Zone maps provide clues to what plants will grow where

Flip through outdoor plant books, seed catalogues or plant tags and beside every plant there is the word zone and a number. These numbers correspond to a government map that was developed by the Department of Agriculture in 1967 and updated in 1990 and again in 2000.

Flip through outdoor plant books, seed catalogues or plant tags and beside every plant there is the word zone and a number. These numbers correspond to a government map that was developed by the Department of Agriculture in 1967 and updated in 1990 and again in 2000.

Climatic information was gathered across Canada concerning the number of frost-free days, average minimum temperature in winter, average maximum temperature in summer, precipitation between June and November, average maximum snow depth and maximum wind gusts.

Information about 174 varieties of trees and shrubs from 108 locations across Canada was gathered. All the information was then plugged into a formula and zones were mapped.

Central Alberta is divided into Zones 2, 2a, 2b, 3, 3a. Zone 2 being the coldest and 3a the warmest. Plants that are rated for colder zones will also grow well in warmer areas.

The Department of Agriculture has never tested perennials or bulbs for zone hardiness.

Some new perennials are placed in trial gardens throughout Canada and the United States. These results are made public and give a fair indication of where the plant will thrive.

It would be prohibitively expensive to test all perennials and bulbs.

People within the plant industry rely on each other and their customers to find out what plant grows well in a given zone. This information will make its way into catalogues, books or tags, which is why the same plant can be classified under different zones depending on who is publishing the information.

The zone map is not an exact science. Micro-climates are everywhere as they are influenced by a number of different factors. Areas that are sheltered by trees, fences or buildings are warmer than ones in the open. Low land has less frost-free days than slopes. A slope facing south or west is warmer than one facing north or east.

Snow cover of six inches (15 cm) or more offers insulation from the cold and fluctuating temperatures making a big difference to plant’s ability to overwinter.

Soil type also plays a huge part in micro climates. Clay soil holds the heat at night while sandy soil warms up faster in the spring. Soil pH affects nutrient levels. A high pH often ties up nutrients, preventing plants from absorbing nutrients that are in the soil.

Sufficient moisture throughout the spring, summer and fall is needed to produce a healthy plant. Plants that are stressed by improper amounts of water and nutrients will be less likely to survive regardless of the weather.

Temperature affects the plant’s ability to grow, flower and set seed. The warmer the temperature, the quicker a plant will mature.

In areas like Central Alberta where there are not a large number of days between frost, warm weather can make a difference between a plant maturing before the first frost or not.

Plants that start from seed each have a slightly differ genetic makeup than others that came from the same seed pod. Growers will plant seeds in hope that one of the seedlings has the desired characteristics that make it superior to what is already available.

Gardeners who start plants, perennials, annuals, trees and shrubs from seed have the same chance of finding a seedling that is better or hardier than others of the same variety. The results can be a plant rated as zone 4 growing in zone 2.

For a plant to be hardy in a zone, it has to do more than survive. It had to produce new growth that lasts for more than a season. It has to be attractive and worthy of space in the garden, not just an oddity. The plant should also grow for a number of years.

To look at the government zone map type, go to

Linda Tomlinson is a horticulturalist who lives near Rocky Mountain House. She can be reached at or

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