Sericocarpus, commonly known as whitetop asters, is a small genus of five perennial herbaceous plants in the daisy family Asteraceae, placed in the tribe Astereae and the order Asterales. The genus is entirely native to North America, with species distributed across two distinct geographic ranges: the eastern United States and the Pacific Coast region from British Columbia south through Oregon, Washington, and California.
Plants grow 15 to 120 cm tall from rhizomes or stout, branching woody caudices. Stems are erect and simple, ranging from nearly hairless to moderately hairy. The basal leaves form rosettes and typically wither by flowering time; cauline leaves are alternate, sessile, and reduced upward along the stem, with blades that vary by species from narrow and linear to obovate or ovate, with margins that may be serrate or entire and finely ciliate. Heads are radiate and borne in clusters of two to five per branch, arranged in flat-topped (corymbiform) arrays. Ray florets number one to six per head, are pistillate and fertile, and bear white corollas — a trait that gives the genus its common name. Disc florets number five to nineteen and are bisexual, with white to cream corollas.
The most distinctive feature of the genus is the achene (cypsela), which is fusiform to obconic, 7- to 10-ribbed, and densely covered in strigose (silky-pressed) hairs — a characteristic directly encoded in the genus name, from the Greek sericos (silky) and karpos (fruit). The persistent pappus consists of 25 to 50 barbellate bristles, white to tan or rust-colored, arranged in two to four series.
Sericocarpus was described by Nees in 1832. For much of the twentieth century taxonomists followed Cronquist in sinking the genus into the broadly conceived Aster, treating it as a subgenus or section. DNA sequence studies by Noyes and Rieseberg (1999) and Semple et al. (2002) demonstrated that Sericocarpus is phylogenetically more closely related to Solidago (goldenrods) than to the core North American asters, supporting its recognition as a separate genus. The five accepted species — S. asteroides, S. linifolius, S. tortifolius, S. oregonensis, and S. rigidus — are all restricted to North America.
Etymology
The genus name Sericocarpus derives from the Greek sericos (silky) and karpos (fruit), describing the densely strigose — silky, pressed-hair-covered — achenes that distinguish these plants from related asters. The genus was established by Nees in Genera et Species Asterearum (1832). The common name "whitetop aster" refers to the flat-topped clusters of white-rayed flower heads.
Distribution
Sericocarpus is confined to North America and occupies two geographically separate ranges. Eastern species (S. asteroides, S. linifolius, S. tortifolius) range through the eastern United States from the Gulf Coast states north to New England and the Great Lakes. Western species (S. oregonensis, S. rigidus) occur along the Pacific Coast: S. oregonensis grows from California north through Oregon and Washington, including the Columbia River Gorge; S. rigidus ranges from southern Vancouver Island (British Columbia, Canada) south through lowland western Washington and into southwest Oregon, west of the Cascades crest.
Taxonomy Notes
Sericocarpus has a complex taxonomic history. Although recognized as a distinct genus by Nees (1832) and maintained by early North American botanists including Gray and Fernald, it was subsumed into Aster in the broad sense by Cronquist and followed by most mid-twentieth-century treatments. Molecular phylogenetic work — especially chloroplast DNA restriction fragment analysis (Xiang & Semple 1996) and nuclear DNA sequencing (Noyes & Rieseberg 1999; Semple et al. 2002) — showed conclusively that the group is more closely allied to Solidago than to Eurasian Aster, supporting reinstatement as a separate genus. The chromosome base number is x = 9. Pappus structure (a triseriate arrangement similar to Doellingeria and Solidago) is an additional morphological character supporting generic separation.
