Invasive 'dog strangling vine' is spreading and choking out native Ontario plants

An aggressive, invasive species that's known to “strangle” native plants and small trees is spreading across Ontario, and despite its name, it doesn't actually pose a threat to your four-legged friends.

Dog-strangling vine refers to two invasive plants that are native to Eurasia: the black swallowwort and pale swallowwort. The plants closely resemble other members of the milkweed family and were first introduced to gardens in the northeastern U.S. in the mid-1800s.

Since then, the plants have spread aggressively across central and southern Ontario, thanks to their ability to grow up to two metres high and wrap themselves around neighbouring plants and trees, effectively “strangling” them.

These plants prefer open and sunny areas, producing bean-shaped pods that are between four and seven centimetres long between late June and July. From late August through September, these mature pods then split open to release feathery white seeds.

Dozens of sightings in southern Ontario have already been reported this week and earlier this summer through the biodiversity social network, iNaturalist.

Sighting reported in Kendal, Ont., this week. Photo: erica_barkley/iNaturalist.

Unfortunately, these seeds are easily spread by the wind and can easily invade hillsides, fence lines, ravines, stream banks, and plantations of pine trees and natural forests.

According to the provincial government's website, dog-strangling vines are particularly problematic due to their ability to form dense stands that quickly overwhelm native plants. The plant's leaves and roots can also be toxic to deer and threaten other species, including the monarch butterfly, which can mistake dog-strangling vine for a safe host plant.

The aggressive plant, which smothers native milkweed plants, starves out larvae, as they are unable to eat its leaves.

Sighting reported in Mississauga last week. Photo: songyy/iNaturalist.

If you suspect that there's dog-strangling vine on your property, look for oval-shaped leaves with a pointed tip that are between seven and 12 centimetres long and grow on opposite sides of the stem. The vines also produce flowers that are pink to dark purple and star-shaped, with five petals anywhere from five to nine millimetres long.

The province notes several ways to control the spread of dog-strangling vine, such as disposing of invasive plants in the garbage, staying on trails and keeping pets on a leash when hiking, and only buying native or non-invasive plants from reputable garden suppliers.