Invasive Plants In the Ravine

Ecologist Katherine Baird proposes sustainable native alternatives to Dog-strangling Vine

One of the most aggressive invasive plants in the Wilket Creek ravine is dogstrangling vine. During surveys of the ravine, 58 invasive non-native plants were identified, with an additional 38 on invasive watch lists. Often abbreviated to DSV, this invasive is also known as swallowwort, and includes two species:

• Pale swallowwort (Vincetoxicum rossicum)

• Black swallowwort (Vincetoxicum nigrum)

What are invasive plants?

Invasive plants are non-native species that escape cultivation and invade natural areas, negatively impacting the environment.

Ecologist Katherine Baird proposes sustainable native alternatives to Dog-strangling Vine

Pale swallowwort (Vincetoxicum rossicum)

Black swallowwort (Vincetoxicum nigrum)

What are invasive plants?

Invasive plants are non-native species that escape cultivation and invade natural areas, negatively impacting the environment.

What impacts do they have?

DSV is identified as a Category 1 invasive species, having serious negative impacts on natural areas by creating dense monocultures to the exclusion of native species (CVC 2012 1 ). DSV is allelopathic, meaning it releases chemicals to inhibit the growth of other plants (Anderson 2012 2 ). DSV also degrades wildlife habitat, inhibits native tree regeneration, and threatens species at risk (Anderson 2012 2 ); (DiTommaso et al. 2004 3 ). Monarch butterflies can mistakenly lay eggs on dog-strangling vine, instead of their true host plants (native milkweeds), killing larvae (Casagrande & Dacey 2007 4 ).

Dog-strangling Vine seed pods

How did they get in the ravine?

DSV arrived in Toronto from Eastern Europe and Asia in the 1800s as ornamental plants. Now a ‘restricted species’ in Ontario, it is illegal to import, deposit, release, breed/grow, buy, sell, lease or trade plants. Despite regulations, DSV continues to spread from established patches through wind dispersed seeds and root fragments. In fact, DSV can produce up to 28,000 seeds per square metre.

What do they look like?

DSV is a perennial herbaceous twining vine in the milkweed family (Asclepiadaceae). It has opposite tear-drop or lance shaped leaves with smooth edges, pale to dark purplish five-part flowers late June to July, and slender seed pods that open in late summer to expose feathery tufted seeds dispersed by wind. DSV reaches up to 2 m long and is often seen twining (coiling) around trees, shrubs, ground flora or even itself to form a dense thicket.

How do they differ from native milkweeds?

As seedlings, DSV may resemble native milkweeds, but they are only distantly related. Unlike DSV, native milkweed species grow upright and erect, and do not twine (coil) like DSV. Native milkweeds also play an important role in supporting native wildlife such as Monarchs that depend on native milkweeds as a host plant.

What should I do if dogstrangling vine is in my garden?

It is important to learn to identify DSV and ensure it is removed early on before it becomes established. Long distance spread of wind dispersed seeds, extensive roots and resprouting ability make DSV difficult to control. Removal can be done by digging (rather than pulling) to remove the entire root to prevent resprouting, or through solarization. If you are unable to remove the entire plant, repeated cutting to prevent seeding is essential to reduce spread. DSV material should be sealed in garbage bags and disposed of in the landfill – not composted. To learn more about controlling DSV, reference Ontario Invasive Plant Council best management practices: https:// www.ontarioinvasiveplants.ca/ resources/best-managementpractices/

Alternative options:

There are a number of beautiful native milkweeds, which we can plant to provide Monarchs with their host plant, rather than DSV. These include the following plants native to southern Ontario:

• Swamp milkweed (Asclepias incarnata) – clumping

• Butterfly milkweed (Asclepias tuberosa) – clumping

• Common milkweed (Asclepias syriaca)

• Whorled milkweed (Asclepias verticillata)

Common milkweed (Asclepias syriaca)

Butterfly milkweed (Asclepias tuberosa) – clumping

Whorled milkweed (Asclepias verticillata)

Swamp milkweed (Asclepias incarnata) – clumping