Inflorescence unit a cyathium; cyathia solitary or borne in terminal or axillary cymes or clusters or pseudoaxillary cymes. Cyathia are flower-heads resembling single flowers, each cyathium with a solitary terminal female flower surrounded by 4 or 5 male cymes (male flowers in cymes of 1–several flowers), all enclosed by a cup-shaped involucre with lobes or teeth alternating with 1–5 glands, the latter sometimes bordered by petaloid appendages; male cymes 1–10-flowered. Male flowers each with 1 stamen, perianth usually absent. Female flower consisting of a stalked (pedicellate) 3-locular ovary, styles 3, free or basally fused, mostly bifid; perianth 3–6 scale-like sepals or absent.
Capsule exserted beyond involucre, separating into three 2-valved segments; seeds with or without caruncle.
| | Key to the species | |
| 1 | Stems erect; leaves usually alternate on lower stems, subopposite or opposite above; stipules minute or absent | 2 |
| Stems prostrate, decumbent or ascending; leaves opposite; stipules present, interpetiolar (includes species formerly treated as Chamaesyce) | 18 |
| 2 | Shrubs or subshrubs, cyathia solitary (except in Euphorbia sarcostemmoides); native species | 3 |
| Erect herbs; cyathia in compound inflorescences or solitary; naturalized weedy species
Back to 1 | 8 |
| 3 | Apparently leafless shrub, multistemmed; cyathia in compound cymes | Euphorbia sarcostemmoides |
| Leafy shrubs or subshrubs, with a single main stem; cyathia solitary
Back to 2 | 4 |
| 4 | Leaves oblong-lanceolate to oblanceolate, ovate or oblong-spathulate; length:width ratio less than 6.5:1 | 5 |
| Leaves linear to narrow-lanceolate, length:width ratio more than 7:1
Back to 3 | 7 |
| 5 | Capsules broader than long, more or less depressed-globose, c. 5 mm long, 6 mm diam.; seeds ovoid without a caruncle; leaves falling early | Euphorbia stevenii |
| Capsule as long as or longer than broad, globose to triangular, 4–4.5 mm long, c. 4 mm diam.; seeds more or less oblong, carunculate; leaves more persistent
Back to 4 | 6 |
| 6 | Leaves with lamina to 25 mm long and 8 mm wide; seeds covered with pustules, caruncle crescent-shaped; an inland species | Euphorbia parvicaruncula |
| Leaves with lamina to 50 mm long and 17 mm wide; seeds smooth to undulate; caruncle hat-shaped; a coastal species
Back to 5 | Euphorbia tannensis |
| 7 | Capsule oblong-ovoid, to 6 mm long, 4 mm diam.; seeds grey-brown, smooth to deeply wrinkled, caruncle hat-shaped, persistent, yellow-brown | Euphorbia tannensis |
| Capsule more or less globose, c. 3 mm long, 3.5 mm diam.; seeds dark blue-green, deeply pitted-reticulate, caruncle small, crescent-shaped
Back to 4 | Euphorbia planiticola |
| 8 | Leaves mostly 40 mm or more long | 9 |
| Leaves mostly less than 40 mm long
Back to 2 | 13 |
| 9 | Stem leaves sessile or shortly petiolate, margins entire or minutely toothed | 10 |
| Stem leaves distinctly petiolate, margins entire to widely lobed or toothed
Back to 8 | 12 |
| 10 | Stem leaves entire, 60–160 mm long, 5–12 mm wide, opposite and decussate | Euphorbia lathyris |
| Stem leaves finely toothed, particularly towards apex, 10–60 mm long, 8–25 mm wide, alternate, not decussate
Back to 9 | 11 |
| 11 | Stem leaves linear to oblong-elliptic, sessile; capsule c. 3.5 mm long, tuberculate; seeds smooth | Euphorbia depauperata |
| Stem leaves obovate to spathulate, petiole c. 1 mm long; capsule c. 2 mm long, smooth; seeds reticulate-pitted
Back to 10 | Euphorbia helioscopia |
| 12 | Lamina entire to widely lobed, often reddish at base of upper leaves; glabrous to pubescent | Euphorbia cyathophora |
| Lamina coarsely toothed, not reddish at base of upper leaves; pubescent
Back to 9 | Euphorbia davidii |
| 13 | Stem leaves sessile, variable, distinctly different in shape and/or colour from the leaves of the fertile branches | 14 |
| Stem leaves petiolate (very shortly so in Euphorbia helioscopia), obovate to spathulate or ovate, not conspicuously different from leaves of fertile branches
Back to 8 | 17 |
| 14 | Stem leaves more than 10 mm wide; leaves of fertile branches with conspicuous white margins; capsule pubescent | Euphorbia marginata |
| Stem leaves less than 10 mm wide; leaves of fertile branches without white margins; capsule glabrous
Back to 13 | 15 |
| 15 | Stem leaves more or less fleshy, crowded and imbricate; terminal cyathia solitary | Euphorbia paralias |
| Stem leaves not as above; terminal cyathia 2 or 3 (occasionally solitary in Euphorbia falcata)
Back to 14 | 16 |
| 16 | Glaucous annual; leaves of fertile branches broad-ovate, 1–10 mm wide, margins crenate; stem leaves oblong-cuneate to oblanceolate, 10–30 mm long, 2–5 mm wide, entire | Euphorbia falcata |
| Green perennial; leaves of fertile branches ovate-triangular, 10–30 mm wide, margins finely toothed; stem leaves linear-lanceolate, 10–40 mm long, 2–8 mm wide, finely toothed
Back to 15 | Euphorbia terracina |
| 17 | Stems leaves with distinct petiole to 8 mm long; lamina obovate, mostly 10–15 mm long, margins entire; glands of involucre with horns at each end | Euphorbia peplus |
| Stem leaves shortly petiolate, petiole c. 1 mm long; lamina obovate to spathulate, 10–40 mm long, margins toothed towards apex; glands of involucre entire
Back to 13 | Euphorbia helioscopia |
| 18 | Leaves 2–15 mm long; cyathia solitary in axils of leaves | 19 |
| Leaves 10–40 mm long; cyathia in axillary or terminal cymes
Back to 1 | 26 |
| 19 | All or part of plant hairy; stems prostrate, often forming dense mats | 20 |
| Plants glabrous; stems prostrate to ascending or rarely erect
Back to 18 | 22 |
| 20 | Entire plant covered with soft white hairs; native herb of inland plains | Euphorbia australis |
| Plants sparsely hairy, mainly on the stems and capsules; naturalized herbs of the coast and Western Slopes
Back to 19 | 21 |
| 21 | Stems with longitudinal bands of short white, often curled, hairs; capsule glabrous except for ciliate angles | Euphorbia prostrata |
| Stems with more or less scattered, longer, mostly spreading hairs; capsule sparsely hairy
Back to 20 | Euphorbia maculata |
| 22 | Annual 15–30 cm high with ascending to erect slender stems; leaves in more or less distant pairs along stems, leaves mostly 8–15 mm long | Euphorbia wheeleri |
| Perennials with prostrate, ascending to almost erect or decumbent stems mostly to 20 cm long, often forming a woody rhizome; leaves more or less crowded along stems; leaves 2–10 mm long
Back to 19 | 23 |
| 23 | Leaves oblanceolate to oblong, margins often strongly recurved when dry; stipules mostly 1–1.5 mm long, more or less conspicuous, entire or divided, thread-like | Euphorbia inappendiculata var. queenslandica |
| Leaves ovate-oblong, margins not recurved when dry; stipules usually less than 1 mm long, inconspicuous, not thread-like (occasionally so in Euphorbia papillifolia var. polyandra)
Back to 22 | 24 |
| 24 | Appendages of glands conspicuous, crenate-sinuate; involucre c. 1.5 mm long; leaves glaucous | Euphorbia papillifolia var. polyandra |
| Appendages of glands usually inconspicuous, mostly entire; involucre 0.5–0.7 mm long; leaves not glaucous
Back to 23 | 25 |
| 25 | Interpetiolar stipules triangular to broadly triangular, 0.2—1.1 mm long, entire or bipartite; capsule with a fringed receptacle appendage appressed to base (hypogynous disc laciniate or entire); seed faces more or less smooth | Euphorbia dallachyana |
| Interpetiolar stipules subulate, 0.5–0.9 mm long, deeply bipartite; capsule without a fringed receptacle appendage (hypogynous disc entire); seed faces wrinkled
Back to 24 | Euphorbia drummondii |
| 26 | Plants persistently and densely hairy; cyathia in dense, stalked, head-like axillary cymes | Euphorbia hirta |
| Plants glabrous or sparsely pubescent often glabrescent with age; cyathia not in dense heads
Back to 18 | 27 |
| 27 | Upper and lower leaves more or less consistent in size and shape; length:width ratio of leaves 2:1; lamina thick; prostrate plant forming mats on coastal sands | Euphorbia psammogeton |
| Upper leaves usually longer and narrower than lower leaves; length:width ratio of upper leaves more than 2:1; lamina thin; decumbent to ascending or semi-erect plants
Back to 26 | 28 |
| 28 | Hairs on stems scattered on all sides; length:width ratio of upper leaves 3:1 to 5:1; appendages of glands conspicuous, 1–2 mm wide, petaloid, white | Euphorbia bifida |
| Hairs on stems usually confined to 1 or 2 sides; length:width ratio of upper leaves 3:1 or less; appendages of glands 0.5 mm wide, entire, white or pink
Back to 27 | Euphorbia hyssopifolia |
