Black swallow-wort (Cynanchum louiseae)
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 and Heather Faubert helped rid a Rhode Island farm of cypress spurge, an invasive weed, in the late 1990s. The spurge is a pretty thing but a thug nonetheless, and poisonous to cattle. Their weapon: a cadre of hungry beetles, biocontrol agents so keyed into spurge they won’t eat anything else. “Then,” says Casagrande (Univ. of Rhode Island), “along came swallow-wort.” Now Casagrande is leading a team to help find biocontrol foes to take on swallow-wort, research backed by Northeast IPM Partnership funds.
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. At sites with desirable grasses that should be conserved, triclopyr ester would be the herbicide of choice. Follow up treatments will be required. Insituations 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.
Black swallowwort is an herbaceous, twinning, perennial vine. Leaves are opposite, dark green, oval, shiny, entire, 3-4 in. (7.6-10.2 cm) long and 2-3 in. (5.1-7.6 cm) wide. Flowering occurs in June to July, when dark purple, 5-petaled, star-shaped flowers appear in clusters. Flowers are approximately 0.25 in. (0.6 cm) across and covered with white hairs. Fruit are pods, similar to milkweed pods, which are slender, 2-3 in. (5.1-7.6 cm) long and split to reveal small seeds with tufts of white hair. The hair allows the seeds to be readily dispersed. Plants have rhizomes that sprout new plants. Black swallowwort invades upland areas with a wide range of light and moisture conditions. (www.invasive.org)
Shoots of black swallow-wort 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, fiberous 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.
Dispersal of black swallow-wort is primarily by wind, which carries and disperses the parachute-shaped seeds.
Class B Noxious Weed
Photos: (c) L. Mehrhoff University of Connecticut
- Black swallow-wort (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 swallow-wort 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 swallow-wort seed pods . But swallow-wort 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 swallow-wort, completely changing their physical structure, possibly impacting nesting birds in the process.