Swallow wort

Black & Pale swallow-wort 

Invasive Plant Factsheet: 

Vincetoxicum nigrum; syn. Cynanchum louiseae
Vincetoxicum rossicum; syn. Cynanchum rossicum

By Victoria Wallace, Alyssa Siegel-Miles, and Klaudia Sowizral, UConn Extension

Identifying Features

• OVERVIEW: Perennial, herbaceous climbing vine. 3-9 ft. long, spiraling around self and up other plants in extensive patches. Vines can also trail along the ground; the plant cannot stand upright without support. Laboratory studies have shown allelopathic effects (release of chemicals that suppress neighboring plants’ growth).

• LEAVES: Long, oval, and opposite, with a pointed tip; 3-4 in. long, 2-3 in. wide, with smooth edges; dark, glossy green, drying to bright yellow. Stems are twining; covered in downy hairs, clear sap runs when stem is broken.

• FLOWERS: Flat, star-shaped, with 5 fused petals, forming a short tube at the base. Flowers form in early summer and grow in clusters along the length of the stem. Black swallow-wort has dark purple, triangular petals about 1/8 in. long; pale swallow-wort has slightly longer, dull pink to burgundy petals.

• SEED: 1.5-3 in. long pods; smooth, slender, and resembling milkweed pods in both species. Pods change from green to yellow to brown, then split on one side to release mature windblown seeds in late summer to autumn. Flat, brown seeds are attached to white fluffy hairs, which help the seeds disperse.

• ROOTS: Dense and fibrous root system.

• REPRODUCTION/SPREAD: Capable of self-pollinating, although sexual reproduction is more common. Colonizes new sites by seed; once established, spreads locally via rhizomes. A square-meter stand can produce 1,000-2,000 seeds per year. Plants initially grow slowly and do not reproduce by seed for the first several years.