Flower:

Flowers are tiny and indistinct in clusters at the tips of branches. A small, green cup, 1/8 inch across, holds several (typically 3) male (staminate) flowers, each with a single stamen with two yellow anthers, surrounding  a single female (pistallate) flower with a six-parted pistil in the center. The round, three lobed ovary below the pistil develops quickly on a stalk that extends it out and down from the flower center. The rim of the cup has a single, thickened, oblong gland - green turning red. The leafy bracts surrounding the flower are typically bright red at the base end. The structure of this flower is called a cyathium and common to all the Euphorbias.

Leaves and stems:

Leaves mostly alternate, though a few upper leaves can be opposite, highly variable from linear to broadly oblong, lobed to fiddle-shaped, up to 4 inches long and 2 inches wide, the edges smooth to finely toothed, very short stalked near the flowers and longer stalked below. The upper surface is mostly hairless and the lower sparsely hairy. Stems are erect, unbranched to much branched, and hairless. Leaves and stems exude a milky sap when broken.

Fruit:

Fruit is a deeply three lobed capsule, 1/5 inch wide, that develops rapidly from the center of the cyathium, typically before the surrounding stamens are fully developed. The fruit hangs down on a short stalk becoming erect at maturity, each lobe containing a single seed.

Seeds are egg-shaped to oval to cylindric, 2 to 3 mm long, mottled gray, brown and black, the surface irregularly covered in shallow pits and small bumps, or, when cylindric, more evenly ridged in rows.

Notes:

In Minnesota where Euphorbia cyathophora (sometimes known as Poinsettia cyathophora) is at the northern fringe of its range, this is a small statured, marginal plant of sand prairie and bluffs along the Minnesota and Mississippi rivers in the southwest central and southeast counties, respectively. In the more southern and eastern parts of its North American range, it can inhabit moister and shadier habitats, getting much larger and showier. It is cultivated for its showy red bracts, similar to the Christmas poinsettia to which it is closely related. Introduced to many parts of the world it has become weedy in much of the South Pacific from Taiwan to Australia.