Southwestern Europe along the Mediterranean coast (Italy, France, Portugal, and Spain). The first collection of black swallowwort in North America was from Ipswich, Essex County, Massachusetts, in 1854. An 1864 Essex County collector recorded that it was "escaping from the botanic garden where it is a weed and promising to become naturalized." The fifth edition of Gray's Manual of Botany reports black swallowwort to be a weed escaping from gardens in the Cambridge area.
Black swallow-wort is a perennial, twining herbaceous vine in the milkweed family. The leaves are oval shaped with pointed tips; 3 to 4 inches long by 2 to 3 inches wide, and occur in pairs along the stem. The small, five-petaled, star shaped flowers are dark purple to almost black with white hairs, about inch across, and are borne in clusters. The fruits are slender, tapered pods, 2 to 3 inches long by about inch wide, turning from green to light brown as they mature. Plants have rhizomes (underground stems) that sprout new plants and grow in clumps forming extensive patches. Black swallow-wort emerges in spring and flowers from June to July.
When ripe, the fruits open along a seam and release flattened seeds equipped with downy parachutes that aid in wind dispersal. Black swallow-wort can spread long distances by seed and clones arising from root systems. Thick infestations in full sun can produce 2,000 seeds per square meter. The seeds can give rise to multiple plants per seed which greatly increases the likelihood of seed survival and establishment. Wind dispersal of seed begins in late July to early August in open areas and continues throughout late summer and fall. Populations growing under dense wooded canopy may have inadequate resources to produce flowers or seeds. Black swallow-wort dies back to the ground every winter.
Black swallow-wort can form extensive patches that crowd out native vegetation, including native milkweed, goldenrod's, and other field grasses and wildflowers. Monocultures of swallow-wort completely change a field's physical structure. Loss of native plant species reduces biodiversity and can delay or redirect succession, as well as severely limit the value of habitat to wildlife. Monarch butterflies lay eggs on swallow-wort, but have zero percent hatching success. Look-alikes: There are many native species of Cynanchum, including honeyvine (Cynanchum laeve) which could be confused with black swallow-wort. Honeyvine has white flowers, and its leaves have a distinct heart-shaped base.
The seeds of this plant disperse very easily. If you are working in an area that has reproducing black-swallowort, be sure to check your clothes and tools for seeds. Manual cutting and removal of the plant can be an effective method depending on the size of the effected area. Using the 'cut-and-dab' method on thicker stems and vines is also effective. Make sure to bag and kill all vascular plant material. Mowing over stems will not kill the vine, but may break the reproductive cycle. Foliar spray is not recommended as it can be harmful to the surrounding flora and fauna. See the invasive removal page for how to carry out these methods. Any removal within 100 feet of wetland resource areas, including certified vernal pools, or within 200 feet of a perennial stream may require approval from the Concord Natural Resources Commission. Please contact the Division of Natural Resources before you begin.
The following native vines can serve as a good replacement to black swallow-wort in a garden:
- Native honeyvine (Cynanchum laeve)
- Trumpet creeper (Campsis radicans)
- Trumpet honeysuckle (Lonicera sempervirens)
- Dutchman's pipe (Aristolochia macrophylla)
- Virginia creeper (Parthenocissus quinquefolia)
- Native wisteria (Wisteria frutescens)