Speckled Wood, Tircis, Waldbrettspiel, Bont zandoogje (Pararge aegeria) on Swamp Milkweed, Asclépiade rouge, Sumpf-Seidenpflanze, Rode zijdeplant (Asclepias incarnata)
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Swamp milkweed is an upright, 100- to 150-centimeter (39- to 59-inches) tall plant, growing from thick, fleshy, white roots. Typically, its stems are branched and the clump forming plants emerge in late spring after most other plants have begun growth for the year. The oppositely arranged leaves are 7 to 15 centimeters (2.75 to 6 inches) long and are narrow and lance-shaped, with the ends tapering to a sharp point.
The plants bloom in early to mid-summer, producing small, fragrant, pink to mauve (sometimes white) colored flowers in rounded umbels. The flower color may vary from darker shades of purple to soft, pinkish purple and a white flowering form exists as well. The flowers have five reflexed petals and an elevated central crown. After blooming, green seed pods, approximately 12 centimeters (4.5 inches) long, are produced that when ripe, split open. They then release light to dark brown, flat seeds that are attached to silver-white silky-hairs ideal for catching the wind. This natural mechanism for seed dispersal is similar to that used by other milkweed plants.
Swamp milkweed prefers moisture retentive to damp soils in full sun to partial shade and typically, is found growing wild near the edges of ponds, lakes, streams, and low areas—or along ditches. It is one of the best attractors of the Monarch Butterfly, which feeds on the flowers and lays her eggs on the plants. The emerging caterpillars feed on the leaves.
The plants have specialized roots for living in heavy wet soils. The scented, thick, white roots are adapted to live in environments low in oxygen. Blooming occurs in mid to late summer and after blooming; long, relatively thin, rounded, pods are produced that grow uprightly. The pods split open in late summer to late fall, releasing seeds that are attached to silky hairs, which act as parachutes that carry the seeds on the currents of the wind.
This species is cultivated frequently and a number of cultivars are available. They are used especially in gardens designed to attract butterflies. The nectar of the plant attracts many other species of butterflies and insects as well. The plants are also sold as freshly cut flowers, mostly for their long-lasting flower display, but sometimes, for the distinctive seed pods.