Beware: The Dog-Strangling Vines!

Contact: Kristine Averill, Cornell University |
Toni DiTommaso, Cornell University |

Beware: The Dog-Strangling Vines!
Pale swallow-wort in bloom.
Photo: Kristine Averill, Cornell University

ANNAPOLIS, MD (August 5, 2009) – Vincetoxicum nigrum and V. rossicum (black and pale swallow-wort or dog-strangling vines) are two vines causing concern for natural and disturbed areas in Maryland. The swallow-worts, cousins of the milkweeds, threaten agricultural lands, terrestrial ecosystems, and native plant and animal species. Their roots likely contain vincetoxin, a chemical compound that is considered poisonous to mammals, including humans. They are aptly referred to as dog-strangling vines because they twine around themselves and other vegetation, creating dense mats that are difficult to traverse. These plants are hard to control with current methods, so finding and identifying them quickly is important. We know that black swallow-wort infestations just over the Pennsylvania state line are now forming fruit. Because of their potential harm to ecosystem integrity and the likelihood of finding them in Maryland now, the swallow-worts have been chosen as MISC’s August Invader of the Month.

The two swallow-worts are distinct species, but they share many similarities, including that they are equally invasive. The vines can grow up to 2 m in a single growing season. They have opposite hairless leaves. The plants produce pink to purple flowers from June through September. From August to October, each stem produces many 2-3 inch long pods with about 8-15 seeds in each. The pods are green when immature and yellowish brown when they are ripe. The swallow-worts produce as many as 400 viable seeds per stem. Dense populations can have over 100 stems per square meter, so they can produce upwards of 30,000 seeds! The species reproduce and invade new sites by these seeds. Seeds have silky, parachute-like appendages that aid in long-distance wind dispersal. In September, badly infested areas look as though they have gotten a very early snowstorm. About half of the seeds are polyembryonic, meaning one seed can give rise to multiple seedlings (up to six), which probably helps them establish new populations. They are herbaceous perennials, so they rejuvenate vegetatively from year-to-year with below-ground rootstocks and thick, spaghetti-like roots. Each individual can likely survive for hundreds of years.

Swallow-worts invade a range of habitats and have the capacity to dominate native vegetation. A variety of areas are affected, from sunny areas (for example, old-fields and roadsides), to forest edge habitats, and even heavily shaded forest understories. They are increasingly problematic in old fields, pastures, Christmas tree plantations, forest understories, and along roadsides. Pale swallow-wort invasion threatens the globally-rare alvar communities (shallow limestone barrens), which cover over 10,000 acres in the Lake Ontario region (U.S. and Canada). Since alvars are sensitive ecosystems with periodic flooding and drought, management activitiesin these and other sensitive areas should be undertaken cautiously, as they contain rare species. Pale swallow-wort’s current North American distribution extends from the Atlantic coast to southern Michigan and northern Indiana, and from southern Ontario, Canada, to southern Pennsylvania. Black swallow-wort has a similar distribution but occurs as far west as Missouri and Kansas. They are both present in most northeastern US states and the Canadian Provinces of Ontario and Quebec. Both species are banned or prohibited in CT, MA, and NH.

Beware: The Dog-Strangling Vines!
Pale Swallow-wort  Photo: Kristine Averill, Cornell University

Pale swallow-wort is native to Ukraine and southwestern Russia, where it grows in stony meadow habitats and rock outcrops. Black swallow-wort is native to France, Spain, and northern Italy. The swallow-worts were introduced into North America late in the 1800’s, probably as ornamentals, which subsequently escaped. In the past 30 years, these plants have greatly increased in abundance as agricultural fields have been deserted and since populations are now rapidly expanding. Before field abandonment, trampling by farm animals may have suppressed these species. Some evidence suggests that dense swallow-wort populations discourage grassland birds from nesting in the summer and prevent predation of small rodents who may take refuge under the thick mass of dead vines in the winter. Monarch butterflies sometimes mistakenly lay eggs on the swallow-worts because they are similar to the butterflies’ primary food source, common milkweed. However, butterfly larvae end up dying on the swallow-worts. Research has also shown that pale swallow-wort can modify fungal communities, which could alter forest regeneration and succession.

Physical, mechanical, and chemical management of the swallow-worts has been challenging and may be difficult or uneconomical for long-term control. Pulling is time-consuming and destructive to other plants and animals. Furthermore, the brittle stems break very easily from the root crown and removing the below-ground biomass is difficult. Mowing is a good temporary control, but plants resprout and can still produce seeds if mowed too early in the season. Although a combination of several management strategies should be utilized as part of an ecological weed management program for these species, few techniques offer long-term suppression, while also being economically and environmentally feasible. With hopes of providing longer-term control, the USDA-ARS began a classical biological control program for these species in 2004 (see links below). However, the best way to control the swallow-worts and not get tripped up is to be on the look-out for new introductions to prevent population establishment in the first place.

For more information, see:

Black Swallow-Wort. Plant Conservation Alliance, Alien Plant Working Group

Vincetoxicum nigrum.

Vincetoxicum rossicum.

Biological Control of Invasive Plants, USDA ARS

For more information about this and other Invasive Species of Concern, visit