Cynanchum louiseae (Black Swallow-wort)
|Also known as:||Black Dog-strangling Vine|
|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):|
|National distribution (click map to enlarge):|
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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:
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.
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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Where to buy native seed and plants ↓
- Black Swallow-wort vine
- Black Swallow-wort vine
- dense thicket of Black Swallow-wort
- early growth
- root crown, rhizomes and buds of new stems
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?