Invasive dog-strangling vine spreading across Ontario, choking out native plants
Two swallowwort species from Eurasia have infested southern Ontario and threaten monarch butterflies by overrunning milkweed.
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An aggressive invasive species known as dog-strangling vine is spreading rapidly across Ontario, threatening native plants and wildlife. The name refers to two swallowwort species native to Eurasia—black swallowwort and pale swallowwort—that were originally introduced to gardens in the northeastern U.S. in the mid-1800s.
The plants have since spread aggressively across central and southern Ontario, growing up to two metres high and wrapping around neighbouring plants and trees to strangle them. They produce distinctive bean-shaped pods between four and seven centimetres long, which mature between late June and July. From late August through September, the pods split open to release feathery white seeds that spread easily by wind, invading hillsides, fence lines, ravines, stream banks, and plantations.
Dozens of sightings have already been reported across southern Ontario this week and earlier this summer through iNaturalist, a biodiversity social network. According to the provincial government, dog-strangling vines are particularly problematic because they form dense stands that quickly overwhelm native plants. The plant's leaves and roots are toxic to deer and pose a significant threat to monarch butterflies, which can mistake dog-strangling vine for a safe host plant. When they do, the larvae are unable to eat its leaves and starve.
If you suspect dog-strangling vine on your property, look for oval-shaped leaves with a pointed tip measuring seven to 12 centimetres long, growing on opposite sides of the stem. The vines produce pink to dark purple star-shaped flowers with five petals measuring five to nine millimetres.
The province recommends several control measures: dispose of invasive plants in the garbage, stay on trails and keep pets on a leash when hiking, and only purchase native or non-invasive plants from reputable garden suppliers. HighOnCity reported in June on a Toronto effort to fight invasive species through canine detection programs.
The facts
What are the two species that make up dog-strangling vine?
Black swallowwort and pale swallowwort, both native to Eurasia. They were originally introduced to gardens in the northeastern U.S. in the mid-1800s.
How tall does dog-strangling vine grow?
Dog-strangling vine grows up to two metres high, wrapping around neighbouring plants and trees to strangle them.
When do the plant's seed pods split open?
The pods split open from late August through September, releasing feathery white seeds that spread by wind.
Why is dog-strangling vine a threat to monarch butterflies?
Monarch butterflies can mistake dog-strangling vine for milkweed, a safe host plant, but the larvae cannot eat its leaves and starve.
