Dog Strangling Vine – Cynanchum rossicum (formerly Vincetoxicum rossicum)

What is it?

Dog strangling vine, also known as European swallowwort, pale swallowwort or just swallowwort is a perennial non-woody flowering plant, native to southeastern Europe and Asia, and introduced to North America in the 1800s for gardening but which has since become invasive.

Contrary to its menacing name, this vine like plant cannot strangle man’s best friend but its rapid spread across southern Ontario and parts of the US, makes it a threat to local biodiversity.

Why you should care

Dog strangling vine can quickly grow and settle in new areas with tough, thick roots via seeds carried by the wind. Once established, the dog-strangling vine can easily regrow from even small fragments of its root system making it difficult to remove from infestations.

Although it prefers open sunny areas, dog strangling vine is also common in areas disturbed by humans such as old fields, railways, and a broad variety of urban environments, and will aggressively wrap around native plants and trees to form dense mats of vegetation in the underbrush, literally ‘strangling’ their access to sunlight and suppressing other plants from growing.

The result is a reduction in habitat and food available to birds and larger animals. Dog strangling vine also poses a threat to insects, being avoided by pollinators, and is often mistaken for native milkweeds by the already endangered monarch butterfly. Since the monarch butterfly needs to lay its eggs on native milkweed plants for its larvae to survive, by being tricked into leaving its eggs on a look-alike, the spread of dog strangling vine could further reduce the population of these beloved butterflies.

What can we do?

Early detection is key! – if we can remove instances of dog strangling vine before they become too well established, dog strangling vines can be much more easily managed!

How to identify dog strangling vine

dog-strangling vine | As lovely as it is invasive. I've neve… | Flickr — Photo by Steve Begin @flickr (CC)

Dog strangling vine can be confused for many other plants, including the black dog strangling vine (Vincetoxicum nigrum), which is within the same genus and visually similar but has darker coloured flowers.

It may also be confused for native milkweeds like common milkweed (Asclepias syriaca) which can be distinguished by its warty seeds, butterfly milkweed which has orange flowers and alternate leaves, and swamp milkweed. However, note that all native milkweeds are erect plants that do not twine like true dog-strangling vine.

Similarly, dog strangling vine may also be confused for dogbane (Apocynum cannabinum), but which has erect reddish stems, droopy leaves, and white flowers.

To identify a true specimen of dog strangling vine, look out for these characteristics:

Height

Dog strangling vine can grow up to heights of about 0.6 – 2m

Stem

Herbaceous (non-woody) – will twine around surrounding objects, climbing around trees, and even among themselves to form dense vegetated mats

Leaves

Green, opposite, smooth, and wavey margins, ovoid with a pointed tip
Leaves are rounder at the base, largest (7 – 12cm) at the middle of the vine, and smaller (5 – 7cm) and narrower at the top

Flowers

Pointed ovoid buds with twisted petals pre-blooming
Occur at the vine tips
5 – 7mm individually
Five-fold symmetry
Usually red-brown, maroon, or pink though the undersides may be a lighter pink, orange, or yellow
Blooms in late June and July
Emerge in stalked clusters of 5 – 20 flowers with

Fruit

long pointed chili- like seed pod – 4 – 7cm long and 0.5cm wide
contain milky sap
green when un-ripened
pale brown when ripe – will split open to release multiple fluffy seeds with coma (tufts of feathery hair) – seeds are easily distributed by wind contributing to their invasive status

Can you remove dog strangling vine?

Yes! Just like any other plant, if it is found on your property in small quantities, dog strangling vine plants can be physically pruned away to reduce its reproductive potential and ability to spread, but only digging it up or killing it, roots and all, will permanently remove plants already present. When digging it up, avoid tilling the soil excessively to prevent fragmenting and spreading the dog strangling vine’s roots.

Please see the Landowner’s Guide to Controlling Invasive Woodland Plants or the Toronto Botanical Garden’s guide for more detailed instructions on how to physically remove instances of dog strangling vine, as well as other guides here.

Chemical control via pesticides can also be effective on a larger scale but different herbicides can have different levels of effectiveness and more often just allows for other non-native species to move in. While you can buy the right pesticides from licensed vendors (see here for further details), it may be better to check with your local conservancy for volunteers they may need to help out!

After any attempts at removing dog strangling vine, remember to check your clothing, any pets you may have, vehicles, and other equipment to ensure that you are not inadvertently spreading the seeds of this invader. Dispose of the remains in the garbage rather than the compost or in natural areas as discarded flowers may contain seeds.

If you have removed any dog strangling vine on your property, consider planting native flowers in their place, which are just as beautiful and much more helpful to the local ecosystem.

If you see any plants that match the description above, please contact the Invading Species Hotline at 1-800-563-7711 or the Natural Resources Information Support Centre at 1-800-667-1940 or 1-800-387-7011 to report your sighting! Also consider visiting the EDDMapS Ontario which has partnered with both of the organizations above to submit observations!

Additionally, importation of dog strangling vines and other invasive plants is illegal and can be reported to the MNRF TIPS line at 1-877-TIPS-MNR (847-7667) toll-free anytime. You can also call Crime Stoppers anonymously at 1-800-222-TIPS (8477).

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