Trees make you healthier, but choose wisely

Arboretum Willows
Female willow trees are pollen free. Photo: Richard Hinchcliff

by Jennifer Humphries

A major research report was released in October 2017 on the link between trees and greenspaces and human health.

Urban Greenness and Mortality in Canada’s Largest Cities: A National Cohort Study is the culmination of a multi-year process engaging 11 researchers. It looked at health outcomes of 1.3 million Canadians over 11 years. The conclusion: living in proximity to trees and greenspace reduced the risk of dying from several common causes of death, including cardiovascular and respiratory diseases, by 8 to 12 per cent.

Researcher Dr. Dan Crouse of the University of New Brunswick told CBC News, “The results we got were a lot larger than I was expecting. We thought we would see more like one to five per cent reduced risk of mortality.” He pointed to several reasons for the beneficial health impact, including reduced exposure to air pollution (trees clean the air), lower traffic noise, the summer cooling effect and enhanced opportunities for recreation. Crouse also said that simply having a view of nature from a home or hospital window had a positive impact by offering a restorative and stress-reducing influence that enhanced overall health.


But is there help for allergy sufferers?

Without diminishing the importance of these findings and hoping that they will spur much more greenspace identification and tree plantings in our city, it is also imperative to recognize that some trees and plants have a downside for allergy sufferers. To be clear, it’s not all trees and plants, and not at all times of the year.

Pollen is a top cause of allergy symptoms and Canada’s urban centres are hotbeds of trees and plants that are the biggest pollen producers. That’s because little or no attention has been paid to pollen emission when selecting trees and plants for cityscapes, parks and institutions such as schools and hospitals.

What can be done?

Peter Prakke
Peter Prakke, horticulturist, researcher and writer, promotes the planting of trees that minimize allergic reactions to pollens. Photo: Larry Arnal

Last August the Glebe Report received a letter to the editor from Peter Prakke, a horticulturist, researcher and writer from Ancaster, Ontario, near Hamilton. He wrote that he had initiated a project called Allergy Friendly Schoolyards for Canada, USA and EU Countries. He also referenced the importance of the Ogren Plant Allergy Scale (OPALS®) in making schoolyards and neighbourhoods safer for people with pollen allergies and asthma.

I spoke with Prakke and was impressed by his wealth of knowledge about gardening and his dedication to making community greenspaces healthier for everyone. While now living in southern Ontario, he has a local connection, having worked many years for Ottawa’s Ritchie Feed and Seed. Now retired, he is a keen proponent of OPALS for healthy schoolyards, urging school boards to implement OPALS in their greenspaces, making presentations and writing articles and a soon-to-be-published book, Veterans Gardening Guide.

Prakke is deeply concerned about the lack of awareness among officials responsible for institutions, parks and other urban places regarding allergy ratings. The result is that the vast majority of trees in all of Canada’s big cities are male, which are pollen emitters not pollen receivers, and many are trees high on the allergy scale. The Canadian Cities Allergen Audit, conducted in 2015, showed that Ottawa’s trees are 94 per cent male. While that’s pretty much on par with other major centres, it’s nothing to boast about. The audit uses the term “botanical sexism” to describe the preference for male trees, which are “easier care” as they don’t produce fruit or seeds – or not as much. Tellingly, the audit found that trees in wild lands are about 50 per cent male and 50 per cent female, an equilibrium that has been ignored in urban planning.

What can communities do? Prakke says we need to persuade municipal officials to make OPALS a key element of their tree selection process. Make sure that local parks get trees that make them healthier places. Encourage citizens to insist on tree species that rank low on the allergy scale and to ask for females of the species, which absorb allergens.

It’s strange that something so obvious has been so completely neglected. The issue of pollen and allergic reaction is nowhere mentioned in Ottawa’s new Urban Forest Management Plan. But the plan is iterative so there’s still an opportunity to make this critical health issue part of our city’s policies and practices. Then Ottawa can be a place where there are healthy trees and plants for everyone.

Jennifer Humphries is co-chair of the Glebe Community Association’s Environment Committee. You can contact her at


Learn more:

Living near green spaces linked to longer lives, study finds
CBC News October 11, 2017

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Urban Greenness and Mortality in Canada’s Largest Cities in The Lancet Planetary Health October 2017

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Creating Allergy-Friendly Schoolyards:

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Veterans Gardening Guide, forthcoming early 2018, by Peter Prakke:

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What is OPALS?

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PolleNation™ – Canadian Urban Allergy Audit:

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National Institute of Environmental Health Sciences notes on ragweed, grass and tree pollen:

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