Article & photos
by Emily Howell


In Canada, the perennial dog-strangling vine or pale swallow-wort can grow from 1-2 metres in length, in dense colonies that strangle out other trees and plants by creating a monoculture of extremely strong stems. Although it prefers open sunny areas, dog-strangling vine grows very well in filtered shade, often choking out forest under stories.

It also inhabits disturbed areas such as fence lines, wastelands and travel corridors. Dog-strangling vine is typically associated with calcareous soils, but its highly adaptive nature allows it to grow in varying soil types.

This ability to flourish under a vast number of environmental conditions is impacting not only natural areas and species, but also causing problems within the agricultural sector as well.

At the peak of its flowering season (June & July) dog-strangling vine forms an almost woody stem, with opposite leaves that are oval with pointed tips. It is yellow-green to dark green in colour, and produces pink to dark purple star shaped flowers in small clusters at the tips of its stems and leaf axils. It also produces distinctive seed pods that contain hundreds of white air born seeds, similar to native milkweed. From August to October these seeds are released and can blanket an area spreading with the wind.

This prolific production of seeds is the key method of dispersal for dog-strangling vine, but they also use extensive rooting systems to spread over a vast area, even after cutting of above ground shoots.

     In the fall of 2010, Ecosystem Management Technology students at Sir Sandford Fleming College , under the direction of the Victoria Stewardship Council, initiated a study of dog-strangling vine in the Kawarthas region.  Because the Orono Crown Lands are just south of the Kawarthas' and contain substantial populations of dog-strangling vine, they were included in the study.  Data monitoring sites were set up , and information will be collected yearly.  The purpose of the study is to record changes in the populations of this invasive weed species.

If you encounter stakes in the ground, please do not disturb them.  They represent the location of the study site.  To maintain scientific integrity they must remain intact.

*Note

• Dog-strangling vine is referred to with different names and spellings.  The common and botanical names in this article are used by the Ontario Invasive Plant Council. 
  
• The plant has many different species,  three in North America and eighteen or more  in Europe.

• The Genus is alternately referred to as Cynanchum or Vincetoxicum and are considered synonyms. 
  
• In the year 2002 a range of species exist in North America, and molecular evidence indicated they were Vincetoxicum nigrum, V. rossicum, and V. hirundinaria.  The Ontario Invasive Plant Council mentions another species named louiseae.
  

• The common names for Vincetoxicum rossicum or Cynanchum rossicum are dog-strangling vine or pale swallow-wort

References

Ontario Federation of Anglers and Hunters. http://www.ofah.org/

Ontario Invasive Plant Council. http://www.ontarioinvasiveplants.ca/

Ontario Ministry of Natural Resources. http://www.mnr.gov.on.ca/en/

Tewksbury L., Casagrande R., Gassman A., (2002) Biological Control of Invasive Plants in the Eastern USA, Forest Service Publication FHTET 2002. 04, 413P. 16 Swalloworts