Black swallow-wort (Cynanchum louiseae) and pale swallow-wort (Cynanchum rossicum) - Early Detection Species

Black swallow-wort with twining stems and seedpods.

 

 

Black and pale swallow-wort are two similar vines 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 and pale swallow-wort, but once the caterpillars hatch they are unable to survive on these non-native milkweeds and die. In Minnesota, both black and pale swallow-worts are prohibited noxious weeds on the eradicate list so any plants found must be killed.

 

Description

Appearance

Both black and pale swallow-wort are perennial, herbaceous vines that can grow up to six feet long. They are in the milkweed family and produce 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 and pale 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. Pale swallow-wort has light pink flowers. The flowers of both species 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 and pale swallow-worts have deep, fibrous roots and a thick, short underground rhizome (horizontal stem that can put up new plants).

Biology

Black and pale swallow-worts are perennial vines. They are not woody, so the herbaceous stems die back over the winter and new growth occurs starting in the spring from the surviving roots. Both swallow-worts can have some vegetative spread through short rhizomes putting up new plants, but most spread is from seed. Black and pale swallow-worts produce 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 and pale swallow-wort are native to Europe and were introduced to the United States as ornamental plants. They are widespread in the northeastern United States, but both swallow-worts are early detection species in Minnesota as they have limited distribution in the state.

Refer to EDDMapS distribution maps for current distribution of black swallow-wort and pale swallow-wort.

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 and pale swallow-worts are vines.
  • 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.
  • Honeyvine, Cynanchum laeve (non-native to Minnesota, native to the United States) – Honeyvine has heart-shaped leaves, white flowers, and smooth seed pods. All vining milkweeds should be reported in Minnesota.
 

Regulatory classification

Both black and pale swallow-worts are Minnesota Department of Agriculture Prohibited Noxious Weeds 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 and pale swallow-worts 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 or pale swallow-wort, the butterfly larvae are unable to develop on the non-native milkweed and die. It is unknown how large of impact this has on monarch populations.
  • Black and pale swallow-worts grow aggressively and forms dense patches that can cover other species in forests, grasslands, and savannas. These species release 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

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