What is a hardiness zone?
Depending on how harsh and cold your local climate is, certain trees and plants will be able to thrive on your land, and others will languish, die or will not have time to mature their fruits. Since Canada has a very wide range of different climates and microclimates, hardiness zones are a way of simplifying the task of choosing plants and trees that are suitable for your area.
Every region is designated with a particular hardiness zone indicating how harsh the climate is in that region. Each tree species is likewise given a zone number, which indicates the coldest zone in which that tree can grow. Zones range from 0 to 9, with 0 being the coldest and 9 the warmest. For example, a tree labelled ‘hardy in zone 3’ can be planted in zones 3-9, but not in zone 2, 1 or 0.
The Canadian hardiness zone map
In 1960, the United States Department of Agriculture created the first hardiness zoning map of North America. This map was constructed based on the minimum winter temperatures of an area. In 1967, the scientists of Agriculture Canada created a hardiness zone map of Canada. This map did not only consider the minimum winter temperature: it also accounted for the length of the frost-free period, summer rainfall, January rainfall, maximum temperature, snow cover, and the maximum wind speed.
The hardiness zone map divides the country into 10 zones (0-9, 0 being the coldest zone and 9 the warmest). Each zone consists of 2 sub-zones (a and b, a being the coldest and b the warmest). For example, Ottawa is in zone 5b, Edmonton is located in zone 4a, La Loche is in zone 2b, and Yellowknife is in zone 0b. On the map, which you can download here, the zones are indicated by a colour code.
Finding your hardiness zone
Every Canadian gardener should know their cold hardiness zone, or they will have some nasty surprises in spring. If you don’t know your growing zone, we created this tool to help you find out. Simply write the name of your municipality to see which zone you are in. Note that this tool is in construction, and some municipalities are still missing. If your municipality is not yet identified with a particular zone, you can download the latest hardiness zone map here.
The importance of cold-hardiness for fruit trees
Hardiness is not quite as important for growing vegetables, as seedlings can be sprouted indoors to compensate for a short season. Trees and shrubs, however, must be able to survive outdoors throughout the winter, and over long periods of time rather than merely a year or two. One unusually cold night is enough to badly damage or even kill the tree you have been taking care of for years. It is also important to understand that frost damage greatly affects the health of a tree, even if the damage is not enough to kill it. The injury slows down the tree’s growth, and provides a doorway through which disease can enter in the following years.
Choosing the right trees for your zone
Bearing all of the above in mind, you should have an idea by now of how important it is to choose trees that are at least hardy in your zone. Particularly if you are in the ‘a’ subzone and you wish to err on the side of caution, you might want to choose trees in a lower zone. For example, if you are in zone 4a, it can be better to choose trees for zone 3, 2 or 1 if you wish to avoid risking any winter injury. For more choice, you may also choose trees for zone 4 – they may occasionally suffer some frost damage during particularly cold winters but should still survive.
Planting a non-hardy tree is comparable to going out in winter in Canada in flip-flops and a t-shirt! It’s a good plan for hypothermia and potential death.
This image represents a zone 5 tree planted in zone 3. The tree will not survive the winter. While we know that we can’t plant an avocado tree, an orange tree or banana tree – they will die for sure in winter. But when it comes to peach trees, pear trees, cherry trees and so on, it is more complex, because they may survive the winter in some regions, but not others. Thus, the importance of knowing your zone.
Something is clearly wrong with how this young girl is dressed for facing the Canadian winter! Her hat and scarf are comically useless when she insists on wearing only shorts and sandals on her legs!
Absurd as this seems, it is a good representation of what is sold in a lot of nurseries! People tend to be fairly well-informed about the need for hardy cultivars, so nurseries will offer Norkent apple trees (z.2) and Ure pear trees (z.3) for example. But on which rootstock are these grafted? Most of the time, on rootstock hardy only to zone 5 or even zone 6, produced in the USA or in Europe! The cultivar may well be hardy, but if its roots and the base of its trunk are not it is the entire tree that will die. Hence the importance of knowing one’s zone and checking the hardiness of the cultivar AND the rootstock!
In the same way, choosing a super-hardy rootstock, only to graft it with a cultivar that is not hardy to your zone, will not get you very far. It will mean almost certain death for the grafted cultivar. However, in this case, at least the rootstock will survive, and will grow its own trunk. Depending on the rootstock that is used, in most cases the fruit will not be of much interest. You can, however, try to re-graft it with a new, more cold-resistant cultivar if you would like to try your hand at the art of grafting!
Finally somebody who is well-dressed for winter! At Hardy Fruit Tree Nursery, we specialise in cultivars that are hardy in at least zone 4 – if we cannot grow something ourselves, we do not sell it. Besides this, we take care to always choose a rootstock that is at least as cold-resistant as the cultivar, if not more so. In this way, you don’t have to worry about whether a tree will survive the winter at your place, as long as it is marked as hardy to your zone.
So do not expect to find peach, nectarine, fig, or sweet cherry trees on our site – nor banana, mango or lemon trees for that matter! None of these species can grow in northern regions, unless they are in a heated greenhouse or have some other elaborate form of protection. If we can’t grow a tree ourselves in zone 4, we do not sell it.
On the other hand, it is possible and easy to grow many fruit and nut trees in northern climates, even up to zone 2! These include, but are not limited to, apple, pear, plum, sour cherry, several kinds of berries, black walnut, hazelnut and Korean pine nut.
To see a full list of all our trees that will be likely to grow well in your area, you can go to the page ‘See all our trees’ and use the filter to see only the trees hardy to your zone.
Hardiness zones and climate changes
The hardiness zone map has been updated since that time to account for climate change, which has resulted in some rather significant changes to the climates of certain areas.
Fort McMurray, for example, was in zone 1a until the most recent update to the map, but is now located in zone 3a. It’s worth noting, however, that this does not hold true everywhere, and you should not expect that trees meant for warmer climates will now suddenly survive at your place.
Make sure to verify your hardiness zone before deciding which fruit trees to plant.
The limitations of hardiness zones
The hardiness zones on the map are a very good approximation but are not always perfect. In particular, the variation of local climates, called micro-climates, is not easily taken into account due to the spread of the weather stations from which data is gathered.
As an example, a frost pocket in a valley between mountains is likely to be colder than the zone identified on the map.
Presence of water bodies in an area results in cooler temperatures in summer and warmer temperatures in winter than if the water body was not present. It is an advantage for the fruit tree grower because the vegetation bud growth is delayed in spring. This lessens the risk of flower death caused by frost that many areas experience.
The hardiness zones are also valid mainly for trees and shrubs, while perennials should be treated differently. For example, a perennial or a very low bush zoned 6 would probably not survive in zone 5 without protection. However, it can sometimes survive in zone 3 with thick snow cover which acts as insulation.
Wood hardening and cold-resistance
Even if you plant a fruit tree in the correct zone and the winter is quite warm, the tree can still be injured by frost. In fact, the wood of the tree can only tolerate the winter after having undergone wood-hardening correctly.
Wood hardening is the hardening of the wood fibre. If the fibre of the wood has not had time to correctly harden in fall the tree can be injured by frost even if it is hardy.
Here is an example of pear tree bark damaged by frost. The bark dies in the winter, dries out and peels off the trunk.
In a case like this, it is the main trunk that has frozen and the tree is completely dead. It will need to be replaced with a different, more cold-hardy tree.
When a tree that was healthy the previous summer develops a branch or branches that are blackened like this one the following spring, it indicates frost damage. It can be due to lack of cold-hardiness, or sometimes over-fertilisation.
In such a case, if there is only one branch affected, it is dead and should be pruned off. It is an indication that the cultivar is at the limit of its hardiness. Although it will survive, it will suffer damage in especially cold winters.
This hickory, although it manages to survive, obviously lacks hardiness in the zone in which it is planted. Each year, the head and a large number of its branches die. The tree is weak, but stays alive. The frequent frost injuries are also places for disease to enter. The tree ends up reaching its limit and dying, as its effort to repair itself is in vain and it freezes once again each winter.