Cynanchum louiseae (Black Swallow-wort)

Plant Info
Also known as: Black Dog-strangling Vine
Family:Apocynaceae (Dogbane)
Life cycle:perennial
  • Invasive - ERADICATE!
  • Prohibited or Restricted species
Habitat:part shade, sun; garden escapee; disturbed soil, waste areas, roadsides, woods, thickets
Bloom season:June - August
Plant height:2 to 6 feet
Wetland Indicator Status:none
MN county distribution (click map to enlarge):Minnesota county distribution map
National distribution (click map to enlarge):National distribution map

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Detailed Information

Flower: Flower shape: 5-petals Cluster type: panicle

[photo of flowers] Loose, branching clusters of up to 10 stalked, star-shaped flowers arising from leaf axils. Flowers are about ¼ inch across, dark purple with 5 fleshy petals that are variously covered in minute white hairs on the upper surface. The pale yellow crown in the center is surrounded by a dark purple collar.

Leaves and stems: Leaf attachment: opposite Leaf type: simple

[photo of leaves] Leaves are opposite, 2 to 5 inches long, ½ to 3 inches wide, narrowly to broadly egg-shaped, tapering to a sharply pointed tip, rounded to somewhat heart-shaped at the base, toothless, hairless, shiny, dark green, on short stalks.

[photo of minutely hairy, twining stem] Stems are multiple from the base, unbranched, minutely hairy, lack tendrils, erect when young, later twining around itself and other vegetation for support, creating dense thickets.

Fruit: Fruit type: capsule/pod

[photo of fruit] Fruit is a slender, spindle-shaped pod about 2 inches long. Inside are numerous brown seeds each with a tuft of white hairs to carry it off in the wind.


Related to milkweeds and dogbanes and known as Cynanchum louiseae or Vincetoxicum nigrum, Black Swallow-wort was first introduced in New England or eastern Canada by European settlers in the 1800s. A far more serious pest plant to our east, Black Swallow-wort is on the “eradicate” noxious weed list for Minnesota. In 2014, only 2 known populations were known to exist in the state and both were reportedly actively managed. Since then, several more sites have been located; the most current reports can be found on EDDMapS. Perhaps we actually will eradicate it, though the population on the University of MN St Paul campus has been around since at least 1975 and, as far as we know, still persists.

The root crown is a dense clump of thick rhizomes that allow it to spread vegetatively, and after cutting it resprouts vigorously from buds on the underground crown. Digging out the root crown can be an effective control method, as are certain herbicides. Besides its alleopathic properties that inhibit the growth of native plants, Black Swallow-wort has another claim to fame: as a “fatal” host plant for Monarch butterflies. Monarch larvae that feed on Swallow-worts have a higher mortality rate than those feeding on native milkweeds. Bad plant!

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More photos

Photos by Peter M. Dziuk taken at the University of Minnesota St. Paul campus.


Have you seen this plant in Minnesota, or have any other comments about it?

Posted by: T Young - My yard - Standish neighborhood Minneapolis
on: 2017-05-27 11:21:47

I believe this plant has invaded my yard. A year or so ago, first spotted in front, growing with some kind of small shrub. Last year found more plants along side of house and growing under neighbor's juniper. This year, even more have popped up. Unfortunately after trying to just repeatedly pull out plants, read this method will likely have it spread further. I also think I saw this plant at another house in Powderhorn neighborhood. Looked like it was planted in place rather than emerging on its own.

Posted by: K. Chayka
on: 2017-05-27 19:11:20

T Young, you should contact the MN Dept of Agriculture weed line about this (see email addr and phone# at the bottom of their page). They should be able to help with eradication.

Posted by: Kelly D - St. Paul and south Minneapolis
on: 2017-06-16 14:46:47

Found in St. Paul (and destroyed) and south Minneapolis private fence growing as a weed. I did report to the department of AG. Plants are flowering right now; do they really continue flowering all season?

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