Know Your Natives – Green Milkweed

Green milkweed (Asclepias viridiflora) of the Dogbane (Apocynaceae) family, formerly of the Milkweed (Asclepiadaceae) family, is one of 14 Asclepias species found in Arkansas. It occurs across the U.S. except for six western states and five northeastern states. In Arkansas, it occurs throughout much of the state, except for lowlands of the Mississippi Alluvial Plain and the West Gulf Coastal Plain. The genus name is based on the Greek god of medicine (Asklepios, a.k.a. Asclepius). The specific epithet is from Latin for “green flowered”. Other common names include green comet milkweed, green-flowered milkweed, and short green milkweed (in comparison with “tall green milkweed”, Asclepias hirtella). Habitat preference is mostly sunny, mesic to dry conditions found in upland rocky glades and hilly prairies where it is not overshadowed by competition.

Green milkweed, typically found as isolated occurrences, is an herbaceous perennial with a straight taproot and one or two typically unbranched stems that reach a height of 2 or 3 feet. Stems, mostly erect, are a light to medium green, but may have purple shading. They are round in cross-section and densely covered with short, matted pubescence (tomentose). With sufficient moisture, apical stem growth continues well into mid-summer and short lateral branching may occur. Plants produce a sticky white sap like many (but not all) of the other milkweeds.

Leaves in lower portion of plants occur in opposite, decussate pairs, while upper leaves may be paired or alternate. Leaf spacing is fairly uniform along the entire stem, with separation being 1 to 2 inches. Leaves are short pubescent on upper and lower surfaces, 4 to 5 inches long and ¾ to 1½ inches wide (often larger leaves in wetter sites), and lanceolate to broadly lanceolate with tapering bases and apexes. The blade tends to be bowed-up on the edges and the entire margins irregularly wavy to undulating. Leaf blades, on short petioles (⅛ inch), extend horizontally at maturity. The margins have the same pubescence as the blade surface. Upper midribs are depressed; lower, light green midribs are expressed. Venation is off-set pinnate with secondary veins extending from midrib to near leaf margins where they connect into a single vein that follows the margin. With summer heat, venation becomes less distinct and leaves feel rough and leathery.

Green milkweed - Asclepias viridifloraPhoto 1: Lower leaves are in decussate pairs while later leaves may be in pairs or alternate. Photo May 30.

Inflorescence, in June into July, consists of umbels slightly below and between leaf pairs or opposite a single leaf (one umbel per leaf pair or per leaf). Depending on site conditions and weather, several to a dozen umbels may occur. Umbels have short, stubby peduncles that divide into many short, slender pedicels. Pubescence of stems extends onto pedicels. Umbels, with a convex “front” side, a flattened base and a round circumference (when viewed from the front), are down-bent (pendulous) and immobile. Umbels, with 20 to 80 tightly spaced flowers, are 1 to 2 inches wide. Pedicels are subtended by a pair of lanceolate bracts to ⅜ inch long.

Green milkweed - Asclepias viridifloraPhoto 2: Umbels attach to the stem between a pair of leaves or, with alternate leaves, opposite the leaf. Inset shows a plant with reddish umbels. Photo Jun 19.

Flowers of green milkweed at anthesis, ½ inch long and ¼ inch wide, are typically light green to light yellowish green, but may be reddish. Flowers each have five, ⅛-inch, narrowly triangular and strongly reflexed calyx lobes and five, ¼-inch, lanceolate, cupped and strongly reflexed corolla lobes that surround an upright corona. The corona surrounds a central flat-topped column that bears both 5 stigmatic surfaces and 10 pollinia (pollen packets). Pollinia occur in divergent pairs at the ends of threadlike “translator arms” joined to a “clip” that is positioned above each of 5 slits in the central column. The column is surrounded by 5 nectar-bearing, oblong hoods attached at its base. Access to each slit is located between each pair of hoods. When a bee collects nectar, its leg can slip through a slit to the clip which then snags onto the leg. As two pollinia are attached to each clip, upon visiting other flowers, the bee’s leg may insert a pollinium into a slit, where it comes into contact with a receptive stigmatic surface, breaks away from its translator arm, and effects pollination. Flowers have two prong-like, greenish white pistils, the tips of which fuse to the anthers to form the central column. Unlike most milkweed species, green milkweed corona hoods do not have horns.

Green milkweed - Asclepias viridifloraPhoto 3: A divided flower showing two bracts (long ones on left side), five calyx lobes (scattered), five petals (two upper and three lower in display), two pistils attached to pubescent pedicel, and a corona/column that has been cut for an interior and exterior view (upper right). Exterior view shows a clip (the tiny dark structure) above a minute line, the slit.

Green milkweed - Asclepias viridifloraPhoto 4: Umbels are stoutly peduncled, immobile and down-bent. Reflexed petals hide sepals while exterior bracts remain visible. Several pollinia are attached to the tips of the bee’s legs.

Green milkweed produces smooth, round (in cross-section) pods that are constricted and round-pointed at both ends and covered with short pubescence. Pods are 3 to 5 inches long with a width of ½ to ¾ inch. Numerous flat, round, brown seeds are attached shingle-style, in the lower portion of the pod, to a central rib-like placenta. Seeds bear long white hairs tightly pressed together in the upper portion of the pod. When pods become brown and dry, they split along one side (follicles) and hairs fluff-up in the breezes and individual seeds are pulled free, each with a buoyant tuft of long hair.

Green milkweed - Asclepias viridifloraPhoto 5: While pods are ascending, the pod’s stem remains down-bent. A monarch caterpillar hides below an upper leaf. Upper leaves of this plant are alternate. Photo August 27.

Green milkweed may be mostly unnoticed in a well-drained, sunny garden setting. However, this non-aggressive species would add interest when mixed with other more visible milkweed species and is a host plant for monarch butterflies and a nectar plant for bumblebees.

Green milkweed - Asclepias viridifloraPhoto 6: Green milkweed (Asclepias viridiflora) alongside butterfly milkweed (Asclepias tuberosa subsp. interior) in a garden setting .

Thirteen other species of the genus occur in Arkansas. Of these, six species can have greenish flowers (though some of these may be whitish, yellowish, or pinkish instead or in addition to greenish), namely green milkweed or spider milkweed (Asclepias viridis), tall green milkweed or prairie milkweed (Asclepias hirtella), curly milkweed or blunt-leaf milkweed (Asclepias amplexicaulis), whorled milkweed (Asclepias verticillata), savanna milkweed (Asclepias obovata), and narrow-leaf milkweed (Asclepias stenophylla). Green milkweed (Asclepias viridiflora) can be distinguished from all other Arkansas milkweeds except A. hirtella by its multiple lateral umbels and corona hoods without horns.

Article and photographs by ANPS member Sid Vogelpohl

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