Threadleaf Fleabane

The densely white-strigose stem bases, linear-filiform leaves relatively unreduced distally, and relatively few heads with coiling, usually blue rays are distinctive for Erigeron filifolius. Proximal leaves are not clustered as a basal rosette; they are inserted on closely spaced nodes that are slightly more separated distally. Plants identified as var. robustior (with more ray florets, fewer heads, and thicker stems, centered in Oregon and Washington) intergrade with the typical form and apparently are separated arbitrarily.

Perennials, 10–30(–50) cm; taprooted, caudices multicipital, often with relatively short and thin, stemlike, lignescent branches. Stems erect, sparsely to densely strigose (hairs loosely appressed to ascending, fine), densely white-strigose at least proximally, eglandular. Leaves basal (usually persistent, not clustered as rosettes) and cauline; basal blades linear to filiform, 10–80 × 0.5–2.5(–3.5) mm, cauline gradually or little reduced distally, margins entire, faces sparsely to moderately strigose, eglandular. Heads 1–5(–10+) from distal branches, tending to be in corymbiform arrays. Involucres 4–7 × 5–-18 mm. Phyllaries in (2–)3–4 series, loosely strigose to hirsute or nearly villous, densely to sparsely minutely glandular. Ray florets (15–)20–125; corollas usually blue, sometimes white, pink, or pale lavender, 3–13 mm, laminae coiling. Disc corollas 2.5–5.5 mm. Cypselae 1.4–1.8(–2) mm, 2-nerved, faces sparsely strigose to glabrate; pappi: outer of setae, inner of 20–30 bristles. 2n = 18.

Diplopappus filifolius Hooker, Fl. Bor.-Amer. 2: 21. 1834; Erigeron filifolius var. robustior M. Peck

Erigeron filifolius is a North American species of flowering plants in the family Asteraceae known by the common names thread-leaf fleabane.

Erigeron filifolius is widespread across much of western North America. It has been found in western Canada (British Columbia) and the western United States, primarily east of the crest of the Cascade Range of Washington, Oregon, and northern California extending eastward to southern Idaho and northeast Nevada. Within its distribution, it occupies a large range of elevations from near sea level to 2,400 m (8,000 ft).

Erigeron filifolius is a branched perennial herb up to 50 cm (20 in) in height. Leaves are long and thin, often thread-like, up to 8 cm (3 in) long. Each stem sometimes produces only 1 flower head per stem, sometimes flat-topped clusters of 10 or more. Each head may contain as many as 125 white, blue, pink, or lavender ray florets surrounding numerous yellow disc florets.