A species of the diverse genus Senecio (Asteraceae), originally identified as the Australian S. quadridentatus Labill. and reported naturalized in Orange and San Diego counties, California, was determined to be correctly identified as the South African Senecio angustifolius (Thunb.) Willd. True Senecio quadridentatus is confirmed to be naturalized in southern California in Santa Barbara County, both in mainland coastal regions and on Santa Rosa Island. The two species differ in plant habit (perennial herb in S. quadridentatus, shrub in S. angustifolius), leaf length (60–130 mm in S. quadridentatus, 10–35 mm in S. angustifolius), head type (disciform in S. quadridentatus, discoid in S. angustifolius), and head shape and size (involucre narrowly cylindric and 6.5–10 mm in S. quadridentatus, involucre widely cylindric and 5–6 mm in S. angustifolius). Evidence for these determinations, a description for Senecio angustifolius, and a revision to the key to Senecio in California are presented. The eradication of these exotics in natural habitats of southern California is ongoing, but further monitoring is required.
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Vol. 69 • No. 3