Flora of North America species comparison

Subspecies 2 (2 in the flora).

Baccharis pilularis can be distinguished by its dark brown stems, and serrate, obovate to oblanceolate leaves. In addition, plants from some dunes of the California coast are prostrate, a growth form unique to this genus in our region.

The common, weedy, widespread form is subsp. consanguinea, which is typically erect, with its larger leaves 15–40 mm. Subspecies pilularis is known only from exposed sandy dunes and bluffs along the central coast of California. Its growth habit is matlike, and its larger leaves are 5–15 mm. The prostrate habit of subsp. pilularis is strikingly different from the upright habit of subsp. consanguinea.

C. B. Wolf (1935) demonstrated that in at least some populations, the distinction between prostrate and erect forms has a genetic basis. Transplants from the wild of the prostrate and erect forms retained their respective growth habits when grown together in a sheltered location and the morphology of seedlings reflected the habit of the parents. Wolf’s arguments for recognizing the forms as subspecies are further supported by the existence of prostrate cultivars in the horticultural trade. On the other hand, both erect and prostrate forms grow in proximity throughout the range of subsp. pilularis. In many areas the forms intergrade completely; in others they can be easily distinguished. Two subspecies are recognized here, notwithstanding difficulties in identifying habit from pressed specimens, or by observations of populations where both growth forms coexist. Further study is needed, perhaps utilizing molecular characters and detailed observations of native populations.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Native to the Atlantic and Gulf Coastal Plain, Baccharis halimifolia is recognized by its broad, distinctly rhombic, coarsely serrate basal leaves, pyramidal, leafy arrays, and cypselae with large fluffy pappi. The plants are often to 600 cm; the basal leaves might be missed by collectors. Forms with relatively narrow leaves are especially common in Arkansas, Louisiana, and east Texas. These may be the result of hybridization and introgression with B. neglecta, in areas where they are known to hybridize (D. J. Zanowiak 1991), or with B. angustifolia. Hybrids between B. halimifolia and B. angustifolia are known from Florida as well. Baccharis halimifolia has been introduced to Australia and France. In Australia it infests large areas along the coast of southern Queensland and New South Wales. Its success as an invasive weed is attributed to production of a large number of seeds that are widely dispersed by the wind, shade tolerant germination and seedlings, tolerance to wet soils and salinity, and ability to resprout after a fire (W. E. Westman et al. 1975). The leaves of B. halimifolia contain a cardiotoxic glycoside known to cause the death of sheep if they eat about one percent of their body weight in leaves.

(Discussion copyrighted by Flora of North America; reprinted with permission.)