Our generic delimitation follows Wagner, Hoch, and Raven (2007). This is mostly similar to the treatment in Michigan Flora, except that our common fireweed is now Chamerion angustifolium (L.) Holub, and the small genera Calylophus and Gaura are included in Oenothera.
Species of Clarkia are showy, frequently cultivated annuals, also used in “wildflower” mixes, and may persist for a short while but are at best barely an element of our flora as yet. Clarkia unguiculata Lindl. was collected in 2009 as “persisting” from a “wildflower” planting in Ontonagon Co. It has alternate leaves (at least on the upper part of the stems), dehiscent capsular fruits, lacks an elongated floral tube, and has striking large clawed purple (rarely white) petals.
1. Floral parts (sepals, petals, stamens) in 2's; fruit ± densely pubescent with hooked hairs, indehiscent; leaves opposite.
1. Floral parts in 4's; fruit glabrous or with hairs lacking hooked tip, dehiscent or indehiscent; leaves opposite or alternate.
2. Floral tube prolonged beyond the ovary for about half as long as the ovary itself, or more; leaves alternate.
2. Floral tube scarcely if at all prolonged beyond the end of the ovary; leaves opposite or alternate.
3. Petals none or yellow to greenish and minute (at most, in one species, equaling the sepals); calyx lobes persistent on the oblong to nearly spherical fruit; stamens 4; seeds without hairs.
3. Petals white to magenta, exceeding the sepals; calyx lobes deciduous from the linear fruit; stamens 8; seeds with a tuft of silky hairs.
4. Leaves all alternate; flowers in long, terminal, showy racemes, the buds reflexed; floral tube none; stems and leaves glabrous or very minutely pubescent.
4. Leaves, except sometimes the upper ones, opposite; flowers all or mostly in the axils of leaves or leafy bracts, the buds not reflexed; floral tube ca. 0.3–3.5 mm long beyond the ovary; stems glabrous to spreading-hairy (if upper leaves alternate and flowers showy, the plant with horizontally spreading pubescence).