Fl. South. U.S. ed. 2, 647. 1883.
Herbs, perennial, delicate, with woody, thickened taproot, 15 mm diam.. Stems prostrate, ascending, or erect, often numerous and wiry, less than 0.1 mm diam., 5–20 cm, glabrous, puberulent, canescent, villous, or hirsute, shorter hairs often uncinate and longer hairs straight or irregularly twisted. Leaves opposite; stipules distinct, triangular, sometimes lacerate or ciliate, 0.2–0.3 mm, glabrous or hairy; petiole 0.3–1 mm, glabrous or hairy; blade narrowly to broadly deltate, cordate, or reniform, 2–5(–7) × 1–4.5(–5) mm, base asymmetric, cordate to rounded, margins entire, ± revolute, apex obtuse or rounded, surfaces glabrous or hairy; only midvein conspicuous. Cyathia solitary at distal nodes; peduncle 0.7–1.5 mm. Involucre turbinate to campanulate, 0.8–1 × 1.1–1.3 mm, glabrous or hairy; glands 4, green to yellow-green, oblong to subcircular, 0.2–0.4 × 0.4–0.6 mm; appendages absent or white, forming narrow rim at edge of gland, rarely slightly wider than gland, (0–)0.1(–0.3) × 0.4–0.6 mm, distal margin entire. Staminate flowers 8–14. Pistillate flowers: ovary glabrous or hairy, subtended by triangular pad of tissue; styles spreading, 0.3–0.4 mm, 2-fid 1/2 to nearly entire length. Capsules broadly deltoid, 1.2–1.5 × 2–2.2 mm, glabrous or hairy; columella 0.9–1.3 mm. Seeds reddish brown, ovoid, 4-angled in cross section, 0.8–1.2 × 0.5–0.6 mm, obscurely wrinkled.
Subspecies 4 (4 in the flora).
Euphorbia deltoidea comprises four narrowly endemic subspecies, all of which are endangered due to habitat loss and fragmentation. The subspecies occur in pine rockland habitat that is free of shrubby undergrowth. Periodic fires are required to keep the rockland habitat open. Subspecies serpyllum is restricted to Big Pine Key, Monroe County, whereas the other subspecies are found only in Miami-Dade County.
|1||Stems erect or ascending.||> 2|
|2||Leaves and stems villous, hairs uncinate or irregularly twisted, 0.2–0.5 mm; leaf blades green.||Euphorbia deltoidea subsp. adhaerens|
|2||Leaves and stems villous-hirsute, hairs straight and spreading, 0.6–0.7 mm; leaf blades silver-green.||Euphorbia deltoidea subsp. pinetorum|
|1||Stems prostrate.||> 3|
|3||Leaves and stems glabrous or very sparsely hairy, hairs 0.1–0.2 mm, appressed, uncinate; leaf blades abaxially reddish, adaxially bright green.||Euphorbia deltoidea subsp. deltoidea|
|3||Leaves and stems canescent or villous, hairs either less than 0.1 mm or 0.2–0.5 mm, uncinate or irregularly twisted; leaf blades green or silver-green.||> 4|
|4||Leaves and stems villous, hairs uncinate or irregularly twisted, 0.2–0.5 mm; leaf blades as long as wide, green.||Euphorbia deltoidea subsp. adhaerens|
|4||Leaves and stems canescent, hairs less than 0.1 mm; leaf blades 2 times longer than wide, silver-green.||Euphorbia deltoidea subsp. serpyllum|