Leaves opposite, stipules often leaf-like, and leaves appearing to be in whorls of 4–8.
Flowers small, mostly in axillary or terminal cymes or clusters, occasionally solitary, mostly unisexual and plants dioecious. Calyx usually absent. Corolla 4-lobed, funnel-shaped with the tube usually longer than the lobes. Stamens with filaments scarcely fused to the corolla; anthers exserted. Ovary 2-locular, each with 1 ovule; style 2-lobed with a capitate stigma.
The stem angles referred to in the key below are the ridges at the corners of the otherwise square stems. Where the angles are very broad, they appear to be the faces/sides of the stems, but are separated by grooves rather than ridges.
| | Key to the species | |
| 1 | Flowers bisexual; corolla lobes blue inside and less than half the length of the tube | Asperula arvensis |
| Flowers unisexual (but with structures of non-functional sex present) and plants dioecious; corolla white all over, lobes c. equal to or longer than tube | 2 |
| 2 | Leaves and leaf-like stipules all or mostly in whorls of 2 or 4 | 3 |
| Leaves and leaf-like stipules all or mostly in whorls or 5 or more
Back to 1 | 7 |
| 3 | Stipules < 3/4 the length of leaves or absent | 4 |
| Stipules > 3/4 the length of leaves
Back to 2 | Asperula gunnii |
| 4 | Stipules absent or less than 10% the length of leaves | 5 |
| Stipules present and more than 20% the length of leaves
Back to 3 | 6 |
| 5 | Corolla tube about a third to a fifth the length of the corolla lobe | Asperula gemella |
| Corolla tube equal to or longer than the corolla lobe
Back to 4 | Asperula geminifolia |
| 6 | Trailing herbs; leaves spreading, narrow spathulate (North Coast) | Asperula asthenes |
| Subshrubs (becoming woody basally); leaves erect, narrow-oblong to linear
Back to 4 | Asperula ambleia |
| 7 | Leaves with an almost pungent hyaline tip 0.2–0.5 mm long and with a terminal hair, if present, 0.05 mm long (magnifying glass needed to see the hair) | Asperula scoparia |
| Leaves with hyaline tip absent or hyaline tip <0.2 mm long; or if rarely slightly longer then with terminal hair 0.1–0.3 mm long
Back to 2 | 8 |
| 8 | Leaves of stems and major branches > 10 mm long and with a l:w ratio > 10 | Asperula charophyton |
| Leaves < 10 mm long, or if longer, with a l:w ratio < 10
Back to 7 | 9 |
| 9 | Stem angles with moderately dense fine hairs (generally > 30 hairs per mm of angle); hairs short, not extending more than 0.1 mm out from the surface of the stem | 10 |
| Stem angles glabrous or hairs sparser than above, or if somewhat dense then the hairs coarse and broad-based and/or etending more than 0.1 mm out from the surface of the stem
Back to 8 | 16 |
| 10 | Leaves and stipules sub-erect to erect, less than or equal to 1 mm wide; plants becoming woody at base with spongy fissuring bark | 11 |
| Leaves and stipules generally spreading, usually largest ones > 1 mm wide; plants not becoming woody at base or developing spongy fissuring bark
Back to 9 | 12 |
| 11 | Stem hairs straight; faces of stems generally much narrower than angles; apex of leaves narrow acute to filiform | Asperula subulifolia |
| Stem hairs generally curving retrorsely; faces of stems generally as broad as or broader than angles; apex of leaves rounded to acute
Back to 10 | Asperula cunninghamii |
| 12 | Leaves acuminate | Asperula acuminata |
| Leaves not acuminate
Back to 10 | 13 |
| 13 | Stem hairs retrorsely curved | Asperula gunnii |
| Stem hairs more or less straight
Back to 12 | 14 |
| 14 | Leaves to 1.8 mm wide; plants typically drying very dark brown; hairs needle like in profile | Asperula pusilla |
| Largest leaves mostly > 2 mm wide; plants drying green; hairs generally narrow-triangular
Back to 13 | 15 |
| 15 | Leaves narrow-cuneate (< 1.5 mm wide 3 mm from base, with l:w ratio of 4–10); larger stems usually with at least one whorl 7- or 8-partite; anthers > 0.5 mm long (male plants); style 2–2.2 mm long (female plants) | Asperula polymera |
| Leaves cuneate (>1.5 mm wide 3 mm from base, with l:w ratio of 2–5); stems usually with all whorls 6-partite; anthers ≤ 0.5 mm long (male plants); style 1.2–1.8 mm long (female plants)
Back to 14 | Asperula euryphylla |
| 16 | Stem-angles with moderately dense, more or less spreading hairs > 0.2 mm long; upper surface of leaves with hairs > 0.2 mm long | 17 |
| Stem angles with either sparse hairs or hairs not spreading, or hairs <0.2 mm long; upper surface of leaves glabrous or with hairs to c. 1 mm long
Back to 9 | 18 |
| 17 | Leaves and stipules ascending (serpentinite, NE NSW) | Asperula hoskingii |
| Leaves and stipules generally spreading
Back to 16 | Asperula pusilla |
| 18 | Leaves tapering mildly towards base (width of base ≥ 1/2 of maximum width), with midrib on underside robust and tending to protrude relative to margins; upper surface of leaves glabrous or rarely with hairs in proximal half; ovary commonly broader than long (face view) | 19 |
| Leaves tapering moderately to strongly towards base (width of base ≤ 1/2 of maximum width), with midrib on underside generally slender and recessed relative to margins; upper surface of leaves often with scattered minute hairs, particularly towards apex; ovary commonly slightly longer than broad (face view)
Back to 16 | 20 |
| 19 | Leaves and stipules weakly angled forwards, spreading or pendant, not arching towards the stem; upper surface usually with a smooth medial glossy strip | Asperula conferta |
| Leaves and stipules strongly angled upwards and arching towards the stem (more so in dried specimens); upper surface lacking a glossy strip
Back to 18 | Asperula wimmerana |
| 20 | Stem hairs retrorsely curved; leaves generally nearly straight sided, spreading to pendant, with upper surface generally glabrous | Asperula pusilla |
| Stem hairs ± straight; leaves narrowly to very narrowly elliptic, spreading or ascending, with upper surface usually with some hairs
Back to 18 | Asperula gunnii |
