Centromadia, commonly known as spikeweeds, is a small genus of annual flowering plants in the sunflower family (Asteraceae), placed in the tribe Madieae. The genus comprises three to five species native to western North America, occurring principally in California and extending into southwestern Oregon, the broader western United States, and Baja California in Mexico.
Plants are annuals growing 10–120 cm tall, with stems erect to prostrate. Leaves are mostly cauline; the lower leaves are opposite (often forming winter-spring rosettes), while most upper leaves are alternate. Blades are oblanceolate to linear or lance-linear, the lower ones usually once or twice pinnatifid, with toothed or entire margins; the tips of the upper stem leaves are characteristically spine-tipped, a feature that gives rise to the common name "spikeweeds." Flower heads are radiate, aggregated in glomerules or arranged in spiciform-paniculiform to umbelliform arrays. The involucres are obconic or urceolate, 3–8 mm across, subtended by calyculi of spine-tipped bractlets. Ray florets number 5–75 or more, are pistillate and fertile, and bear yellow corollas; disc florets number 6–200 or more and are usually functionally staminate. The base chromosome number is x = 13.
Centromadia was formerly subsumed within the broad genus Hemizonia. Following the molecular phylogenetic work of Bruce G. Baldwin (1999) and subsequent studies by Carlquist and colleagues (2003), Centromadia is recognized as a distinct genus more closely related to Calycadenia, Deinandra, Holocarpha, and Osmadenia than to Hemizonia in the strict sense. Taxa in the genus are self-incompatible and show low to high interfertility with one another. Most species favor somewhat poorly drained or alkaline soils.
Distribution
Centromadia is native to western North America. Species occur primarily in California and extend into southwestern Oregon, the broader western United States, and the Baja California Peninsula (including Baja California Sur) in Mexico. Most species favor somewhat poorly drained or alkaline habitats.
Ecology
Most species of Centromadia occur in somewhat poorly drained or alkaline sites. They are annuals that typically grow in disturbed or seasonally dry grassland and valley habitats within their California-centered range.
Taxonomy Notes
Centromadia was historically treated as part of the broader genus Hemizonia. Baldwin (1999) and Carlquist et al. (2003) established that Centromadia is more closely related to Calycadenia, Deinandra, Holocarpha, and Osmadenia within tribe Madieae than to Hemizonia sensu stricto, leading to its recognition as a distinct genus. Taxa within Centromadia are self-incompatible and exhibit varying degrees of interfertility.
