(Vincetoxicum nigrum; syn. Cynanchum louiseae)
Also called Dog-strangling Vine, Climbing Milkweed
Black swallow-wort and pale swallow-wort look similar, though differ in flower color and shape. Swallow-wort vines twine 3 to 6 feet high, often smothering nearby vegetation. Leaves -- 2 to 5 inches long -- are opposite, toothless, oval-shaped with pointed tips, and dark green with smooth, shiny surfaces. Flowers of black swallow-wort are dark purple and each of the five pointed petals are triangular -- about as long as wide -- and finely pubescent with downy white hairs. Pale swallow-wort flowers are maroon to pale pink, and each petal is at least twice as long as wide and lacks hairs. Slender, tapering seed pods - 1.5 to 3 inches long -- resemble those of other milkweeds. Ripe seeds are wind dispersed on silky filaments.
HABITS & HABITAT
Black swallow-wort occurs at a few sites in southern Wisconsin. Pale swallow-wort, while not yet documented in the state, is reputed to be a more aggressive invader forming impenetrable, tangled thickets. Both prefer upland habitats such as forests, woodland edges, old fields, fencerows, and roadsides. They can persist in sun or shade.
Eradication is difficult once a colony is established because plants form a dense, knobby mass of underground roots. Initial control efforts should concentrate on plants in sunny areas since they produce the most seeds. All pods should be removed before they open, and then burned or landfilled to prevent seed release. Triclopyr or glyphosate with a surfactant can be applied to foliage during the growing season. Cut-stem treatment with glyphosate is also effective but labor intensive. If plants are hand-dug, all roots must be removed. Monitor for populations in late summer when plants turn golden yellow and pods become prominent.