Beverley Wagar

FireSmart landscaping for cleaner air and safer yards


This article was inspired by Jane Bailey’s presentation to the 1000 Islands Master Gardeners on September 22, 2025. The list of references and many of the technical details are her work

Wildfires: ecological role and health risks

As the climate crisis deepens, extreme and volatile weather is becoming normal, with droughts and heat waves becoming hotter, more frequent, and longer lasting. For gardeners, each summer is more challenging than the last, not simply because of high temperatures, lack of soil moisture, and water-use restrictions but also the haze of smoke from wildfires.

Air Quality Health Index in Kingston Ontario on June 6, 2025. The risk was high all day.

Three major factors influence wildfires:

  • frequent lightning strikes that start fires
  • dry windy weather that fans the flames
  • a lot of dry fuel to burn

Long-established fire suppression practices have left an abundance of fuel on forest floors. Before governments began large-scale firefighting, fire was the forest’s ally, removing litter (duff, dead branches, dry stumps and snags) which limited the amount of fuel available to a wildfire. 

Authors of a 2016 report to the Royal Society noted the futility of thwarting this process: “We cannot completely remove fire from the landscape…. That is the misconception that led to the ‘100% fire suppression’ policies in the US and elsewhere that have made things worse in many cases.”

The Climate Atlas of Canada outlines the role of fire in a healthy forest

Wildfire is a natural part of the boreal forest life cycle. Fires help clear litter from the forest floor, recycle nutrients back into the soil, open up gaps in tree stands to promote new growth, and kill invasive species and forest pests

What is wildfire smoke?

Wildfire smoke is mixture of gases (carbon monoxide, hazardous air pollutants, and water vapor) but the main component is particle pollution. This “particulate matter” (PM) is what poses the biggest threat to the health of humans—and other creatures with lungs.

https://www.bccdc.ca/health-info/prevention-public-health/wildfire-smoke

Wildfires cause spikes in the amount of fine particulate matter (2.5 microns or smaller) and ultra-fine particles (less than .1 micron) in the air. Ultra-fine particles are small enough to pass through lung tissue and into the bloodstream. No matter what the size, particulate matter can harm your health.

Plant choices mitigate the danger

Research has shown that plants, especially conifer (needle-bearing) trees, can protect us from air pollution. With each needle functioning as a leaf, it’s no surprise that a conifer can provides an exceptionally large surface area for capturing PM. Eastern white-cedar (Thuja occidentalis), a proven biomonitor of heavy metal pollution, is also good at trapping PM from wildfires. Thuja’s soft, flat, scale-like needles overlap in sprays which act like horizontal fans to intercept drifing PM. Particles accumulate on needle surfaces and, due to a waxy cuticle around each stomata, do not penetrate into the cells. 

Conifers outperform deciduous trees for filtration not just because of the number of needles (and hence the larger number of stomata) but also because the needles keep filtering the air year-round, which deciduous trees stop doing when the leaves drop. As well, when rainfall washes the needles clean, conifers are quicker than deciduous trees in resuming their scrubbing function. 

After a rain the PM ends up in the soil. The effect on soil has not been well studied, but there is evidence that, depending on the amount deposited, PM has no effect on soil pH but it does increase soil nutrient content and decrease bacterial diversity.

Using shrubs and conifers

Studies of urban forests and roadside vegetation show that plants shrub-sized and smaller accumulate significant PM. Home landscapes can go one further with multi-layered plantings that include trees, shrubs, and groundcovers. Multi-layered filtration thwarts the deposition of PM on the soil surface, where it can be blown by the wind and become resuspended in the air.  

After choosing a variety of trees and large shrubs, underplant with smaller, dense, native shrubs and a ground-cover layer. The secret to the success of the other filtration layers is the number of leaves per plant. Site these perpendicular to the prevailing winds and, if possible, upwind of air intakes and living areas. Avoid planting in straight lines. Instead stagger the placement to form “porous hedges” that will slow the passing airflow and increase PM deposition. Be sure to plan for maintenance access—not only pruning but also cleaning gutters and screens after smoke events.

Good choices for evergreens include Eastern white-cedar (Thuja occidentalis) and Eastern hemlock (Tsuga canadensis). These species, both densely branched, cold hardy and native to Eastern Canada, not only make excellent specimen trees but also can withstand pruning into hedges.

leaves of Eastern white-cedar (Thuja occidentalis)

If your landscape plans call for something deciduous, consider shrubs with dense leaves and branching such as serviceberry (Amelanchier spp), red osier dogwood (Cornus sericea), ninebark (Physocarpus opulifolius), and chokecherry (Prunus virginiana). Layer with durable groundcovers to limit dust resuspension. A low-growing conifer such as creeping juniper (Juniperus horizontalis) is excellent for this purpose

FireSmart plantings near buildings

FireSmart Canada recommendations
https://firesmartcanada.ca/


Keep a noncombustible 1.5 m (5 ft) buffer around building foundations—use gravel, pavers, or bare mineral soil. In this fire-safe zone do not use wood or bark mulch and resist the urge to add shrubs or “foundation plants”.

In the next zone ( from 1.5 to 10 m) if there are existing mature trees you may want to raise the crown to about 2 m (6 ft). Sometimes called “limbing-up”, this practice can prevent fire from climbing a combustible “ladder” to upper stories. After pruning, a healthy crown should be at least 2/3 of the tree’s overall height.

To avoid having to limb-up trees in the future, plant them at a fire-safe distance from buildings. Deciduous trees should be at least 5 m (15 ft) from structures and conifers at least 10 m (30 ft) away. 

FireSmartTM recommends limbing up trees even if they aren’t within 2.5 m (30′) of a structure, to remove ladder fuels. This could be important to our little forests and in conservation areas, where deadfall and dead lower branches could still fuel significant fires.  

A stark example of a Zone-1 buffer area. Note the conifers in the background for mitigation of particulate matter.

The future of fire prevention

Fire prevention has always been about enforcing burn bans and campfires, educating about careless smoking, and early detection—whether via drones, towers, satellites, or household smoke alarms. When wildfires happened, authorities took quick action with resource-intensive suppression tactics. But the climate crisis is forcing change in the way we respond to fire in forests and landscapes.

As the growing intensity and frequency of wildfires overwhelms the capacity of traditional firefighting methods, land managers are increasingly likely to use prescribed burns to remove “fuel” from the forest floor. Deliberately setting and managing fires are traditional practices for many Indigenous people. And burns are routinely used in managed prairie remnants to mimic the regular fires that shaped these ecosystems. But using prescribed burns in boreal forests and vast areas of parkland is a relatively new strategy in wildfire prevention. The Canadian Council of Forest Ministers’ 2024 Wildland Fire Prevention and Mitigation Strategy offers comprehensive details and insight on Canada’s new approach to the threat of wildfires.

Traditional practices in urban landscaping are similarly under scrutiny. FireSmartTM Canada’s recommendations (endorsed in the Ministers’ Report linked above) for non-combustible zones around buildings are sure to be a hard-sell for the landscaping industry. We can encourage fire-safe practices not only by creating non-combustible zones in our own yards but also by promoting the use of native conifers, especially Eastern white-cedar. And, for the tenacious, by using social media and local gardening networks to challenge the tedious landscaping convention of “foundation plantings”.

References

  • Chen L. et al. 2017. Variation in Tree Species Ability to Capture and Retain PM. Atmosphere 8(5):91.
  • Mo L. et al. 2015. Assessing Capacity of 35 Plant Species to Accumulate PM. PLoS ONE 10(10):e0140664.
  • Xu Y. et al. 2018. PM accumulated on leaves by 17 urban species; ultrafines & stomata. Environ Sci Pollut Res 25:16520–16529.
  • Diener A. et al. 2021. How vegetation protects us from air pollution—a critical review. Sci Total Environ 796:148605.
  • Steinparzer M. et al. 2023. Evergreen conifers show highest PM accumulation. Environ Pollut 326:121324.
  • Conservation Ontario & MGOI: Rain gardens, waterfront swales, alvar adaptations.
  • Ontario MNRF & FireSmart Canada. FireSmart Landscaping Guides (buffers, mulch, spacing).
  • Ontario Invasive Plant Council. Grow Me Instead (native alternatives).
  • Popek R. et al. 2022. Not Only Trees Matter—lower forest layers accumulate PM. Sustainability 14(5):2973.
  • Popek, R., Przybysz, A., Łukowski, A., Baranowska, M., Bułaj, B., Hauke–Kowalska, M., … Kowalkowski, W. (2024). Shields against pollution: phytoremediation and impact of particulate matter on trees at Wigry National Park, Poland. International Journal of Phytoremediation, 27(4), 448–461. https://doi.org/10.1080/15226514.2024.2426771
  • Santin, C. and Doerr, S. 2020. Global trends in wildfire and its impacts. https://royalsociety.org/blog/2020/10/global-trends-wildfire/
  • Wróblewska K. et al. 2021. Green infrastructure for PM removal—review. Environ Sci Europe 33:114.
  • Zhongpan Zhu, et al. (2024) Effects of forest fire smoke deposition on soil physico-chemical properties and bacterial community. Science of The Total Environment, Volume 909.

Further Reading