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Invasive Species Sheet - Swallow-wort

Invasive Species Identification Sheet

Black Swallow-wort (Cynanchum louiseae Kartesz & Gandhi)
European Swallow-wort (Cynanchum rossicum (Kleo.) Borhidi)

Alternate Latin names: Black = Cynanchum nigrum (L.) Pers., Vincetoxicum nigrum (L.) Moench;
European = Vincetoxicum medium (R. Br.) Decne., V. rossicum (Kleo.) Barb., Cynanchum rossicum Kleo., Antitoxicum rossicum (Kleo.) Pobed.

  • twining, herbaceous, perennial vines; 3'-9' long, spiraling around self and up other plants
  • leaves opposite; base never heart-shaped; color dark, glossy green, drying to bright yellow
  • fruit resembles smooth, slender Milkweed pods 1 1/2"-3" long; surface is never warty
  • pods change from green to yellow to brown then split on one side to release windblown seeds
  • flowers about 1/8" across; flat, star-shaped, light pink to dark purple with yellow centers

Swallow-wort flowers have the petals united to form a short tube at the base of the flower while the outer ends of the petals form a flat, 5-parted star. Petal color and shape distinguish our 2 species of Swallow-worts: Black Swallow-wort = dark purple, triangular (with microscopic hairs); European Swallow-wort = light purple, maroon, or pinkish, longer than broad (and yellow center does not stand out). The 2 invasive Swallow-worts in North America have a confusing history of name changes. European Swallow-wort (also called Pale Swallowwort) sometimes has been lumped with White Swallow-wort (under V. hirundaria Medik. or Cynanchum medium R. Br.). However, White Swallow-wort lacks a twining stem and, although it has escaped from gardens, it has not been found to be invasive in North America.

Black and European Swallow-worts grow in uplands under a wide variety of moisture and light conditions. They readily choke out native plants. If incompletely removed, they sprout from fibrous, spreading root systems (Black) or directly from the base (European). When Milkweed-seeking Monarch Butterflies lay eggs on Black Swallowwort, the larvae do not survive after hatching.