Calycadenia is a genus of annual flowering herbs in the family Asteraceae (order Asterales), known colloquially as the western rosinweeds. The genus comprises roughly a dozen species, all native to California and closely associated with the Central Valley and surrounding foothills; the ranges of two species — Calycadenia fremontii and Calycadenia truncata — extend northward into Oregon.
Plants are aromatic annuals that range dramatically in stature, from just a few centimetres to well over a metre tall. The leaves are narrow and linear, resembling grass leaves, and are alternately arranged on most species; C. oppositifolia is a notable exception with opposite leaves. Leaf margins are sometimes slightly toothed. Flower heads are solitary or carried in open clusters in the leaf axils, held on peduncles (flower stalks) that bear distinctive tack-shaped glandular bracts — the feature that most reliably distinguishes the genus. Each head typically has one to six ray florets, three-lobed at their tips and coloured white, cream, pink, or yellow, sometimes with a dark red spot near the base. Up to 25 disc florets occupy the centre of each head, in similar pale shades, with anthers that are commonly purple or black. The fruit is an angular cypsela; those from disc florets usually bear a pappus of several scales.
The genus name is derived from the Greek kalyx ("cup") and aden ("gland"), a direct reference to the tack-shaped glands on the bracts and phyllaries that characterise the genus. Well-known members include Calycadenia multiglandulosa (sticky western rosinweed), Calycadenia truncata (Oregon western rosinweed), and Calycadenia villosa (dwarf western rosinweed).
Etymology
The genus name Calycadenia is formed from the Greek kalyx ("cup") and aden ("gland"), alluding to the distinctive tack-shaped glands borne on the peduncle bracts and phyllaries — the morphological hallmark of the genus.
Distribution
Calycadenia is native almost entirely to California, with its centre of diversity in and around the Central Valley. Two species, C. fremontii and C. truncata, extend their ranges northward into Oregon. No species are known to occur outside western North America.
Taxonomy Notes
GBIF recognises Calycadenia as an accepted genus within Asteraceae (order Asterales), with 13 descendant taxa. Calycadenia tenella Torr. & A. Gray was formerly included but is now placed in the monotypic genus Osmadenia as Osmadenia tenella Nutt.
