Black and pale swallow-wort were introduced in the US from Europe in the 1800s. These European swallow-worts are highly invasive; they form dense monocultures in shade or full sun. Swallow-worts also produce compounds with anti-bacterial and anti-fungal properties, and inhibit the growth of many pathogens. The roots contain haemolytic glycosides, which are toxic to mammals, including livestock. Swallow-worts are also distasteful to many insect larvae and toxic to some, including monarch butterfy caterpillars. It is not clear how often female monarch butterflies lay eggs on swallow-wort, but when they do, the caterpillars die. In general, insect diversity and abundance is significantly lower in dense stands, with cascading effects on the entire food chain.
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Swallow-wort vines will grow over larger plants, blocking out sunlight and strangling plants. Photo: Leslie J. Mehrhoff, University of Connecticut, Bugwood.org
Identification
  • Twining vine up to 6 ft. long.
  • May climb adjacent plants.
  • Pepper-like seed pods split open in mid-summer.
  • Dark green leaves with a waxy coating.
  • Flowers give off slight rotting odor.
  • Bloom in June and July.
  • Black swallow-wort: flowers are small, dark purple, and have 5 petals covered in fine light hairs with petals that are as wide as they are long. 
  • Pale swallow-wort: flowers are pink to dark burgundy. The narrow petals are longer than they are wide.
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Twining vines strangle trees and block out undergrowth. Photo: John M. Randall, The Nature Conservancy, Bugwood.org
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Black Swallow-wort. Photo: Rob Routeldge, Sault College, Bugwood.org
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Pale Swallow-wort. Photo: Rob Routledge, Sault College, Bugwood.org
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Seed pods are often confused for milkweed by pollinating and egg laying insects, such as the monarch butterfly. Caterpillars hatched on European swallow-worts will not survive. Photo: Leslie J. Mehrhoff, Bugwood.org
Habitat: Black swallow-wort vines thrive in both shade and sun and are found in disturbed areas along roadsides, pastures, old fields and gardens as well as alvar and along fens.

Additional Information - 
Invasive Swallow-wort Best Control Practices
A Homeowner's Guide to Swallow-wort Control
MISIN Species Training Module - Black swallow-wort
MISIN Species Training Module - Pale swallow-wort
Banner Credit: Leslie J. Mehrhoff University of Connecticut, Bugwood.org