A familiar and attractive genus; all of our native species known also in cultivation.
1. Calyx (except rarely in P. paniculata) glabrous and stem (at least below the uppermost internodes) glabrous or nearly so.
2. Leaves with evident anastomosing lateral veins and with hispidulous or ciliate margins; escapes from cultivation.
2. Leaves with lateral veins obscure or not at all visible and with smooth glabrous margins; rare natives of forests or wetlands.
3. Stem unspotted; cauline leaves 3–5 pairs below the inflorescence.
3. Stem purple-spotted; cauline leaves ca. 8–25 pairs below inflorescence.
1. Calyx and stem puberulent to pilose.
4. Plants ± densely matted or tufted, with branches from nearly woody depressed or trailing stems; leaves rather needle-like (especially in P. subulata).
5. Corolla lobes notched ca. (3–) 3.5–7 mm; largest leaves ca. 2.5–4.5 cm long, glabrous or ciliate only at the base.
5. Corolla lobes notched less than 2 (–2.5) mm; largest leaves (0.8–) 1.1–1.6 (–2.2) cm long, often ciliate at least to the middle.
6. Style very short (less than 4 mm); stamens all included in the corolla tube; limb of corolla ca. 2–2.4 cm broad.
6. Style longer (sometimes long-exserted); stamens at least partly exserted at maturity; limb of corolla ca. 1.4–1.9 (–2.1) cm broad.
4. Plants not densely tufted, with erect flowering stems (only P. divaricata with depressed subligneous stems and vegetative shoots); leaves with flat blades, without subulate tips; styles less than half as long as the calyx.
7. Plant a slender-rooted annual, rarely escaped from cultivation; corolla normally rose-red; leaves on upper part of stem alternate.
7. Plant perennial, native; corolla normally blue to pink or violet; leaves all opposite.
8. Corolla entirely glabrous; calyx lobes at most with a subulate tip 0.5 mm long; larger leaves 2.3–5.6 times as long as broad; leafy vegetative shoots conspicuous, arising from depressed stem; fresh corollas normally blue (if not white).
8. Corolla tube pubescent; calyx lobes with awn-like tip at least 0.5–1.5 mm long; larger leaves (5.6–) 6–10 (–18) times as long as broad; leafy vegetative shoots none or very inconspicuous; fresh corollas pink to purple.