Holocarpha is a small genus of flowering plants in the daisy family (Asteraceae), order Asterales. The genus contains four species of plants known collectively as tarweeds, all of which are annual herbs endemic to California, USA. They are distinguished by their glandular stems and foliage, a strongly aromatic scent, and yellow composite flower heads typical of the Asteraceae family.
All species are native to California and occur nowhere else in the world. The four recognized species are Holocarpha virgata (yellowflower tarweed), Holocarpha heermannii (Heermann's tarweed), Holocarpha macradenia (Santa Cruz tarweed), and Holocarpha obconica (San Joaquin tarweed). Species authorities within the genus are principally associated with D.D. Keck, who formalized several combinations.
Holocarpha macradenia, the Santa Cruz tarweed, is the most conservation-notable member of the genus. It was considered nearly extinct by 1960 and is now listed as a federally threatened species under the US Endangered Species Act, as well as a California Rare Plant Rank 1B.1 species. It occupies coastal prairie and valley grassland habitats in Santa Cruz, Santa Clara, and Santa Barbara counties. Its strongly aromatic resin — variously described as citrusy, tangerine-scented, or reminiscent of a Christmas tree — is thought to deter herbivores.
Distribution
All four species of Holocarpha are endemic to California, USA. H. macradenia is restricted to coastal prairie and valley grassland habitats in Santa Cruz, Santa Clara, and Santa Barbara counties. H. heermannii and H. virgata are also California endemics with ranges within the state.
Ecology
Holocarpha species are glandular annual herbs that produce a pronounced aromatic resin. In H. macradenia, this scent has been described as citrusy, tangerine-like, or reminiscent of a Christmas tree, and is believed to deter herbivory. The genus occupies open habitats including coastal prairie and valley grassland in California.
Conservation
Holocarpha macradenia (Santa Cruz tarweed) carries the highest conservation concern in the genus. It is listed as a federally threatened species under the US Endangered Species Act and holds California Rare Plant Rank 1B.1 (rare, threatened, or endangered in California and elsewhere). The species was considered nearly extinct by 1960. The other species in the genus are not known to carry comparable formal protection status.
Taxonomy Notes
Species in Holocarpha were transferred from related tarweed genera, principally Hemizonia and Deinandra, with D.D. Keck responsible for several of the final combinations. H. macradenia was originally described as Hemizonia macradenia DC. The common name "tarweed" is shared with several other genera in Asteraceae, including Deinandra, which are not closely related to Holocarpha despite the shared vernacular.
