Swallow-wort is an aggressive, invasive plant. While some inquire about its potential benefits, its significant ecological concerns and documented negative impacts are the primary focus. This article provides an overview of swallow-wort, its nature, and any historical claims of benefit.
Understanding Swallow-wort and Its Invasive Nature
Swallow-wort primarily refers to two highly invasive species: black swallow-wort (Vincetoxicum nigrum or Cynanchum louiseae) and pale swallow-wort (Vincetoxicum rossicum or Cynanchum rossicum). Also known as dog-strangling vine due to their vining growth, both belong to the genus Vincetoxicum within the dogbane family, Apocynaceae.
Black swallow-wort originates from the western Mediterranean region, while pale swallow-wort is native to Ukraine and southeastern European Russia. These perennial, herbaceous climbing vines were intentionally brought to North America in the 1800s, likely for ornamental purposes or as botanical curiosities. However, they quickly escaped cultivation due to their highly effective spread mechanisms.
Swallow-wort propagates aggressively through prolific seed production, with a single square meter stand generating 1,000 to 2,000 seeds annually. These seeds have fluffy, parachute-like hairs, enabling wide wind dispersal. They are also polyembryonic, meaning each seed can contain multiple embryos, increasing establishment success. Plants also spread locally through extensive root systems and rhizomes, forming dense mats that quickly dominate areas.
Investigating Claimed Benefits
Historically, some Vincetoxicum genus plants, including swallow-wort, were attributed medicinal properties, though these claims lack modern scientific validation. The name Vincetoxicum translates to “poison-beater,” reflecting a past belief in its antidotal effects against snakebite. Traditional folk medicine also used these plants as emetics or for presumed diuretic, depurative, and sudorific qualities. These historical applications were often based on anecdotal evidence and occurred without a full understanding of the plant’s toxicity.
Any perceived ecological benefits in its native habitat are highly localized and do not translate to non-native environments. While swallow-wort may host certain insects in its original range, this interaction does not outweigh its negative impacts when introduced elsewhere. Widely recognized and safe benefits of swallow-wort are extremely limited or non-existent, especially considering its ecological harm. The plant’s toxic compounds make it unsuitable for most uses and highlight the dangers of relying on unproven historical claims.
Major Detrimental Impacts
Swallow-wort poses significant threats to ecosystems by outcompeting native vegetation and reducing biodiversity. Its aggressive vining growth smothers other plants, forming dense monocultures where little else survives. This displaces diverse plant communities, including rare habitats like alvar ecosystems. The plant also exhibits allelopathy, releasing chemicals from its roots that suppress neighboring plant growth, further hindering native species.
The impact on wildlife is severe, especially for monarch butterflies. Despite its similarity to native milkweeds, swallow-wort is a “false host” plant; monarch larvae cannot survive on it after eggs are laid. This mistaken host selection contributes to monarch population declines, particularly as swallow-wort invades native milkweed habitats. The plant is also toxic to livestock and herbivores like deer, which avoid feeding on it, exacerbating its spread as other palatable plants are consumed.
Human health risks are a concern, as the plant contains poisonous alkaloids. While direct ingestion is rare, contact can cause irritation. Swallow-wort disrupts the natural food chain and ecosystem balance, impacting insects, large animals, and bird populations that rely on diverse plant communities. Its rapid spread and ability to thrive in various conditions make it a serious environmental threat.
Identification and Control
Identifying swallow-wort is crucial for effective management. Both black and pale swallow-wort are perennial vines with twining stems up to 6 feet long. Their leaves are typically dark, glossy green, opposite, oval to lance-shaped with pointed tips, measuring 3 to 4 inches long by 2 to 3 inches wide. Broken stems may exude a milky white sap.
Small, star-shaped flowers appear in clusters from June to September. Black swallow-wort produces dark purple or brownish flowers, while pale swallow-wort flowers range from pink to reddish-brown or magenta. After flowering, slender, milkweed-like seed pods (1.5 to 3 inches long) develop, turning from green to brown before splitting open in late summer or fall. These pods release numerous flat, brown seeds, each with silky hairs for wind dispersal.
Controlling swallow-wort requires persistent effort due to its robust root system and prolific seeding. For small infestations, manual removal is possible, but digging up the entire root crown and associated rhizomes is essential to prevent resprouting. Simply pulling the plant often leads to regrowth. Proper disposal of removed plants, especially those with seed pods, is necessary; bagging and sending them to a landfill or burning them prevents further seed dispersal.
Mowing alone is insufficient to eradicate swallow-wort, as plants regrow from roots. However, timely mowing, especially when immature seed pods form in early to mid-July, can prevent seed production and reduce spread. For larger infestations, chemical treatments using systemic herbicides like glyphosate or triclopyr are effective when applied after flowering, allowing absorption into the root system. Repeated applications may be necessary.
Currently, no widely adopted biological controls exist. After removal, replanting with native groundcover species is recommended to reduce re-infestation.