Tall Green Milkweed

Miss Chen

Description: This perennial wildflower is 1½–3' tall, producing one or more unbranched stems at the base that are erect to ascending. The stems are light green to reddish green and glabrous to densely pubescent. Along each stem, there are numerous alternate leaves that are ascending to widely spreading. Individual leaves are 2-6" long and 1/8-1/2" (3-12 mm.) across; they are linear to linear-lanceolate and smooth along their margins. Their upper surfaces are medium green and glabrous. The leaves often curve upward along their central veins. The petioles of the leaves are about ¼" long and light green. Globoid umbels of greenish white flowers about 1½-2" across develop from the axils of the middle to upper leaves (one umbel per leaf). Each umbel has 30-100 flowers on pedicels about ½-¾" long. The slender pedicels are light green and pubescent. Individual flowers are about 1/8" (3 mm.) across and 1/4" (6 mm.) long. Each flower has 5 sepals, a corolla with 5 narrow lobes, 5 upright hoods without horns, and a short central column containing the reproductive organs. The lobes of the corolla hang downward from the face of the flower (pointing toward the center of the umbel). The lobes are mostly greenish white, although they become purplish toward their tips. The peduncles of the umbels are about 1-1½" long, light green, and pubescent. The blooming period occurs from mid- to late summer and lasts about 2 months. Fertile flowers develop into follicles (seedpods that split open along one side) about 4-5" long and up to ½" across that are narrowly lanceoloid in shape. The surface of each follicle is canescent and smooth. Each follicle contain numerous seeds with tufts of white hair that are released to the wind at maturity. The root system consists of a taproot. This wildflower spreads by reseeding itself.

Cultivation: The preference is full sun, moist to dry conditions, and sandy or gravelly soil. Soil containing loam or clay-loam is also tolerated.

Range & Habitat: Tall Green Milkweed is occasional throughout Illinois. It is a native herbaceous plant. Habitats include dry-mesic railroad prairies, sand prairies, rocky glades, edges of sandy wetlands, roadsides, pastures, and abandoned fields. This milkweed can be found in both high quality habitats and disturbed areas that are sunny.

Faunal Associations: The flowers of Tall Green Milkweed are cross-pollinated primarily by long-tongued bees and wasps. Bee visitors include honeybees, bumblebees, and leaf-cutting bees (Megachile spp.). To a lesser extent, the flowers also attract small butterflies and day-flying moths (e.g., Cisseps fulvicollis). These insects obtain nectar from the flowers. Milkweeds attract a special group of insects that are oligophagous on various parts of these plants. These insect feeders include caterpillars of the butterfly Danaus plexippus (Monarch) and caterpillars of the moths Cycnia inopinatus (Unexpected Cycnia), Cycnia tenera (Delicate Cycnia), and Euchaetes egle (Milkweed Tiger Moth). Some of these moths seem to prefer some Asclepias spp. over others. The larvae of some long-horned beetles bore through the stems and roots of milkweeds, specifically: Tetraopes tetrophthalmus (Red Milkweed Beetle), Tetraopes femoratus (Large Red Milkweed Beetle), and Tetraopes quinquemaculatus. This last species has been found on Tall Green Milkweed. When the follicles of milkweeds are present, the seeds are eaten by Lygaeus kalmii (Small Milkweed Bug) and Oncopeltus fasciatus (Large Milkweed Bug). Occasionally, aphids are found on the upper stems and leaf undersides; these species include Aphis asclepiadis, Myzocallis asclepiadis, Myzocallis punctatus, and Aphis nerii (Introduced Milkweed Aphid). Because the milky latex of the leaves and stems contain cardiac glycosides and possess a bitter flavor, they are usually avoided by mammalian herbivores. The White-Footed Mouse eats the seeds of milkweeds to a minor extent, while the Eastern Goldfinch uses the silky hairs of the seeds in the construction of its nests.

Photographic Location: A prairie in Fayette County, Illinois. The photograph of the flowering plant was taken by Keith & Patty Horn (Copyright © 2009). A caterpillar of the Monarch butterfly is conspicuous in this photo.

Comments: Tall Green Milkweed is one of the lesser known species of milkweed that is occasionally found in upland prairies. It is distinguished primarily by its greenish white umbels of flowers, lack of horns in the hoods of the flowers, and narrow alternate leaves. It also produces more umbels of flowers per plant than most milkweeds. Other milkweeds usually have opposite leaves. A somewhat similar species, Asclepias viridiflora (Short Green Milkweed), is a shorter plant with pairs of opposite leaves. It produces only 1-2 umbels of flowers that nod downward. Another similar species, Asclepias verticillata (Whorled Milkweed), is a smaller plant with whorls of grass-like leaves; these leaves are more narrow (filiform-linear) than those of Tall Green Milkweed.
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