Black Swallow-wort (Cynanchum louiseae): Identification & Characteristics

Black Swallow-wort (Vincetoxicum nigrum / Cynanchum louiseae)

Family: Apocynaceae (Dogbane)

Growth habit: Perennial, twining herbaceous vine, 1–2 m tall; stems smooth, green to purplish, often glaucous (waxy).
Leaves: Opposite, simple, lanceolate-ovate to narrowly ovate, 5–12 cm long, entire margins, acute tips, short petioles.
Flowers: Small (3–5 mm), dark purple to black, 5 reflexed petals fused at base, 5 tiny sepals; borne in axillary cymes (clusters) June–September.
Fruit: Paired follicles (slender pods), 4–6 cm, smooth, green → brown; split longitudinally to release numerous flattened seeds with long, silky white hairs (wind-dispersed).
Roots: Rhizomatous, forming dense clonal mats.

Native: Europe, Asia.
Introduced to North America ~1850s as ornamental.
Now invasive in 35+ U.S. states (Northeast, Midwest, Mid-Atlantic) and southern Canada.
Thrives in forest edges, fields, roadsides, disturbed soils, alvars, limestone bluffs; tolerates sun to deep shade.

Forms dense monocultures, outcompeting native flora for light, water, nutrients.
Reduces biodiversity; degrades habitat for monarch butterflies (larvae die on leaves—oviposition sink).
Toxic cardiac glycosides deter herbivores; no effective natural enemies in NA.
Alters soil microbial communities.

Early detection & rapid response – pull seedlings before seed set.
Manual removal – dig entire root crown; fragments resprout. Bag & landfill (do not compost).
Repeated cutting/mowing – depletes root reserves; must be frequent (every 2–3 wks) for years.
Herbicide – Triclopyr or glyphosate (cut-stump or foliar) per label; aquatic-safe near water.
Biological control – Hypena opulenta (moth) and Chrysomelid beetle released in some states; long-term hope.
Monitoring – revisit sites 3–5 yrs; seed bank persists.

Persistence is key—eradication takes years of integrated effort.