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Black Swallow-wort Vincetoxicum nigrum – the Monarch Menace

by Rebecca Chizzo

If you looked out over the wetland border and onto the Sudbury River in August of 2012, you would have seen a few determined souls cutting and bagging Black swallow-wort pods. It was a hot month, and most people were busy with vacation activities, but some took time to join a local effort to correct the environmental wrong caused by the non-native milkweed relative. Volunteers dug and bagged several large root crowns until it became obvious that without more help only seed pods could be collected. They then stuffed thirty large feed bags with pods containing many thousands of winged seeds that would have taken flight to new locations.

Swallowwart2.240This summer, the Black swallow-wort collection and eradication begins again, and many community organizers throughout Massachusetts have sprung up to confront Black Swallow-wort. They all need help. The group working along the Sudbury River, SWEET, Inc. (Sudbury Weed Education and Eradication Team), is a small group of residents and a few support volunteers from Sudbury and surrounding communities. The group started as a grassroots association and is now a non-profit organization recognized by the state of Massachusetts pending final 501C3 status. The group’s work sites include historical and conservation lands, parks, and a large wooded property at the local high school. The Sudbury location with the unwelcome Black swallow-wort is near four important conservation sites and sports a fabulous view of the iconic Sudbury River.

Plant Characteristics

Black swallow-wort is a perennial twining vine that is related to milkweed and native to Spain and Portugal. Black swallow-wort is also called, “Louis” and was classified as Cynanchum Louiseae; however, careful analysis showed distinct differences between Black swallow-wort and the Cynanchum species. Research still yields information under both of these scientific names.

Black swallow-wort develops vines from dense root crowns that grow three to four feet in height or in length on the ground. According to a University of Maine extension Invasive Plant bulletin, leaves of Black swallow-wort are opposite, lanceolate (shaped like a lance head ) to heart shaped. These have smooth margins (edges), range from two to five inches, and come to fine points. Leaves may appear shiny and dark. Small, five-petaled, star-shaped, purple flowers develop in early to mid-summer. The centers are bright yellow. Pale swallow-wort, an equally invasive related species, has mauve-colored flowers. In July through August, slender smooth pods develop and contain winged seeds. In August to September, pods dry, then crack. Seeds take flight on “feathered parachutes” destined for new places. The feathered seeds land on cars, deer, or even people and travel further.

Swallowwart3.350Found in wetland vegetation at the back of properties near the Sudbury River, Black swallow-wort also found along chain-link fences, at farms, and in home gardens. Black swallow-wort vines happily snake their way around poles and fencing or form twisted shapes by curling back around themselves. They are highly adaptable and thrive in most environments. Once established, they spread and smother other plants.

Control Measures

Luckily, there is hope in the battle against Black swallow-wort: several moth species have been found to eat invasive swallow-wort. According to a publication by The College of the Environment and Life Sciences, Swallow-wort biocontrol project draws attention, Dr. Richard Casagrande, URI entomologist, and his team conducted research to find a bug capable of eating the noxious species, but not interested in our native plants. Studies show that two species of European moth defoliate the plants in lab tests. Field tests will determine the true value of the moths. Under controlled conditions, they were shown to pose no threat to North American plant species and were potentially effective for control of swallow-wort infestations.

Gardeners should patrol their properties for Black swallow-wort. The species is toxic if ingested by humans, equines, or canines. Monarch Butterflies mistake the invasive plant for common milkweed, a native of MA. When Monarchs lay eggs on swallow-wort, the larvae do not survive because they are not evolved to ingest or use the toxin in either of the non-native swallow-wort plants. So they threaten one of America’s best known pollinators. According to the USDA Natural Resources Conservation Plants Database, Black swallowwort has been found recently in twenty-one states. Less than a decade ago, sources gave a distribution of only a handful of states.

Some people may not understand why invasive species should be of concern. “Why worry? It’s only in one neighborhood….” Not true. SWEET, Inc. has been made aware of plants found in Concord, Lincoln, Marlboro, Brookline, West Cambridge, and Watertown. They appear in local landscapes, restaurant and retail store lots, and parks. Being able to identify and remove individuals before they seed and spread is an important tool in the effort to stop the species.

A Fortuitous Discovery

In 2009, a student at Lincoln Sudbury High School helping in a landscape bedding with the founder of SWEET found an interesting plant and asked if it was invasive. He thought it looked exotic, and it was. Because that student was observant and said something, Black Swallow-wort plants were identified and confirmed. In the large concrete-edged bedding, SWEET found a plant tag for Mediterranean Curry. It appeared that invasive materials unintentionally hitch a ride in pot soil. To avoid this situation, environmental organizations recommend “bare rooted” installation of plants. But that strategy was not yet utilized when the high school bed was planted.

swallowwort1.350Because Black swallow-wort seeds are long-lived and plants regenerate from root pieces, the entire plant bed was topped with heavy covers. To keep the covers in place in the windy location, heavy mulches were added. The bedding sat empty for three years. With some hesitation, SWEET began replanting the bed in Spring 2013 with native flowers attractive to butterflies by installing through cuts in the covers. The invasive plants could develop again and find their way out. Such projects require repeated monitoring in order to be successful. Two stray plants continue to survive in cracks in the concrete surrounding the bed. This is a rugged species. Groups must be more determined than the Black swallow-wort plants to get rid of them.

Continuing Efforts

SWEET, Inc. is permitted to use manual means to suppress seeds. Hand digging the entire root crown out can eliminate an individual plant, but if it has seeded, further work to deal with offspring is required. Weed barriers and mowing can be effective to stop seed pods from developing. Cutting and collecting pods further reduces spread, but the work has to be timed correctly to prevent seeds from escaping. Outreach is just as important. It’s up to everyone to share identification and control information with others.

Consider helping to remove Black swallow-wort at the Sudbury River border area on August 17 9:30-12:30 or August 21 2:30-4:30. Students from several area high schools can earn community service credit by working with SWEET, Inc. on this project. The group is in constant need of help for outreach, removal, and replacement efforts for this and other projects. Contact the group at or www.Facebook/

About the Author

Rebecca Chizzo founded and organized SWEET and actively works to control invasive species in Sudbury, MA, while simultaneously educating residents about the threat of invasives. SWEET seeks volunteers with different perspectives and skill sets to help with education and public outreach efforts. To offer help, contact Rebecca at (978)505-1301 or email the group at