1. Erigeron annuus (L.) Pers. (daisy fleabane, whitetop fleabane, annual fleabane)

Pl. 233 g, h; Map 971

Plants annual or less commonly biennial, with shallow, fibrous roots. Stems 1 to several, (50–)60–150 cm long, usually well branched above the lower 1/3, sparsely to densely roughened with mostly spreading hairs (the hairs sometimes more appressed toward the tip). Basal leaves sometimes withered by flowering time, 3–15 cm long, mostly long-petiolate, the blade broadly oblanceolate to broadly obovate, short- to long-tapered at the base, mostly rounded at the tip, the margins coarsely and sharply toothed, the surfaces and margins sparsely to moderately pubescent with short, relatively stiff, curved hairs. Stem leaves usually relatively numerous, 1–10 cm long, the lower ones short-petiolate, the median and upper ones sessile, the blade oblanceolate to elliptic or lanceolate (the lower leaves rarely obovate), angled or tapered to a mostly sharply pointed tip, angled, tapered, or narrowly rounded at the base, not or only very slightly clasping the stem, the margins of all but the uppermost leaves usually with several sharp teeth on each side, these often produced from below the midpoint to the tip, the surfaces and margins sparsely to moderately hairy. Inflorescences rounded to more or less flat-topped panicles, usually open and often with numerous heads. Involucre 3–5 mm long, the receptacle 6–12 mm in diameter at flowering, the bracts sparsely to moderately pubescent with more or less spreading hairs and often also minutely glandular. Ray florets 80–125, the corolla 4–10 mm long. Disc florets with the corolla 1.5–2.5 mm long. Pappus of the ray and disc florets of 2 types, an inner series of 10–15 threadlike bristles 1.2–2.2 mm long and an outer series of several shorter bristles or slender scales 0.1–0.4 mm long, the ray florets lacking the longer, inner series. Fruits 0.8–1.0 mm long, sparsely and inconspicuously hairy. 2n=27, 54. May–November.

Common throughout the state (U.S., Canada; introduced in Europe). Banks of streams and rivers, margins of ponds and lakes, openings of bottomland and mesic upland forests, and bottomland and upland prairies; also pastures, old fields, fallow fields, crop fields, railroads, roadsides, and open, disturbed areas.

Erigeron annuus is a mostly triploid taxon that reproduces apomictically. Using allozyme markers, Hancock and Wilson (1976) showed that multiple genotypes may be present within populations. Stratton (1991) demonstrated that there was considerable morphological variation within experimental populations grown under greenhouse conditions. This genotypic variation and phenotypic plasticity are responsible in part for difficulties that some botanists have had in distinguishing some plants of E. annuus from E. strigosus in the field and herbarium. Frey et al. (2003) studied introduced populations of these species in Europe and a small number of North American plants, and they concluded that plants attributed to E. strigosus var. septentrionalis (Fernald & Wiegand) Fernald should more properly be considered a variety of E. annuus. In fact, some botanists have suggested that E. strigosus as a whole might be treated better as a subspecies of E. annuus, but this approach merely avoids dealing with the problem rather than the more difficult job of reassessing evolutionary lineages within the group. Further research involving more in-depth sampling of North American populations of both species will be necessary to resolve the situation. See the treatment of E. strigosus for further discussion.

Cronquist (1947c) called rare plants from Quebec, Canada, with the heads lacking ray florets var. discoideus (Vict. & J. Rousseau) Cronquist. This rare mutant should deserve no more recognition than that of a form (if at all).

Steyermark (1963) noted that plants of E. annuus have been used medicinally as an astringent, diuretic, and tonic. He also noted that deer frequently browse the foliage. According to Stratton (1991), a single plant of E. annuus may produce 10,000–100,000 fruits in one growing season, which helps to account for the success of the species geographically and ecologically.