A collection from “Gunther Gardens,” Washtenaw Co. (E. W. Erlanson & F. J. Hermann 5177 in 1927, MICH) is E. obscurum Schreb., a Eurasian and North African species locally naturalized in New Zealand, Australia, and South America, but not previously recorded from North America. It was probably a weed at the site, which was a private garden including greenhouse, but whether outdoors or in the greenhouse is unclear. The leaves of E. obscurum have flat (not revolute) margins and are denticulate, as in E. ciliatum, but the inflorescence and stem have ± dense whitish pubescence of minute incurved non-glandular hairs, as in E. leptophyllum (E. ciliatum has glandular puberulence).
1. Stems pubescent with horizontally spreading (not incurved) hairs.
2. Stigma entire; leaves less than 8 (–10) mm wide, entire or nearly so with slightly revolute margins.
2. Stigma deeply 4-lobed; leaves (at least the larger ones) over 8 mm wide, toothed or at least remotely but distinctly denticulate.
3. Leaves sessile and (except sometimes the smallest) the blades clasping up to halfway around the stem or often (the largest) slightly decurrent; petals (9–) 11–17 mm long.
3. Leaves sessile or subsessile but the blades not at all clasping or decurrent; petals at most 10 mm long.
1. Stems glabrous or with pubescence all or in large part of minute incurved hairs (glandular puberulence may be present).
4. Leaves linear to narrowly lanceolate (usually less than 4 mm, rarely as much as 1 cm, wide), with entire ± revolute margins or rarely (when broader) with flat but entire to obscurely undulate margins; internodes not angled (except sometimes immediately beneath the nodes); puberulence not glandular, appressed-incurved.
5. Taprooted annual of dry habitat; leaves glabrous; plants bushy-branched above the middle (except in depauperate individuals).
5. Fibrous rooted perennials with overwintering rosettes or turions, of wetland habitats; leaves with minute incurved hairs, at least along the midrib; plants often less branched.
6. Leaves (at least many of them) with ± evenly distributed minute incurved hairs on upper surface; tip of stem erect.
6. Leaves essentially glabrous except for minute incurved hairs on or near the midrib (above and/or below); tip of stem ± strongly nodding when flowers are still in bud.
4. Leaves lanceolate to lance-elliptic (the largest often as much as 1.5 cm wide), definitely toothed or denticulate; internodes often ± angled from slender ridges extending well below the nodes; puberulence glandular.
7. Teeth of leaf margin often scarcely more than dark (non-green) gland-like denticulations; petioles of midcauline leaves none or less than 2 (–3) mm long, often doubled or tripled in width by broad wings; hairs of seeds whitish.
7. Teeth of leaf definite (apart from gland-like tips) between quite irregular rounded sinuses; petioles of midcauline leaves mostly 2–5 (–6) mm long, narrowly if at all winged; hairs of seeds copper-colored or brownish at maturity.