Flowering Spurge (Euphorbia corollata) of the Spurge family (Euphorbiaceae) is an herbaceous perennial with cyathia (false flowers) that have outward flared, white petaloid appendages growing from the rim of the involucre (modified receptacle). Genus name recognizes Euphorbus, a Greek physician. Specific epithet is Latin for “like a corolla” in reference to the appendages appearing to be a typical corolla. In the US, occurrence includes all states from Texas to South Dakota and eastward to the Atlantic and Gulf Coasts, excluding Florida. In Arkansas, occurrence is statewide. The species is adaptable to many dry to mesic soil types in full to partial sun, such as, upland woods, prairies, glades, roadsides and disturbed areas.
Plants (2-5 feet tall; typically to 2 feet) have descending, unbranched, ropy roots. Early in the growth-year, new stems are reddish green to light green with bract-like leaves near soil-level and more leaf-like, similarly shaped, leaves a few nodes above. Early stems and leaves are variously pubescent (variable by plant and region) but become glabrous with maturity.
Lower stem leaves are alternate but become opposite higher up the stem and in the inflorescence. About mid-stem, lateral branches may occur singly in opposite, equal-sized branch-pairs (dichotomous branching) with branches set at about 45⁰ off the axis (see Photo 10). More robust stems may divide into whorls of 3-7 equal-sized primary branches. Primary branches may further divide 2-5 times before each ultimate branch terminates with a cyathium. Lengths of branches, all axillary, reduce in length from node to node with lowest internodes being to 10 inches. Stems are straight and erect while branches (primary to ultimate) are straight, spreading and ascending. Mature branches are slender, glabrous (pubescence, if any, having been lost) and pale green to greyish green (lighter than leaves) – sometimes glaucous (bluish haze). Stems and branches, with sticky white sap, are smooth between nodes and round in cross-section (terete). With a large inflorescence, entire stem becomes top-heavy and leans toward sunlight. With drying summer-time conditions, leaves drop-off (progressing from soil-level into the inflorescence) so that the now-naked stems and branches are coarsely roughened by rigid leaf bases. Mature stems/branches in full sun darken with age and have reddish blotches. With extended dry conditions, stems may die to the root but can regrow the same year if conditions improve.
Leaves (to 3 inches long and ½ inch wide) are commonly oblong (lower leaves) to linear-oblong (middle leaves); both shapes with rounded apices and rounded to wedge-shape bases. Leaves of some plants are narrowly lanceolate (to 4 inches and width of 3/16 inch) with a pointed apex and rounded base. The simple, sessile to sub-sessile (petioles to 3/16 inch) leaves have smooth (entire) margins which may be slightly revolute. Leaves are green above and a paler green below. Obscure venation is diffused-pinnate with a prominent lower (abaxial) midrib and a less prominent and lighter colored upper (adaxial) midrib. In the inflorescence, the small to tiny leaves (floral leaves) are mostly opposite with a leaf subtending each branch-division and cyathium. Floral leaves (decreasing in size distally) are elliptic to ovate (to < 1/16 inch by <1/16 wide). Leaves near cyathia may have white upper margins and apices.
Inflorescence architecture for the genus Euporbia is a dichasium characterized by floral units with a central cyathium on a shortened stalk and two lateral main axes. Young plants and plants in poor habitats may have a few loosely arranged floral branches with few cyathia while older plants in rich habitats have more robust stems and may have panicle-like clusters to ¾ feet long and a foot wide with hundreds of cyathia. Cyathia, facing upwards, are ¼-⅜ inch wide and ⅛ inch tall (from base of cup to tip of divided stigma) and have erect pedicels (stalk of a flower) to ⅛ inch long. Branches of the inflorescence and pedicels are light green, glabrous and terete. Flowering may occur from late spring into late summer for a 1-2 month period with timing/duration affected by soil moisture.
Cyathia are the unique, primary inflorescence of the genus Euphorbia. In Flowering Spurge, cyathia have: 1) a bell-shaped (campanulate) involucre, 2) five white petaloid involucral appendages, 3) five green, flattened, ovate, yellowish green nectar glands attached to the appendages, 4) five white, elongate, incurved, fringed (fimbriate) bracteoles between the appendages, 5) a number of apparent-stamens (filament + anther) and, sometimes, 6) an apparent-pistil (ovary + style + stigma). Morphologically, each stamen and each pistil is considered to be an entire flower. Centers of cyathia bear either: 1) a tight group of up to 25 staminate flowers (herein, a staminate cyathium) or 2) a smaller group of staminate flowers surrounding a pistilate flower (herein, a perfect cyathium). Staminate and perfect cyathia have the same involucre, appendages, nectar glands and bracteoles. The ratio of staminate to perfect cyathia and position within the inflorescence vary from plant to plant. Appendages are positioned in a planar manner. Bracteoles may be absent.
Staminate flowers have a single stamen with a joint on its stalk that separates the pedicel (below) and the filament (above); see Photo 19. Staminate flowers have a white filament/pedicel and an anther of 2 pale yellow spherical lobes that unroll to expose a darker yellow pollen. Pistillate flowers have a united, 3-carpel, green ovary and 3 united, whitish green styles/stigmas. The united styles separate from the tip and recurve to expose stigmatic surfaces. Ovaries are on a floral stalk (hidden until fertilization occurs). In the case of the perfect flowers, stigmas are well-spread and recurved before the “underlying” pistillate flowers of the same cyathium appear but remain below pistil-level.
Following fertilization, the enlarging ovary develops into a seed capsule. The capsule is pushed out of the cyathium by an elongating floral stalk where the capsule dangles as the cyathium slowly deteriorates. The glabrous to minutely scabrous capsules, to about ⅛ inch wide and tall, have 3 globoid cells. The thin-skinned capsules become tan and cells split along their common, axial line to release one seed per cell. The mottled brown seeds, about 1/16 inch long, are broadly ovate in outline and terete in cross-section.
Flowering Spurge would be good plant for a native plant garden, pollinator garden or naturalized area. The erect perennial plants provide leafy springtime growth and a wispy display of white cyathia for up to 2 months in mid to late summer. Stems lean when topped with a heavy inflorescence. Plants in sunny sites may drop leaves with the hint of drought; leaf loss potentially extending into the inflorescence. Sap is a skin and eye irritant. Pollen and nectar are collected by small insects and seeds are eaten by turkey and other ground-dwelling birds. Foliage may become reddish in fall. Stems of the low maintenance plant disintegrate quickly. May be propagated by root segments.
Twenty-one additional Euphorbia species occur in the state. Flowering Spurge is easily distinguished by being a relatively tall perennial with mostly oblong leaves that change from alternate (lower on plant) to opposite (higher on plant and in the inflorescence) and relatively large cyathia. Toothed Spurge (Euphorbia dentata) was previously addressed in this series of articles.
Article and photographs by ANPS member Sid Vogelpohl
