Leaves alternate, cauline leaves sometimes reduced to bracts, margins toothed or rarely lobed or entire.
Inflorescence terminal and cymose or raceme-like or flowers solitary, with 1–3 leaf-like or reduced bracts or bracteoles. Flowers unisexual. Calyx shortly tubular, lobed. Corolla tube split almost to base on upper side, 2-lipped, with the upper lip of 2 erect or recurved often much shorter lobes, and the lower one of 3 spreading lobes. Stamens fused to corolla at base; filaments sometimes splitting with age; 3 upper anthers slightly longer and with or without hairs, the 2 lower ones shorter with or without apical hairs. Ovary inferior, 2-locular; stigma 2-lobed.
| | Key to the species | |
| 1 | Fruit dehiscent, capsule opening by 2 loculicidal valves; erect or ascending herbs; few to multistemmed at base, rarely rooting at the nodes (except for L. anceps); corolla 2-lipped with usually one petal larger than the other | 2 |
| Fruit indehiscent or bursting irregularly below the corolla lobes, often succulent; prostate herbs, rooting at the nodes with petal ± the same size | 12 |
| 2 | Flowers at the tip of the shoot in a raceme-like arrangement, subtended by bracts or very reduced leaves | 3 |
| 2 | Flowers solitary in the axils of leaves (but leaves often reducing in size towards the tips of stems in L. anceps) | 8 |
| 3 | Stems decumbent, flowering shoots ascending; leaves ovate, base of lamina truncate to cordate | Lobelia trigonocaulis |
| Stems erect or ascending; leaves oblanceolate to linear-elliptic, sessile, subsessile or tapering to the base
Back to 2 | 4 |
| 4 | Seeds strongly 3-angled, smooth and shiny | 5 |
| Seeds not 3-angled, surface very faintly ridged or reticulate
Back to 3 | 6 |
| 5 | Capsule 7–15 mm long; upper corolla lobes recurved, glabrous | Lobelia simplicicaulis |
| Capsule 3.5–4 mm long; upper corolla lobes incurved and pilose
Back to 4 | Lobelia andrewsii |
| 6 | Lower leaves ovate, margins broadly lobed; seeds ellipsoidal, very faintly ridged to almost smooth | Lobelia dentata |
| Lower leaves with margins usually entire, rarely with a few narrow teeth; seeds irregularly angular, reticulate
Back to 4 | 7 |
| 7 | Anther tube 2.5–4 mm long; staminal filaments 4–6.5 mm long; corolla 10–20 mm long, lower lip spreading, rarely decurved, the central lobe of lower lip usually narrowly elliptic, acute; leaves entire | Lobelia gibbosa |
| Anther tube 1.8–2.3 mm long; staminal filaments 2–3 mm long; corolla 9–14 mm long, lower lip decurved, the central lobe of lower lip oblanceolate to oblong, usually obtuse and apiculate; leaves narrowly toothed or entire
Back to 6 | Lobelia browniana |
| 8 | Young stems usually with a narrow wing 0.2–1.1 mm wide; capsule ± cylindrical, 8–12.5 mm long, 2–2.5 mm wide | Lobelia anceps |
| Stems sometimes ridged but not distinctly winged; capsule ellipsoid to obovoid or obconical, 7 mm long
Back to 2 | 9 |
| 9 | Pedicels 1–5(–7) mm long; leaves crowded | Lobelia darlingensis |
| Pedicels (8–)10–60(–70) mm long; leaves ± well spaced
Back to 8 | 10 |
| 10 | Lamina usually broadly ovate, margins toothed, base rounded; petiole distinct | Lobelia membranacea |
| Lamina linear to narrow-elliptical or elliptical, tapering to an indistinct petiole
Back to 9 | 11 |
| 11 | Pedicels (8–)10–30 mm long; plants with spreading hairs | Lobelia pratioides |
| Pedicels 35–70 mm long; plants glabrous
Back to 10 | Lobelia stenophylla |
| 12 | Peduncles <5 mm long | 13 |
| Peduncles >5 mm long
Back to 1 | 14 |
| 13 | Leaves to 12 mm long; corolla to 5 mm long | Lobelia darlingensis |
| Leaves 10–30 mm long; corolla >5 mm long
Back to 12 | Lobelia concolor |
| 14 | Leaves usually obovate to spathulate, margins mostly entire | Lobelia surrepens |
| Leaves not as above, margins ± toothed
Back to 12 | 15 |
| 15 | Leaves ovate to lanceolate to oblong-lanceolate, 10–30 mm long, glabrous | 16 |
| Leaves ovate to ± circular, mostly <10 mm long, usually hairy or glabrescent
Back to 14 | Lobelia pedunculata |
| 16 | Peduncles 10–70 mm long; calyx lobes with a small basal tooth on each side; leaves mostly petiolate, often purplish on lower surface | Lobelia purpurascens |
| Peduncles 3–13 mm long; calyx lobes without basal teeth; leaves sessile, green on lower surface
Back to 15 | Lobelia concolor |
