Black & Pale Swallow-worts
Black and pale swallow-worts, also known as “dog-strangling vines,” are perennial, herbaceous, twining vines that grow from 2 to 6 1/2 ft in length. Native to Eurasia, these species are adapted to a variety of habitats. Swallow-worts grow rapidly and once established can completely smother native vegetation.
Related to milkweeds, swallow-worts are extremely toxic to livestock and monarch butterfly larvae, which are sometimes fooled into laying their eggs on this plant. Their wind-dispersed seeds can spread over long distances.
Pale swallow-wort prefers limestone-based soils. It is drought tolerant and will thrive in a wide range of soil, moisture, and light conditions with the exception of extremely wet soils.
Swallow-worts aggressively choke out desirable species. They are problematic in Christmas tree plantations, perennial crops, pastures, roadsides, disturbed areas, and natural areas. Turf and annual crops, however, are generally resistant to infestations.
Swallow-worts interfere with forest regeneration. Their wind-dispersed seeds are produced in large numbers and can travel a great distance. Once established, pure stands of pale swallow-wort suppress the establishment of other species. Pale swallow-wort contains substances that are toxic to deer and other grazers so it offers no food value. It also changes the microbial composition of the soil.
Because it is a member of the milkweed family, monarch butterflies sometimes lay their eggs on swallow-wort, but their larvae do not survive.
Habitat: both species 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 to dry soils.
Pale Swallow-wort (Cynanchum rossicum syn. Vincetoxicum rossicum)
Leaves: The 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: 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)
Similar to pale swallow-wort but leaves are dark green, shiny, 3-4″ long and 2-3″ wide. Flowers bloom in June to July and are dark purple. Black swallow-wort is native to Europe and escaped from a botanical garden in Massachusetts.
Prevention and Reporting: 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 to prevent seed dissemination to unaffected areas. Likewise, clean boots, ATVs, and other equipment when coming out of infested areas.
Small patches must be dug out by hand. The entire crown must be removed and destroyed. Hand pulling roots, however, is labor intensive and difficult since the stem base is brittle. To prevent seed dispersal, pods should be removed before they open and then burned.
Large stands can be managed to prevent new seed crops by consistent mowing when pods are very small (early July). Burning plants is not effective and may improve site conditions for seedling establishment. Large stands can also be cultivated on a yearly basis to achieve control over time.
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.
Areas of known infestation
Swallow-worts have been observed in nearly every county of New York, though a distinct epicenter exists in Jefferson County, where immense stands (100 to 500+ acres) of pale swallow-wort can be found on Grenadier Island and Stony Point in the town of Henderson. The plant is also established in the townships of Adams, Antwerp, Brownville, Cape Vincent, Clayton, Ellisburg, Henderson, Lyme, Orleans, and on Fort Drum.
Swallow-wort has slowly progressed outward from this epicenter. Management efforts are focused on preventing the spread toward Tug Hill and the Adirondacks and are therefore focused on Oswego Lewis, and Oneida counties.