Inflorescence consisting of clusters of flowers in upper leaf axils or flowers arranged in terminal spike-like clusters. Sepals ± regularly fused, forming 5 equal or ± equal lobes. Corolla slightly 2-lipped or 4-lobed, then lower lip with 3 ± equal lobes and upper lobe truncate, emarginate or 2-lobed. Stamens 4, fertile, inserted below the throat of the corolla tube; filaments erect; anthers with 2 fertile loculi, not diverging.
| | Key to the species | |
| 1 | Clusters of flowers in upper leaf axils, with branch visible between the clusters; calyx 13-veined (except M. pulegium 10-veined) | 2 |
| Flower clusters terminal, forming a spike-like inflorescence, clusters subtended by narrow bracts, not subtended by leaves, with branch frequently not visible between the clusters; calyx 10-veined | 6 |
| 2 | Calyx lobes villous on inner surface; clusters few-flowered. (Mentha satureioides group) | 3 |
| Calyx lobes with short more or less spreading hairs on inner surface, not villous; clusters few- to many-flowered
Back to 1 | 4 |
| 3 | Branches almost glabrous; calyx glabrous on outer surface; flowers in 3-flowered clusters per leaf axil; leaves narrow-elliptic to linear-elliptic, 0.5–3.5 cm long, 0.2–0.7 cm wide; margins entire | Mentha satureioides |
| Branches and calyx hairy; flowers in 3–8-flowered clusters per leaf axil; leaves ovate, often angular-ovate, rarely narrow-ovate or elliptic, 0.6–2 cm long, 0.4–1.2 cm wide; margins entire or sometimes with distant shallow teeth
Back to 2 | Mentha diemenica |
| 4 | Flowers 2–6, rarely more per leaf axil | Mentha laxiflora |
| Flowers crowded in dense clusters, at least 10 flowers per leaf axil
Back to 2 | 5 |
| 5 | Leaves narrow-ovate, 1.2–6 cm long, 8–20 mm wide, margins with 2–7 teeth, apex acute; calyx 13-veined; corolla white or pink | Mentha australis |
| Leaves obovate, elliptic to more or less circular, 0.6–3 cm long, 4–20 mm wide, margins entire or with small blunt teeth, apex obtuse or rounded; calyx 10-veined; corolla usually pale-mauve to lilac
Back to 4 | Mentha pulegium |
| 6 | Branches and lower surface of leaves densely hairy, with branched and long simple hairs | Mentha x rotundifolia |
| Branches and leaves glabrous or sparsely hairy, especially on the veins
Back to 1 | 7 |
| 7 | Basal leaves sessile; plants producing fruit; calyx lobes about half the length of calyx | Mentha spicata |
| Basal leaves petiolate; plants not producing fruit; calyx lobes a third the length of calyx
Back to 6 | Mentha x piperita |
