Understanding cold-hardiness and hardiness zones

10, 2014

Winter and fruit treesEvery Canadian gardener should know their hardiness zone, or they will have a nasty surprise in spring. Winter frosts will kill a tree or plant if it is not located in an appropriate hardiness zone.

Where does the term ‘Hardiness Zone’ come from?

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. Not only did this map 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. Ever since, the map has been continually updated. You can download the latest map by clicking here.

The hardiness map of Canada divides the country into 9 zones (0-8, 0 being the coldest zone and 8 the warmest). Each zone consists of 2 sub-zones (a and b, a being the coldest and b the warmest). For example, Edmonton is located in zone 3a, Winnipeg is in zone 2b and Ottawa is in zone 5a. Trees can grow in their designated zone or a zone warmer than that specified, but will not survive in a colder zone.

Alberta, Manitoba and Saskatchewan hardiness zones map

Ontario, Quebec, Maritimes Hardiness Zones map

Hardiness zones … always right?

Not always. The hardiness zones on the map are a very good approximation but are not always perfect. Firstly, 3000 meteorological stations are used to collect data for the map. The variation of local climates, called micro-climates, is not taken into account due to the spread of the stations. As an example, a south slope identified as zone 3 on the map could in reality fit the description of a zone 4 area. Conversely, an area experiencing higher wind rates and altitude than the surrounding area could fit the description of a lower zone than that shown 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. This 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 valid for trees and shrubs only. Perennials should be treated differently. For example, a perennial or a very low bush zoned 6 would not survive in zone 5 without protection. However, it could survive in zone 3 with thick snow cover which acts as insulation.

Hardiness and fruit trees

Frost injuries on a pear treeFor vegetable growers, hardiness is not so important as seedlings can be sprouted indoors to compensate for a short season. However, for fruit tree growers, the situation is different. Unless fruit trees are grown in a greenhouse, it is not possible to grow fruit such as oranges and grapefruit in Canada.

Contrary to entrenched views, it is possible and easy to grow certain types of fruit trees in northern climates, even up to zone 2. Pear trees, apples trees, cherry trees, plum trees, nut trees (hazelnut, pine nuts, walnuts), currants, haskap (also known as honeyberry), blueberry, grape and more can be successfully grown. The key to success is choosing a tree rightly adapted to its hardiness zone. In other words, it is essential to choose a tree that is at least hardy to your zone, or preferably to a lower zone. Why? Simply because nature is unpredictable and will sometimes deliver conditions colder than your zone. You only need one overly cold night to damage or kill the tree that you have been taking care of for years. The photo on the right shows a non-hardy pear tree after it suffered from frost. The evident black bark is a sure sign of frost.

Choosing the right variety

When considering growing apple trees in northern climates, favour varities such as Norda, Norkent and Red Haralson. Unlike McIntosh and Red Delicious varieties, these have proven to survive extreme cold conditions. A decent number of pear trees such as Ure, Loma and Krazulya can grow in conditions as harsh as those in zone 3. The sweet cherries grown in western Canada are not suitable outside of this area. However, some sour cherries (pleasant to eat fresh) can grow up to zone 2. Black walnuts are hardy to zone 3 but need to be located in zone 4 for the fruit to become mature. Haskap originates from Siberia, is hardy to zone 1 and the flowers will survive at temperatures above -7 C. It is important to know that a fruit tree damaged by frost will have retarded growth and will be susceptible to diseases in the season to come. This is because the injury is a doorway for disease to enter the tree.

A grafted fruit tree that is well adapted to its zone will be composed of a hardy variety grafted on a hardy rootstock. If the variety chosen is hardy to your zone but the rootstock on which it was grafted is not, you are in trouble. It is difficult to know the origin of the rootstock when you buy a tree in a garden centre, but mostly the rootstock will be imported from Holland and grafted in Ontario. To find trees adapted to your climate, it is best to buy fruit trees from a nursery that grows both the rootstock and the variety, and completes the grafting themselves. 

Wood hardening

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. For more details about cold hardening and fertilisation, read our article Fertilising fruit trees.

In conclusion, the hardiness zone map is an extremely important tool to guide the selection of a fruit tree. Knowing your zone is the first step to success in fruit tree growing. The secret to perfect home-grown fruit is a healthy and hardy fruit tree which is adapted to its environment!