The University of Wisconsin-Extension has issued a bulletin announcing the latest threat to area gardens and native flora. The name of this threat, according to the press release, is "Black Swallow-wort, a twining vine with dark purple star-shaped flowers." It has been spotted at several locations near Fort Atkinson, and is an invasive species that is aggressive. Its vines, the notice continues, "can completely take over upland sites, strangling native trees and shrubs and cascading over flowers and grasses, blocking out the sunlight."
Black swallow-wort is native to France, Italy, Portugal and Spain, according to an exhaustive fact sheet published by the National Park Service. It describes the basic structure of the plant:
The leaves are oval shaped with pointed tips, 3-4 in. long by 2-3 in. wide, and occur in pairs along the stem. The small five-petaled star shaped flowers are dark purple to almost black with white hairs, about 1/4 in. across, and are borne in clusters. The fruits are slender tapered pods, 2 to 3 in. long by about 1/4 in. wide, turning from green to light brown as they mature. Plants have rhizomes (underground stems) that sprout new plants and grow in clumps of several to many stems, forming extensive patches.
The complete fact sheet makes for scary reading.
Its melodramatic colloquial name, dog-strangling vine, exaggerates the extent of its malevolence, but not by a lot. Documented in Rock, Walworth and Waukesha counties (as well as in California, Connecticut, New Hampshire and New York), it has the potential to spread into Dane, Dodge and Jefferson counties. A member of the milkweed family, its seeds are broadcast on the wind -- and each seed can generate three plants.
"Eradication is difficult once a colony is established," the UW Extension release continues, "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. Triclopyror glyphosate with a surfactant can be applied to foliage during the growing season. Cut-stem treatment with glyphosate is also effective but labor intensive." Black swallow-wort has been linked to declines in grassland bird and Monarch butterfly populations, the bulletin adds.
This bulletin goes on to quote a landscaping professional who notes, "Thick infestations in full sun can produce 2,000 seeds per square meter," adding that people can help stem the spread by pulling the plants, sealing them in black plastic garbage bags, labeling the contents "Invasive Plant" and sending them to the landfill. Burning the plants is an alternative.
UW-Extension's Jefferson County office directs interested parties to a fact sheet on the plant, and instructs them to consult its invasive plants field guide for instructions on how to report any black swallow-wort plants they may encounter.