Know Your Natives – Butterfly Weed

Butterfly Weed (Asclepias tuberosa) of the Dogbane (Apocynaceae) family has clear sap (not milky) and showy orange (reddish orange to uncommonly yellow) flowers. The genus name honors the Greek god of medicine, Asklepios. The specific epithet is Latin for “swollen,” a reference to the roots. The species is widespread and common from eastern Texas to Minnesota, east to the Gulf and Atlantic coasts. Disjunct populations occur in the Four-Corner States. In Arkansas, plants grow statewide. Other common names include Butterfly Milkweed, Orange Milkweed, and Pleurisy Root (historically used to treat pleurisy). Preferred habitats are open areas with dry to mesic, well-drained soils: rocky uplands and slopes, open woodlands, prairies, fields, and rights-of-way.

Photo 1: Butterfly Weed has clear sap and nectar that attracts swallowtails and other butterflies. This is the summer form of Zebra Swallowtail (Eurytides marcellus).

Butterfly Weed typically occurs as an erect, perennial herb, 1-2½ feet tall, from a large, tough, knobby crown. Descending taproots may be 1-1½ feet long. Around the first of April, one or more densely pubescent stems sprout directly from the rootstock. With sufficient, late-season soil moisture, spring-like shoots sprout from both the rootstock and mature stems. Plants survive droughts well, but as soils dry out, stems may straggle or die to the rootstock. At the end of the growth year, stems quickly disintegrate.

Photo 2: These mid-April stems, with prominent pubescence, are most likely from a single rootstock. Old stems, as seen, disintegrate over winter.
Photo 3: The erect, pale green spring-time stems are not branched. In early May, most stems of this plant terminate with developing inflorescences.
Photo 4: After a dry period, this plant (may be two plants) ceased growth; some stems died and surviving stems became scraggly (one bearing seed pods). With improved soil moisture, new stems sprouted from the rootstock, but died again with further drying of soil.

Leaves are alternate, oblong-lanceolate to lanceolate, sessile or short-petiolate, 3½ inches long and 1 inch wide. At first crowded together, leaves become well-spaced as stems elongate, gradually decreasing in size upwards. Leaf pubescence is dense on both surfaces, that of the lower surface longer, especially on the midvein––the lower surface feels fuzzy. Margins are narrowly revolute.

Photo 5: Alternate leaves are oblong-lanceolate to lanceolate with narrowly revolute margins. Caterpillar is Unexpected Tiger Moth (Cycnia inopinatus).
Photo 6: Leaves have pubescent surfaces––the lower surface more so. The sessile to short-petiolate leaves have rounded, truncate, or cordate bases. Venation is weakly pinnate. Lower leaf of inset is 1⅞ inches long and ½ inch wide.

Plants produce terminal clusters of flowers in spring and, often, again in summer after rainfall. Clusters consist of multiple umbels that are from 1-2½ inches wide with 8-25 closely spaced flowers per umbel. A loose calyx with 5 light green lobes encloses the flower bud. As buds enlarge, the green color transitions to orange. Within a flower cluster (2-6 inches across), together with a central umbel, several diverging, spreading, floral branches bear a few to a half-dozen additional umbels, in straight-line sequence. Central umbels and innermost umbels on floral branches bloom first, with all flowers of an umbel blooming at the same time.

Photo 7: In uppermost flower cluster, flower bud development of the central umbel (topmost) is more advanced than that of the umbels immediately below––the emerging corollas have grown beyond the tips of the calyx lobes. Monarch caterpillars prefer to forage on the freshest growth.
Photo 8: This terminal inflorescence has a central umbel and two floral branches with the umbels on the branches in a straight-line sequence. All flowers of an umbel bloom at the same time.

Umbels are rounded in outline (as seen from above) and nearly flat (as seen from the side). Peduncles (½–2½ inches long) grow directly from floral branches, often opposite a leaf. Pedicels are about 1 inch long. Peduncles, pedicels and calyx lobes are pubescent, with hairs of peduncles longer and those of the calyx lobes on the exterior only.

Photo 9: The sturdy, pubescent peduncles grow directly from the stem, often opposite a leaf. Straight, slender, less hairy pedicels, attach at the tip of the peduncle to form the umbels.
Photo 10: This dense inflorescence combines that of several, closely spaced stems. Umbel at upper center has begun to fade. Leaves darken with age.

Flowers in bloom consist of a calyx with 5 strongly reflexed lobes hidden by a corolla with 5 strongly reflexed lobes––a perianth typical of many angiosperm flowers. The distinctive structure of the complex milkweed flower is the corona or crown, here comprising 5 erect hoods, each with an exserted, curved horn. An additional morphological anomaly of the milkweed flower is a central column or anther head formed by the fusion of the 5 anthers to the style tip. The petal-like hoods (to ¼ inch long) extend from the base of a tube formed by the fusion of the staminal filaments. The hoods hold the nectar attractive to pollinating insects, in this species, mostly butterflies and bumblebees. A longitudinal section of a flower reveals yet a third unusual structure: the pistil is compound, comprising two carpels, however, they are fused together only above the ovaries to form a single style. Each ovary is separate from its partner.

The adaptive significance of these floral novelties is fascinating. Pollen grains are not granular but cemented together into packets called pollinia––a departure from the norm found only in the milkweed subfamily of the dogbanes and in the unrelated orchid family. There are two pollinia in each anther. As the flower matures, pollinia of adjacent anther-halves become connected by threadlike translator arms to a small secretion of the anther head called a corpusculum or gland. A corpusculum is positioned directly above each of 5 slits––vertical openings that form between the 5 adjacent anthers. When a pollinating insect collects nectar, a leg may inadvertently enter a slit and become snagged by the notched base of the corpusculum. As the insect flies away with an attached corpusculum, the adjacent pair of translator arms with their pollinia is pulled out. The unique contraption––one corpusculum, 2 translator arms, and 2 pollinia––is called a translator. As the insect feeds on other flowers, the translator arms dry and rotate the pollinia, orienting them for a perfect fit into the stigmatic slit of another flower. If a pollinium is then accidentally inserted into a slit, the pollen grains within the pollinium germinate, produce pollen tubes that grow down the style into the ovary, and fertilization of more than 100 eggs is consummated. The stigmatic slits provide both a site where insects can most easily snag a corpusculum as well as an opening into the stigmatic surface of the style, where the pollinium can effect pollination.

Photo 11: Petal-like hoods, forming the corona, are appendages of the staminal filaments, and the exserted horns are appendages of the hoods. Tiny black spots (one at white arrow) between the hoods are the corpusculums that sit above the stigmatic slits.
Photo 12: With corolla and crown hoods removed (#1 and #6, respectively), the following structures can be seen: the anthers (#3) and anther wings (#2) that form the stigmatic slits, stigma head (#4), and stigmatic slit with corpusculum (#5) directly above it.

Typically, only one flower of an umbel develops fruit and only one of the two ovaries of a fertilized flower matures. Fruits are spindle-shaped, single-suture pods (follicles) that stand erect on descending stalks (pedicels). They are smoothly short-pubescent, to 6 inches long and 3/4 inch wide, with seeds along a placenta attached at the suture. When mature and dry, the light tan, papery pods split along the suture from tip to base so that the placenta becomes free-standing. The ⅛-inch, flat, ovate, brown seeds are stacked in the lower part of the pod with long silky, white, apical hairs (1½ inches long) extending toward the narrowing pod tip. As pods gradually open wide on sunny days, breezes tug out the hairs with attached seeds for short- to long-distance dispersal. (Dispersed seeds may not be viable due to predation of ovules by milkweed bugs or dry soil conditions.)

Photo 13: Pods are double-walled when green and feel spongy. Stalks of the pubescent pods are twisted below the somewhat persistent calyx. This pod is 3¼ inches long.
Photo 14: In early October, these Monarch caterpillars eat the freshest leaves. The erect pods are thickened along their lower portion where developing seeds are located.
Photo 15: Large Milkweed Bugs (Oncopeltus fasciatus), both nymphs and adults, feed destructively on developing seeds by inserting their proboscis through the pod wall (see adult on right).
Photo 16: Dry follicles split along their one suture so that the placenta becomes free-standing (see upper pod). Pod on left did not develop fully.

The showy Butterfly Milkweed is a “must have” for well-drained soils of natural areas and most gardens, especially if soil moisture is consistent. The hardy species has outstanding aesthetic appeal with its erect stems and prominent flowers and pods. A nectar source for hummingbirds, swallowtails, fritillaries and the Monarch. Although the sap is considered toxic, the plant hosts caterpillars of the Monarch (Danaus plexippus) and the Unexpected Tiger Moth (Cycnia inopinatus). Ovules within green pods are consumed, from the outside, by adults and nymphs of the Large Milkweed Bugs (Oncopeltus fasciatus) and Small Milkweed Bugs (Lygaeus kalmii). Aphid infestations can stop fresh growth. With drying soils and summer heat, stems become ragged or die back to the ground, but renewed fresh growth may occur with improved moisture. Dry, empty pods can last for years in dry arrangements.

Photo 17: In early June, these Butterfly Weeds blend with other native species in a garden setting including Dittany (Cunila origanoides), Hairy Blazing Star (Liatris hirsuta) and Rattleweed (Astragalus canadensis).

Butterfly Weed, one of 14 species of Asclepias that occur in Arkansas, is readily distinguished by its orange flowers, lack of milky sap, alternate leaves, and pubescence. Four species have been featured previously in this series of articles: Four-Leaved Milkweed (A. quadrifolia), White or Red-ring Milkweed (A. variegata), Whorled Milkweed (A. verticillata), and Green Milkweed (A. viridiflora).

Article and photographs by ANPS member Sid Vogelpohl

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