12. Euphorbia marginata Pursh (snow-on-the-mountain)

Map 1674, Pl. 379 f–h

Plants annual, with a taproot. Stems 30–80(–150) cm long, erect or ascending, usually unbranched below the inflorescence, the branches not flattened toward the tip, green to yellowish green or tan, often somewhat reddish- or purplish-tinged toward the base, moderately to densely pubescent with slender, spreading, somewhat tangled hairs, at least toward the tip, often glabrous or nearly so below the midpoint. Leaves alternate below the inflorescence branches (those of the inflorescence branches opposite or whorled), not appearing crowded, sessile or minutely petiolate. Stipules usually a pair of minute, narrowly conical, light brown glands but these shed as the leaves develop. Leaf blades 10–100 mm long, unlobed, the margins entire or slightly wavy, the surfaces glabrous or sparsely to moderately pubescent toward the base and along the midvein with slender, spreading, somewhat tangled hairs, light green to bright green; those below the inflorescence broadly ovate to elliptic or oblong-obovate, rounded to angled or short-tapered at the base, angled or tapered to a bluntly or more commonly sharply pointed tip; those along the inflorescence branches narrowly elliptic to elliptic, oblanceolate, or narrowly obovate (much longer than wide), angled or tapered at the base, angled to a sharply pointed tip, at least the uppermost leaf blades with conspicuous, broad, white margins, these occasionally slightly pinkish-tinged toward the outer edge. Inflorescences terminal umbellate panicles with a whorl of leaves at the base and each of the 3 or 4(5) primary branches usually branched 2–5 additional times, the cyathia solitary or less commonly in small clusters at the branch tips and solitary at the branch points. Involucre 3–4 mm long, the outer (and inner) surface sparsely to densely hairy, the rim 5-lobed, the lobes deeply divided into a fringe of slender, pale lobes, the marginal glands 5, 1.0–1.8 mm long, oblong-elliptic to nearly circular and often somewhat concave, green to light green or yellowish green, with a relatively conspicuous petaloid appendage 2–4 mm long, this white. Staminate flowers 35–60 per cyathium. Ovaries densely pubescent with ascending hairs, the styles 1.0–2.5 mm long, each divided 2/3–3/4 of the way from the tip into 2 relatively slender lobes. Fruits 4–6 mm long, moderately to densely hairy. Seeds 3–4 mm long, ovate to nearly circular in outline, nearly circular in cross-section, rounded at the base, the surface with a network of irregular wrinkles or blunt ridges and sometimes also low, rounded tubercles, orangish tan to olive gray, sometimes with a thin, white coating, lacking a caruncle. 2n=56. June–October.

Scattered, mostly in the northern half of the state (Minnesota to Montana south to Texas and Arizona; Canada; introduced east to New Hampshire and Florida). Bottomland prairies and loess hill prairies; also pastures, gardens, railroads, and roadsides.

This species is cultivated as an ornamental for its showy, white-margined upper leaves. Steyermark (1963) noted that it grows well in hot, dry, clayey soils. Wherever it is grown, the species tends to escape sporadically.