Purple loosestrife: here to stay?


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Hey there, time traveller!
This article was published 28/09/2021 (612 days ago), so information in it may no longer be current.

Cycling through the community a few weeks ago, I was struck by how attractive the flowers were at and around the retention ponds.

Having watched them grow over the summer I wondered why they were still there, because I knew they were purple loosestrife, something I thought was classified as a noxious weed requiring control.
While waiting for a call back from the city’s naturalist services branch, I did some research on the plant.

Purple loosestrife is native to Europe and Asia, and likely came to North America accidentally in the ballast water of a ship about 220 years ago, during the European colonization of this continent. Two key things make it a problem – high reproduction and no natural predators. It grows and germinates quickly, with adult plants producing more than 2.5 million seeds annually. In its native habitat (it prefers wetlands or riversides) it provides food for more than 100 species of insects that control its population, as well as deer, muskrats, rabbits and several bird species (excluding waterfowl).

Photo by Nick Barnes Purple loosestrife’s high reproduction rate and the fact it has no natural predators means it’s hard to control.

In Manitoba, regional impacts to wetland and riparian areas began in the 1930s as purple loosestrife out-competed native plant species, and in the 1990s it was listed under the provincial noxious weeds act, which required actions to control its spread.

The City of Winnipeg became an active participant in the Manitoba Purple Loosestrife Project to co-ordinate control measures, along with other government and non-governmental agencies, such as Ducks Unlimited, Canadian Wildlife Service and Manitoba Agriculture.

Control measures include physical or mechanical removal, chemical spraying and biological control.

Due to its rate of spread, mechanical removal (pulling up the plants) was not effective beyond a really local level, and it was challenging to find a herbicide that targeted purple loosestrife without killing native species.

Efforts therefore focused on biological control, and finding an insect from its native habitat that selectively fed on the plant. The root-mining weevil and a leaf-eating beetle species were tested in Manitoba and were found to be successful in reducing populations of purple loosestrife.

When natural services did get back to me, the first thing I learned was that purple loosestrife is no longer listed in the noxious weeds act. It’s listed in a similar federal act that focusses on methods of controlling its spread, but there is no provincial legislation requiring its control.

There’s still a program to rear and release the leaf-eating beetles, and evidence that the weevils are still mining the roots, but I guess there’s a recognition that for urban centres like Winnipeg, purple loosestrife is here to stay. Efforts are now focussed on trying to protect important natural wetland areas.

Nick Barnes

Nick Barnes
Whyte Ridge community correspondent

Nick Barnes is a community correspondent for Whyte Ridge.

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