Urban Nature ON Nature

by Christine Beevis

Yet another invasive plant is wreaking ecological havoc wherever it grows–and it’s darn near impossible to run out of town.

Five years ago, while mowing a neighbour’s lawn, Gary Ford was surprised to see a two-metre-high tangle of vines that he hadn’t noticed before. “It’s a jungle now,” he says, noting that the plant, which he identified as swallow-wort, has since shown up in his own garden. Ford believes that if he did not mow his lawn regularly, his yard would have likewise succumbed to swallow-wort, which his neighbour never did manage to eliminate. Despite his diligent mowing, however, swallow-wort stubbornly refuses to leave Ford’s garden altogether. “I keep pulling and pulling it,” he says, “but it keeps coming back.

“Ten years ago, you never noticed it here,” adds Ford, who has lived in Port Hope for 20 years and has noticed the invasive plant creeping into gardens throughout town ove the last decade.

The earliest record of swallow-wort (also called dog-strangling vine) in North America dates back to the mid-1800s, in Massachusetts. Two species now thrive in southeastern Ontario, southwestern Quebec and the northeastern United States: pale swallowwort (Cynanchum rossicum), which is native to the Ukraine and is found mainly in Canada, and black swallow-wort (Cynanchum louiseae), indigenous to the Mediterranean and found mainly in the United States. Here, swallow-wort has been spotted along the edges of eastern Ontario’s rare alvar ecosystems and has even been found in one of Ontario Nature’s nature reserves north of the Oak Ridges Moraine. Conversely, pale swallow-wort is almost nowhere to be found in the Ukraine, says Naomi Cappuccino, a biologist at Carleton University.

In North America in the last 50 to 100 years, swallow-wort has gone from occasional patches of growth to flourishing swatches of the vine covering hundreds of hectares. It has taken off dramatically in Ontario in the last 30 years. Cappuccino points out that once a few plants are able to mature, populations expand quickly. Says Sandy Garside, a member of the Fletcher Wildlife Garden in Ottawa, where volunteers have been trying to control the plant’s presence for the last decade,“It’s very scary, because a couple of plants will sort of sit there for a few years and then all of a sudden it starts growing exponentially like you wouldn’t believe.”

A member of the milkweed family, swallow-wort can grow to almost two metres in height. It has dark green, oval glossy leaves and five-petalled flowers. Its pods produce thousands of windborne seeds, and a single seed can produce as many as six plants. Swallow-wort tolerates most light and moisture conditions and thrives in disturbed ecosystems such as transportation corridors and old fields. But it will also do well in undisturbed areas, particularly shorelines, flood plains and forest understories.

The list of harmful effects associated with swallow- wort is long. Studies have shown the vine deters other plants from growing near it by releasing antifungal chemicals that change the soil composition.

The plant also plays host to a variety of crop insect pests and can smother shrubs and small trees. Abandoned fields infested with swallowwort appear to have a substantially lower diversity of insects compared with nearby, similar sites with predominantly native vegetation. Because swallow-wort closely resembles milkweed, monarch butterflies will lay their eggs on it, but the larvae will not survive. Very few native herbivores are interested in eating the plant, and its seed bank is resistant to burning, pulling,mulching, smothering and even recommended doses of herbicides. In fact, the only way to get rid of swallow-wort without resorting to chemical warfare is by digging out its root crown. Successful destruction of the seed bank can take as long as three to five years.

It appears, too, that swallow-wort has a negative impact on wildlife. In a study commissioned by the Nature Conservancy in the U.S. and the Thousand Islands Land Trust, ornithologist Gerry Smith spent the summer of 2004 monitoring the relationship between swallow-wort and grassland birds on Grenadier Island, a 560-hectare island in Cape Vincent, New York. Smith found that, in heavily infested areas, grassland bird species, including bobolinks, savannah sparrows and eastern meadowlarks, were missing. “You literally had to get to the periphery of the really heavily infested areas before you would detect any grassland birds,” he reports. Smith believes that the dense growth of the vine inhibits the birds from building nests in the grass. “I am absolutely convinced,” says Smith, “that this stuff is really bad news for grassland birds.”

For the past five years, Cara Webster, who works for the Urban Forestry Department of the City of Toronto, has been trying to eradicate swallow-wort from Toronto’s High Park. She and her volunteers have found manually wiping herbicides onto the plant to be the most successful strategy. Even so, says Webster, “There are areas [in the Rouge Watershed, Highland Creek Watershed and Don Valley] where we haven’t been trying to do any control because we feel like it’s already too heavily infested.” Sandy Garside is doubtful that the City of Ottawa has the resources to adopt a similar approach:“When you look at a field of swallow-wort in the middle of a natural area, you wonder what, exactly, they can do.”

Swallow-wort has also been gaining ground in the Ottawa greenbelt. Gershon Rother, a senior manager with the National Capital Commission, which acts as the steward of federal lands and buildings in the National Capital Region, says that in most cases “we try to let nature take its course over the long term.”The City of Toronto’s nvasive species plan proposes targeting areas where only a few plants are present that can be removed manually, but, argues Webster, “nobody thinks it’s an issue in those areas until it already is a problem, so it’s hard to get people to react quickly enough.”

Due to lack of funding, Cappuccino has had to shift her research away from swallow-wort, but she is hoping that U.S. research into biocontrol mechanisms that target black swallow-wort will work on pale swallow-wort as well.

Left to its own devices, swallow-wort is one tenacious vine. Over a 30-year period, black swallow-wort turned more than 800 hectares in Henderson County,New York, into a virtual monoculture.

“There isn’t anywhere in the park that does not have swallowwort. It’s up in the trees and in the fields,” says Fran Lawlor, a researcher with Cornell University. An expert on swallow-wort in North America, Lawlor doesn’t have any easy answers. “Some days, invasives just feel to me like a big hole because of the way they transform the environment,” she says. So far, the only solution may be to teach landowners how to identify the plant before it takes hold of their land – or let nature do what it will.

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Christine Beevis is a Toronto-based freelance writer and the editorial intern at ON Nature magazine.