Swallowwort, Black

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Identification

Appearance

Cynanchum louiseae is an herbaceous, unbranched, perennial vine which can grow up to 6.5 feet in length.

Foliage

Leaves are opposite, dark green, oval, and shiny with entire margins. Leaves are from 3-4 inches long and 2-3 inches wide. A short petiole attaches the leaf to the vine.

Flowers

Clusters of 6-10 flowers bloom from June to September. Five lobed dark purple corollas are approximately 0.25 inches across and covered with short white hairs on the upper surface.

Fruit

Fruit are pods, similar to milkweed pods, which are slender, 2-3 inches long and split to reveal small seeds with tufts of white hairs. The hairs allow the seeds to be readily dispersed by wind. Plants have rhizomes that sprout new plants.

Check out the downloadable fact sheet above.

Biology

Ecological Threat

Black swallowwort (Vincetoxicum nigrum) can colonize two ways, wind borne seeds which can travel for miles or by rhizomes (underground stems) that sprout into new plant clumps and form extensive patches.

Extensive patches of swallowwort grow over other, often native, vegetation, blocking light and creating tangled thickets.

Since this plant is a member of the milkweed family, Monarch butterflies often lay their eggs on swallowwort seed pods. But swallowwort is poisonous to monarchs and its larvae die either when they feed or by starving to death.

Old field habitats of goldenrod and grasses can be replaced almost exclusively by swallowwort, completely changing their physical structure, possibly impacting nesting birds in the process.

Origin

Black swallowwort is native to Europe and escaped from a botanical garden in Massachusetts.

Habitat

Forests, forest edges, disturbed areas, meadows fields, shoreline, river banks

Life Cycle

Shoots of black swallowwort emerge in spring and the plant flowers in June and July. Flowers remain open for 6 to 8 days and smell similar to rotting fruit. The plant forms seed pods in July and early August, sometimes continuing through October. Although the shoots die to the ground each winter, the plant has a very strong, fibrous central rhizome which helps the plant survive the winter.

The life span of individual plants is somewhat unknown, but some plants have been reported to live more than 70 years. Seeds also remain viable through the winter. A healthy stand of black swallow-wort can produce between 1000-2000 seeds per square meter per year. Abundance of sunlight promotes earlier and more prolific seed production. Black swallow-work primarily reproduces by seed however cut plants can quickly replace the cut shoot from buds on the rhizome.

Management Options

This species is Quarantined: Class B Noxious Weed

Check out the downloadable treatment sheet above.

Mechanical Management

Fruits can be manually removed and carried off-site to prevent seed dispersal, but this practice is time-consuming and must be continued until no more pods are produced and the plants reach the end of the growing season. It is more effective to remove the entire plant by mowing or pulling as it takes the plants a long time to recover and they often cannot do so in time to produce more seeds that season. Mowing is best for preventing seed production. However mowing does present the same rapid re-sprouting problem as manual pulling. Mowing frequently (one to two visits per season) just as the pods are beginning to form is ideal to prevent seed production. Digging up root crowns is more effective than hand pulling alone. The stem tends to break easily above the root crown if pulled while the root crown itself is held tenaciously in place by the fibrous root system and can readily resprout if the stems are cut or broken. If the root crown is pulled up, it must be removed from the site and/or destroyed because broken root crowns tossed on the ground have been observed to re-grow.

BIOCONTROL: Swallow-wort Biocontrols Pass Test

Entomologists Richard Casagrande (University of Rhode Island), is leading a team to help find biocontrol foes to take on swallow-wort, research backed by Northeast IPM Partnership funds.

Click here to learn more about this project.

Chemical Management

Be careful not to damage or kill nearby native plants when conducting management work.
Always read and follow pesticide label directions. Application of pesticides may require a certification from the Agency of Agriculture, Food and Markets. The Agency website provides information on what applicator certification is needed.

Foliar spray treatments are shown to be superior to cut-stem treatments. Herbicide choice for foliar spray treatments will depend on site conditions. In degraded patches with little desirable vegetation, glyphosate may be preferred. Follow up treatments will be required. In situations where spraying is impractical, cut-stem applications with follow up treatments should be effective. Repeated follow-up herbicide treatments are necessary for effective control. These herbicides should be applied when plants are actively growing, after flowering has begun. Only when the plants flower will they be large enough to receive enough spray on the exposed leaf surface to deliver a killing dose to the roots.