Description from Flora of China
Shrubs, rounded, up to 1 m tall. Stems many branched, glabrous; pith hollow. Stipules very small, membranous, caducous; petiole 2-4 cm; leaf blade elliptic, 1.5-7 × 1.2-4 cm, thinly papery, both surfaces smooth, glabrous, base usually attenuate, rarely rounded, margin entire, apex rounded or emarginate. Inflorescence a terminal or axillary solitary cyathium. Cyathium stipe ca. 3 mm; involucre campanulate, ca. 3.5 × 3.5 mm, lobes 4 or 5, pilose; glands 3 or 4, light yellow, flat, transversely elliptic, entire, ca. 2 × 1.5 mm. Female flower: ovary ca. 2.5 mm in diam., smooth; styles connate below middle; style arms not lobed. Capsule 3-angular-ovoid, ca. 6 mm in diam., smooth, glabrous. Immature seeds globose, ca. 3 mm in diam., without ornamentation, without caruncle. Fl. Dec.
Though it has cyathia with 3, flat cyathial glands, rather than the solitary cupular or 2-lipped glands typical of Euphorbia subg. Poinsettia, initial molecular data indicates that E. hainanensis is most closely related to that group, as was suggested in the protologue. All other members are native to the New World, and Croizat suggested that the Hainan plant might be introduced in China; however, a second collection (M. G. Gilbert et al. [National Geographic Society 2000 Hainan Expedition] 501, BM, IBSC, MO) shows that this is extremely unlikely. It grows in a very distinctive habitat with a number of other Hainan endemics.
● Among rocks at base of limestone cliffs; ca. 900 m. Hainan (Ledong).