as the corollas, white. Fruits 0.5–1.5 mm long, more or less oblong-elliptic in outline to slightly obovate, circular in cross-section or very slightly flattened, faintly 2–4-nerved, the nerves often difficult to observe, the surface moderately pubescent with fine, ascending, white to somewhat silvery hairs, straw-colored to light tan to light greenish brown, sometimes with slightly darker nerves. Five to 8 species, temperate North America; introduced in Europe, Asia.

Traditionally, Euthamia was considered part of Solidago (Steyermark, 1963). In his doctoral dissertation, Sieren (1970) presented data suggesting that although the plants have superficial similarities to that genus, there are strong differences in some morphological features of the heads as well as the anatomy of the leaf glands. He suggested a closer relationship between Euthamia and a group of genera related to Gutierrezia. Anderson and Creech (1975) studied details of leaf anatomy in the group and also concluded that Euthamia and Solidago should be regarded as distinct genera. More recently, molecular research (summarized by Beck et al., 2004) has supported the hypothesis of separate lineages and generally upheld the notion of a close relationship between Euthamia and the Gutierrezia group.

At the species level, the differences between the eight species accepted by Sieren (1981) are often fairly subtle and there is a lot of morphological variation within and between populations of each species. Some workers have suggested that there are fewer actual species in the genus (Nesom, 2000). There has not been a recent comprehensive review of the genus, thus the specific and infraspecific taxonomy remain somewhat controversial.