Black and pale swallow-worts (Cynanchum louiseae, Cynanchum rossicum ), also known as “dog-strangling vines,” are perennial, herbaceous vines that grow from 2 to 6 1/2 ft in length.
Origin/Introduction into U.S.:
Black swallow-wort is native to Europe near the Mediterranean Sea, while pale swallow-wort is native to Ukraine and parts of Russia. Although swallow-worts’ weren’t highly valued as a horticultural specimen, they escaped cultivation in the 1800s’.
- Swallow-worts aggressively choke out desirable species. They are problematic in Christmas tree plantations, perennial crop fields, pastures, roadsides, disturbed areas, and natural areas.
- Pure stands of swallow-wort suppress the establishment of other species and interfere with forest regeneration.
- Related to milkweeds, swallow-worts are toxic to livestock, deer and monarch butterfly larvae, which are sometimes fooled into laying their eggs on these plants, but their larvae do not survive.
Habitat: Both species of swallow-worts can be found in mixed hardwood forest to heavily shaded woods. They also can be found in disturbed sunny areas, prairies, savannahs, open fields, and along roadsides in moist or dry soils.
Pale Swallow-wort (Cynanchum rossicum syn. Vincetoxicum rossicum)
Leaves: Leaves are opposite in arrangement, oval to wedge-shaped with pointed tips. Generally, the leaves are 2.5″ to 4.5″ long and 2″ to 2.75″ wide, glossy and medium green in color. In summer, the leaves begin to display a warm, yellow color.
Flowers: The star-shaped flowers are small and fleshy, with 5 pink to reddish colored petals. They are borne in loose clusters and are visible in late May through mid-July.
Fruit/Seeds: The fruit is a smooth, slender, pointed pod that looks much like a milkweed pod. The pods are light green in color and are frequently borne in pairs. They are abundant during July and August. Like milkweed, the pods open in late summer, disseminating large numbers of downy seeds. Can produce 2,000 seeds per square yard.
Black swallow-wort (Cynanchum louiseae syn. Vincetoxicum nigrum)
Characteristics are similar to pale swallow-wort, but black swallow-wort leaves are dark green, shiny, and larger in size ranging from 3-4″ long and 2-3″ wide. Flowers are dark purple/blackish in color, and bloom in June to July.
Prevention: Once established, swallow-wort is difficult to control. Monitor for populations in late summer, when the plants turn golden yellow and pods are present. Initial control efforts should concentrate on plants in sunny areas since they will produce the most seeds. Stay out of infested areas during seed dispersal to prevent seed dissemination to unaffected areas. Likewise, clean boots, ATVs, and other equipment when coming out of infested areas.
Physical/Mechanical Control: Small patches can be dug out by hand. The entire plant must be removed and destroyed. To prevent seed dispersal, pods should be removed before they open and then burned.
Large stands can be managed by consistent mowing when pods are very small (May-July). Controlled burning is not effective and may improve site conditions for seedling establishment.
Chemical Control: Triclopyr or glyphosate can be applied to foliage around mid-September. Use of a surfactant helps herbicides penetrate the waxy leaf coating. Cut-stem treatment with glyphosate is effective but labor intensive. Use of systemic “Round Up” herbicide is also effective in removing swallow-wort.
For More Information:
Center for Invasive Species and Ecosystem Health http://www.invasive.org/browse/subinfo.cfm?sub=3398
Michigan Department of Natural Resources Michigan Natural Features Inventoryhttps://mnfi.anr.msu.edu/invasive-species/Swallow-wortBCP.pdf
Title photo and second flower photo of pale swallow-wort: The Michagan Nature Guys Blog, http://www.michigannatureguy.com/blog/tag/.
Title photo of pale swallow-wort flower: Early Detection and Distribution Mapping System, http://www.eddmaps.org/AT/distribution/point.cfm?id=2279104.Title photo of black swallow-wort flower: Kansas Department of Agriculture, http://agriculture.ks.gov/images/default-source/pp-priority-pest-list-weeds/black-swallow-wort-flowers.jpg?sfvrsn=0.
Bottom black swallow-wort photo :Les Mehrhoff, http://www.discoverlife.org/mp/20psee=I_LJM18444&res=mx&flags=no_slide_show:glean:.
Flower photo: Rob Routledge, Sault College, Bugwood.org.
Fruit/Seeds photo: John M. Randall, The Nature Conservancy, Bugwood.org.