Black swallow-wort (Vincetoxicum nigrum)

Synonyms: Cynanchum louiseae, Cynanchum nigrum, Asclepias nigra

Common Names: black dog-strangling vine, Louise's swallow-wort, climbing milkweed

Description:

Black swallow-wort (Vincetoxicum nigrum) is an invasive twining vine in the milkweed family, native to southwestern Europe and introduced to the northeastern United States, parts of the Midwest, and southeastern Canada [1][3]. It twists over other plants into dense tangles that smother native vegetation [1]. MISIN tracks it because it forms thick patches that crowd out native plants, lowers grassland-bird use, and harms monarch butterflies ? which sometimes lay eggs on this milkweed relative even though their caterpillars cannot survive on it [1].

Habit:

Black swallow-wort is a slender perennial vine that twines around itself and other plants, climbing 3 to 6 feet and forming dense tangles [1]. Its leaves grow in opposite pairs and are dark green, shiny, and smooth, oval with a pointed tip, about 2 to 5 inches long [1]. The flowers are the clearest clue: small, dark purple to nearly black, star-shaped with five pointed petals covered in fine white hairs, in small clusters, blooming in early summer (about June to July) [1]. By late summer it forms slender, milkweed-like pods that split open to release seeds with fluffy white hairs that blow away on the wind [1].

It is easiest to spot in early summer when the dark star-shaped flowers are open, and again in late summer when the slender pods stand out before they burst [1]. Look for it twining over old fields, fence rows, roadsides, and woodland edges, often in dense patches [1].

At-a-glance cues:

1. Twining vine (no tendrils) that wraps around itself and other plants into dense tangles [1]
2. Opposite, shiny, dark green oval leaves with pointed tips, about 2?5 inches long [1]
3. Small, dark purple to near-black, five-petaled star-shaped flowers with fine white hairs, in early summer [1]
4. Slender milkweed-like pods that release wind-borne seeds with white fluff in late summer [1]

Leaves:

1. Arrangement: opposite (in pairs) [1]
2. Shape: simple, oval to egg-shaped, tapering to a sharp point, rounded base, on short stalks [1]
3. Margin: smooth (toothless) [1]
4. Surface: dark green, shiny, hairless [1]
5. Size: about 2?5 inches (5?13 cm) long [1]

Stems:

1. Habit: herbaceous perennial twining vine, climbing about 3?6 feet (1?2 m); no tendrils ? it twists around itself and nearby plants for support [1]
2. Stems: several arising from the base, mostly unbranched, finely hairy [1]
3. Roots: forms dense, knobby masses of root crowns/rhizomes that sprout new stems and resprout after cutting [1]

Flowers:

1. Color: dark purple to nearly black [1]
2. Shape: small, star-shaped, with five triangular pointed petals covered in fine white hairs; flowers have both male and female parts (perfect) [1]
3. Arrangement: in small branched clusters of about 6?10 flowers in the leaf axils [1]
4. Bloom time: early summer, about June to July [1]

Fruit and seeds:

1. Fruit: slender, tapered, milkweed-like pods (follicles) about 2?3 inches long, ripening from green to light brown in late summer [1]
2. Seeds: many flat brown seeds per pod, each with a tuft of white hairs (pappus) [1]
3. Dispersal: seeds blow long distances on the wind; the plant also spreads locally from its rhizome crowns [1]
4. Timing: pods form late July?August and split open in late summer to early fall [1]

Habitat:

Black swallow-wort grows in a wide range of sunny to partly shaded places. It is common in old fields, pastures, fence rows, roadsides, and woodland edges and understories, and it also invades Christmas tree plantations and perennial crop fields [1]. It tolerates many soils and both open and shaded ground, which lets it spread from disturbed sites into natural areas [1].

The vine is native to southwestern Europe (the Mediterranean region ? Italy, France, Spain, and Portugal) [3]. In North America it has become invasive across the northeastern United States, parts of the Midwest, and southeastern Canada [3]. As a milkweed relative, it is also largely avoided by deer and livestock, which gives it an edge over more palatable native plants [1].

Reproduction:

Black swallow-wort spreads in two ways homeowners should know. First, by wind: its slender pods release many seeds, each carried on fluffy white hairs that can blow well beyond the parent patch, starting new infestations along fences and field edges [1]. Second, from its roots: the plant builds dense, knobby masses of root crowns that sprout new stems, so a patch thickens and spreads outward and resprouts after the tops are cut or pulled [1].

The vine flowers in early summer and ripens pods from late summer into fall [1]. Two timing rules make control work: clip and bag the pods before the seeds ripen and blow away, and dig out the whole root crown rather than just pulling the tops, since broken crowns resprout (see Management) [1].

Impact and Damage:

Black swallow-wort is mostly a problem for natural areas and land, not a direct danger to people. It twines over and smothers other plants, forming dense single-species patches that crowd out native wildflowers and grasses and can interfere with forest regeneration [1]. As patches expand, grassland birds use the area far less [1].

Its most talked-about harm is to monarch butterflies. Black swallow-wort is in the milkweed family, and monarchs ? which normally lay eggs on native milkweeds ? sometimes lay eggs on this vine instead, but their caterpillars cannot survive on it, so the eggs are wasted [1]. By both luring egg-laying and crowding out the native milkweeds monarchs depend on, heavy infestations are shown to reduce monarch numbers [1]. It is also avoided by deer and livestock and can be a costly weed in Christmas tree and perennial crop fields [1].

Similar species:

A few twining or milkweed-family plants can be mistaken for black swallow-wort.

Pale swallow-wort (Vincetoxicum rossicum) is its close cousin and nearly identical twining vine, but its star-shaped flowers are pink to maroon or reddish-brown rather than the dark purple-to-black of black swallow-wort.

Honeyvine milkweed (Cynanchum laeve) is another twining milkweed-family vine, but it has heart-shaped leaves and white flowers, unlike black swallow-wort's oval leaves and dark flowers.

Native milkweeds (Asclepias spp.) share the milkweed-like pods and wind-borne fluffy seeds, but they grow as upright, non-twining plants with milky sap and broad pink, orange, or white flower clusters, not a dark-flowered climbing vine.

Monitoring and rapid response:

Signs to watch for:

1. A vine with opposite, shiny, dark green pointed leaves twining over other plants into dense tangles [1]
2. Small, dark purple to near-black star-shaped flowers in early summer [1]
3. Slender milkweed-like pods in late summer that split to release wind-borne fluffy seeds [1]

What you can do:

1. Dig out the whole root crown of small patches with a spade ? pulling or cutting only the tops leaves the knobby roots, which resprout; this is easiest in spring or early summer in moist soil [1].
2. Clip and bag the pods before the seeds ripen in mid-to-late summer to stop wind-blown spread ? put them in the trash, not the compost [1].
3. Don't plant black swallow-wort, and check fence rows and field edges downwind of any patch for new seedlings [1].
4. Natural area crews can dig and clip pods across larger patches over several seasons, prioritize outlying plants and seed sources, and keep monitoring, since the root crowns and seed bank persist [1].
5. Report sightings (below) to help track where this vine is spreading [1].

When to call a professional:

1. Large or well-established patches with extensive root-crown masses that keep resprouting, where digging can't keep up ? a land manager or licensed applicator can plan a multi-season approach [1].
2. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Wisconsin Department of Natural Resources. Black Swallow-wort (Vincetoxicum nigrum). dnr.wisconsin.gov
2. Michigan Department of Natural Resources, Invasive Species Program. Black Swallow-wort (Vincetoxicum nigrum). michigan.gov/invasives
3. USDA NRCS PLANTS Database. Vincetoxicum nigrum (L.) Moench (black swallow-wort). plants.usda.gov

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