Your zone is not the number you remember
Ottawa sits in plant hardiness zone 5. What changed is the map. Natural Resources Canada updated Canada's plant hardiness zone maps in July 2025, the first update in more than a decade, and it reports many areas shifting by half a zone or more and some by as much as two full zones.
That makes a zone number remembered from a nursery tag ten years ago an unreliable guide. Check your own address on Canada's Plant Hardiness site, because the zone boundary can fall between one part of the city and another.
Two practical rules survive any map revision. Buy one zone hardier than your rating for anything you cannot afford to lose, because a zone describes an average winter rather than the cold snap that actually kills. And treat a container or a raised bed as a full zone colder than the ground beside it, because roots above grade freeze from all sides.
The three-metre salt band
Every driveway, front walk and municipal sidewalk in Ottawa is salted between December and March, and plough and shovel throw salted snow two to three metres. Within that band, salt accumulates in the soil and salt spray burns foliage and buds.
The plants that fail there are the ones most often planted there. Eastern white cedar hedging along a driveway, boxwood along a front walk, and most broadleaf evergreens brown out on the exposed side and do not recover, because the damaged tissue is the previous year's growth.
The fix is a plant list that changes within three metres of hard surfaces rather than a change of habit about salting. Salt-tolerant deciduous shrubs and ornamental grasses in the band, evergreens set further back, and where a hedge is wanted along a driveway, accept that it needs replacing periodically or move it.
Snow is protection and snow is a load
Consistent snow cover insulates crowns and roots, and a mild, snowless January with hard frost is considerably harder on perennials than a cold one under half a metre of snow. That is why the same plant survives in a back garden and dies in an exposed front bed on the same lot.
Snow is also weight and it is placed. Where is the plough pile going, where is the roof shedding, where does the shovelled path get emptied. A shrub under the driveway pile until mid-April emerges flattened, and an upright evergreen under a roof shed breaks open in the middle. Both are design decisions rather than plant failures.
Frost heave is the third winter mechanism. Repeated freezing and thawing lifts newly planted perennials out of the ground in their first winter. Plant early enough in autumn for roots to take hold, mulch after the ground has frozen rather than before, and check in April for anything that has been pushed up, pressing it back in rather than leaving the roots exposed.
Soil is where the budget should go
A large part of Ottawa sits on Champlain Sea clay, which swells when wet, shrinks as it dries and drains slowly. On a suburban lot the topsoil above it is often 100 mm of builder-grade material over compacted subfill, which is not a growing medium.
Beds want 300 to 450 mm of amended topsoil, and a planting pit for a shrub or a small tree wider than it is deep so roots spread outward rather than circling. Work compost into the clay at the interface, because a clean-sided pit in clay holds water like a bucket and drowns the plant it was dug for.
Water deeply once a week for the first two summers rather than lightly every day. Shallow frequent watering keeps roots at the surface, and surface roots are the ones that die in the first dry August or the first open winter.
Deer, rabbits and where they are
Browsing pressure is a geography question in this city. Kanata, Stittsville, Manotick, Greely and the properties backing onto the Greenbelt or farmland see real deer and rabbit damage through winter, when other food is under snow. Inner-city neighbourhoods largely do not.
The plants eaten first are also the ones most commonly planted: cedar, yew, tulips, hostas and young fruit trees. A planting list that works in the Glebe can be stripped by March on a Stittsville lot backing onto a field.
Practical responses, in order of reliability: choose species that are not browsed, protect young trunks with spiral guards against rabbit and vole girdling under snow, and fence or wrap valuable evergreens for their first several winters. Repellents work until it rains and then they do not.