The newcomers have alluring names, such as Himalayan balsam and Amur honeysuckle, but they're considered dire threats that will drive out local species and reduce biodiversity.
The recent immigrants bring to 45 the total number of invasive alien species now catalogued on properties managed by the National Capital Commission, which includes about 10 per cent of the land in and around Ottawa, including parts of Quebec.
The findings are from a detailed survey commissioned from the Montreal office of consultant Genivar Inc.
The survey report from January this year was obtained by The Canadian Press under the Access to Information Act.
Genivar Inc. for the first time also documented an insect-and-fungi combination, known as Beech bark disease, on the NCC's urban properties.
The survey, conducted by biologist Melanie Lapointe, also had some good news: giant hogweed, whose toxic sap can burn human skin in the presence of sunlight, was found nowhere on the commission's lands despite having been located elsewhere in Ottawa.
And two alien species that had been catalogued in previous years, water chestnut and black swallow-wort, were nowhere to be found.
The report provides a detailed inventory of just what invasive species are located where, and includes recommendations for preventing the arrival of new pests and fighting the ones already here.
Among the suggestions: create a rapid-response team for when new species are detected; ask nurseries not to stock alien species; require maintenance crews to hose down trucks when moving from one location to another; and ban the transport of firewood.
"Given the extent of the colonies detected on NCC lands and the actual and potential impacts of the threat of invasive species on valued natural ecosystems and habitats, prevention measures, early detection and rapid response program, monitoring, and control should be implemented as soon as possible," says Lapointe's report.
"Implementing an early detection and rapid response program should be a priority."
The National Capital Commission has already set itself a deadline of March 31, 2015, to reduce the surface area occupied by alien species by 10 per cent for its urban lands.
Spokesman Stephen Blight says the commission has carried out "random acts of good stewardship" for years, but needed a comprehensive picture to determine where its limited resources should be used against alien species.
"For some invasive species, natural limits and natural predators do emerge over time, but for some they don't," he said in an interview. "Those are the ones we have a real problem with."
Blight, vice-president of capital lands and parks branch, says the commission will review Genivar's findings over the coming months while implementing some measures in the short term, working with the municipalities of Gatineau and Ottawa.
The commission is also responsible for cutting and maintaining the lawns on Parliament Hill, on behalf of the Public Works Department, and is being asked to do remedial work on the escarpment behind the Centre Block of the Parliament Buildings.
Public Works has hired a contractor for $680,000 to stabilize the rock face, using steel mesh, after which the National Capital Commission would clear invasive vegetation and plant native trees in its place.
The commission — sometimes called the "gardeners of the capital" — manages about 610 square kilometres of land, including pristine Gatineau Park in Quebec and the Green Belt around Ottawa.
Ontario and Quebec have the highest number of invasive plant species among all the provinces, 441 and 395 respectively, according to a 2008 federal survey.