Dog-strangling Vine (Cyanchum nigrum and C. rossicum)
Dog-strangling vine is the common name for two very similar perennial herbaceous vines that are also know as black swallowwort (Cyanchum nigrum) and pale swallowwort (C. rossicum). Both species are native to Europe and have become invasive in North America since their introduction in the mid 1800s. However, currently only C. rossicum has been confirmed in the Credit River Watershed.
Dog-strangling vine (DSV) is a member of the milkweed family; it typically grows from 1 – 2 metres in height forming dense mats by twining onto trees, other plants and even itself. It prefers sunny, open areas but will also grow in filtered shade. It is tolerant of a wide variety of growing conditions and can be found in forests, thickets, meadows, and roadsides. The leaves are yellowish green to dark green in color, opposite, and oval to oblong in shape. DSV flowers in late June producing small clusters of flowers at the tip of the stems and where the leaves join the stem. Flower colour differs between the two species, with C. nigrum having purple-brown to dark purple flowers and C. rossicum having pink flowers.
The fruit produced is a 4 – 6 cm long pod that splits open in August to release numerous white fluffy seeds (like a smaller common milkweed seed pod). The seeds are wind dispersed and can travel long distances. Reproduction is achieved both by seed dispersal and vegetatively by rhizomes and shoots from the root crown of the parent plant.
DSV is capable of forming dense colonies that outcompete and overwhelm native ground vegetation in a wide variety of habitats. Once established it can quickly spread due to its wind dispersed seeds and aggressive root system.
DSV has also been found to impact monarch butterfly populations. This is because if an adult lays it eggs on DSV, mistaking it for its true host plants (native milkweeds), then the larvae starve because they do not have their necessary food source.
Early detection and removal of DSV is the best means of controlling it. The plant can be dug out of the ground, with careful attention given to ensuring the roots are removed entirely. After a year or more of growth, DSV becomes much harder to control because it will grow an extensive root system that is difficult to remove entirely. Roots left in the ground will often resprout. Another control strategy that can be used is cutting the plant back multiple times a year. This limits its reproductive output, and stops it from seeding, however it will not totally eliminate plants already present.