Black swallow-wort (Cynanchum louiseae) - Early Detection Species

Black swallow-wort with twining stems and seedpods.

 
 

Black swallow-wort is a vine in the milkweed family that can densely cover other plant species. Minnesota has no native milkweeds that are vines. Monarch butterflies will lay their eggs on black swallow-wort, but once the caterpillars hatch they are unable to survive on black swallow-wort and die. In Minnesota, black swallow-wort is a prohibited noxious weed on the eradicate list so any plants found must be killed.

Description

Appearance

Black swallow-wort is a perennial, herbaceous vine that can grow up to six feet long. Black swallow-wort is in the milkweed family and produces the characteristic milkweed seedpods and seeds.

Leaves and stem

This plant's leaves come off the stem opposite one another in pairs of two. Leaves have smooth edges, can be glossy, vary in shape, and are light to dark green.

Black swallow-wort vines twine around other plants and their stems can grow up to six feet long. The vines have no tendrils (thread-like plant parts that can form curls) at the tips, they simply wrap around what they encounter. Stems are not woody and die back over the winter and new stems grow the next year.

Flowers

Black swallow-wort flowers are purple with a yellow center and are very small (1/8 inch across). Flowers have five petals with small hairs on the petals. The flowers are arranged in clusters hanging from where the leaf meets the stem and bloom from late May to mid-July.

Seeds

Seeds are in milkweed seedpods that are 1.5 - 3 inches long. Seedpods open in late summer and early fall and release seeds on silky threads or filaments that are spread by wind.

Roots and rhizomes

Black swallow-wort has deep, fibrous roots and a thick, short underground rhizome (horizontal stem that can put up new plants).

Biology

Black swallow-wort is a perennial vine. It is not woody, so the herbaceous stems die back over the winter and new growth occurs starting in the spring from the surviving roots. Black swallow-wort can have some vegetative spread through short rhizomes putting up new plants, but most spread is from seed. Black swallow-wort produces seedpods that release seeds with a fluffy tuft that helps the seeds disperse by wind. Plants can grow in areas of full sun to partial shade.

Origin and spread

Black swallow-wort is native to Europe and was introduced to the United States as an ornamental plant. It is widespread in the northeastern United States, but is an early detection species in Minnesota as it has limited distribution in the state.

Refer to EDDMapS distribution maps for current distribution.

Don't be fooled by these look-alikes

  • Swamp milkweed, Asclepias incarnata var. incarnata (native), common milkweed, Asclepias syriaca (native), and other native milkweeds – Minnesota's native milkweeds are not vines, they are upright plants. Black swallow-wort is a vine.
  • Pale swallow-wort, Cynanchum (Vincetoxicum) rossicum (invasive) – Pale swallow-wort has light pink flowers while black swallow-wort has dark purple flowers. Pale swallow-wort has limited known populations in Minnesota and should be reported if found.
  • Wild cucumber, Echinocystis lobata (native) and bur cucumber, Sicyos angulatus (native) – Wild cucumber and bur cucumber are native vines that can be very noticeable and abundant in some years. They have bright, light green leaves that grab people's attention. They have star shaped leaves, white flowers, and spiny fruits.
  • Virgin's bower, Clematis virginiana (native) – Virgin's bower has toothed leaflets and white flowers.
 
Regulatory classification

Black swallow-wort is a Minnesota Department of Agriculture Prohibited Noxious Weed on the Eradicate List meaning that the above and below ground parts of the plant must be destroyed. Additionally no transportation, propagation, or sale is allowed.

Threat to Minnesota
  • Black swallow-wort may have negative impacts on monarch butterflies by displacing the native milkweeds upon which monarchs' depend. Additionally, swallow-wort may serve as a fatal host to monarch larvae. When monarch butterflies lay eggs on black swallow-wort, the butterfly larvae are unable to develop on black swallow-wort and die. It is unknown how large of impact this has on monarch populations.
  • Black swallow-wort grows aggressively and forms dense patches that can cover other species in forests, grasslands, and savannas. Black swallow-wort releases allelopathic chemicals that may reduce growth of nearby plants and alter soils.
  • In the eastern United States there have been reports of economic impacts to the forestry and horticulture industries due to the difficulty of getting news trees to establish in sites with black swallow-wort.
What you should do

One way that invasive plant seeds and fragments can spread is in soil. Sometimes plants are planted purposefully. You can prevent the spread of invasive plants.

PlayCleanGo: Stop Invasive Species in Your Tracks

  • REMOVE plants, animals and mud from boots, gear, pets and vehicles.
  • CLEAN your gear before entering and leaving the recreation site.
  • STAY on designated roads and trails.
  • PLANT non-invasive species.
Native substitutes

Learn about additional native milkweeds (Minnesota Board of Water and Soil Resources)

 
Control methods

Mechanical control can be done by digging out the plant and making sure to remove the root crown. The Minnesota Department of Agriculture provides information on plant disposal. Mowing is unlikely to impact plants.

Herbicide control can be done using systemic herbicides which are taken up by plants and move within the plant, which can kill leaves, stems, and roots. Herbicides with the active ingredient glyphosate or triclopyr ester are likely to be effective. For the most effective control, herbicide application should be timed to be applied when the plant is flowering or when the plant has finished flowering but seedpods are not mature.

Reporting

Report new occurrences by submitting a report through EDDMapS, emailing Report a Pest, calling Report a Pest (1-888-545-6684), or contacting your local county agricultural inspector.

Resources