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Canada's plant hardiness zone map: making a good thing better!

The Canadian Plant Hardiness Zone map has long been familiar to serious gardeners.

First produced in 1967, the hardiness map is a visual tool that depicts geographic areas statistically associated with the survival of native and horticultural plants. In Canada, hardiness is not just about temperature. The Canadian hardiness zone system combines seven climate criteria: C minimum temperatures, maximum temperatures, frost free period, summer rainfall, snowfall, a wind factor and January rainfall, to delineate zones.

Canada now has a new map that retains the original hardiness system and builds upon it. Developed by scientists at Natural Resources Canada's Canadian Forest Service and Agriculture and Agri-food Canada, this "newer" tool employs cutting-edge topographic and statistical mapping techniques.

Major Improvements

For the first time, the Canadian plant hardiness zone map incorporates the effect elevation has on climate and therefore, on plant survival. In order to include this critical feature, Canadian Forest Service scientists employed a computer-generated Digital Elevation Model of Canada, originally developed for forest research applications.

The amount of climate data available to statistically compute hardiness zones has more than doubled. The 1967 map used climate data recorded between 1930 and 1960. The new map keeps all the original data and adds climate measurements taken between 1961 and 1990. Comparing these two data sets has allowed researchers to identify changes to plant hardiness zone locations.

One other important change--the new map includes hardiness zone indications for all of Canada, not just the southern regions. Northern gardeners are now "on the map"!

How Have Hardiness Zones Changed?

From Manitoba westward, hardiness zones have generally increased, by about half a zone. This change suggests conditions have become slightly more "hospitable" for plants. In eastern Canada, hardiness zones appear generally to have declined by half a zone. Southern Vancouver Island and south-western Ontario show more dramatic declines.

Of course, the new map doesn't capture everything that affects plant survival. Canadian gardeners know from experience that well-placed microsite protection, either natural or created, can greatly increase survival of "borderline" selections in a garden or small-scale landscape.

Next Step: Plant-specific Hardiness Maps

Researchers at Natural Resources Canada, Canadian Forest Service are currently developing an Internet-based tool that will allow gardeners to identify the plants that survive in their locality. The ultimate aim is to develop climatic profiles for individual plant species. Once sufficient information has been acquired, range maps will be generated for viewing on-line.

Accessing the New Map:

The new interactive Canadian Hardiness Zone can be accessed on the Canadian Forest Service Web Site at ...

http ://

Those interested in `Going beyond the zones' to learn more about developing new plant hardiness maps can visit:


The Plant Hardiness Zones map outlines the different zones in Canada where various types of trees, shrubs and flowers will most likely survive. It is based on the average climatic conditions of each area. The first such map for all of North America, released by the United States Department of Agriculture in 1960, was based only on minimum winter temperatures.

In 1967, Agriculture Canada scientists created a plant hardiness map for Canada using plant survival data and a wide range of climatic variables, including minimum winter temperatures, length of the frost-free period, summer rainfall, maximum temperatures, snow cover, January rainfall and maximum wind speed.

The new map

Natural Resources Canada's Canadian Forest Service scientists have now updated the plant hardiness zones using the same variables and more recent climate data (1961-90). They have used modern climate mapping techniques and incorporated the effect of elevation. The new map indicates that there have been changes in the hardiness zones that are generally consistent with what is known about climate change. These changes are most pronounced in western Canada.

Interpreting the new map

The new hardiness map is divided into nine major zones: the harshest is 0 and the mildest is 8. Relatively few plants are suited to Zone 0. Subzones (e.g., 4a or 4b, 5a or 5b) are also noted in the map legend. These subzones are most familiar to Canadian gardeners.

Some significant local factors, such as micro-topography, amount of shelter and subtle local variations in snow cover, are too small to be captured on the map. Year-to-year variations in weather and gardening techniques can also have a significant impact on plant survival in any particular location.

For more details and a close-up view of your local plant hardiness zone, visit the Canadian Forest Service page at: or the Agriculture and Agri-Food Canada page at:
Zone hardiness of some indicator trees

0b shining willow                  Salix lucida ssp. lucida
1  white spruce                    Picea glauca
1  lodgepole pine                  Pinus contorta var. latifolia
1  tamarack                        Larix laricina
1b laurel willow                   Salix pentandra
2  European white birch            Betula pendula
2  white elm                       Ulmus americana
2a cranberry                       Viburnum trilobum
2b Manitoba maple                  Acer negundo
2b ponderosa pine                  Pinus ponderosa
2b Ohio buckeye                    Aesculus glabra
2b hackberry                       Celtis occidentalis
3  little-leaf linden              Tilia cordata
3  Rocky Mountain juniper          Juniperus scopulorum
3  red maple                       Acer rubrum
3b black walnut                    Juglans nigra
3b white ash                       Fraxinus americana
4  ginkgo/maidenhair tree          Ginkgo biloba
4  black locust                    Robinia pseudoacacia
4a Rocky Mountain Douglas-fir      Pseudotsuga menziesii var. glauca
4a sugar maple                     Acer saccharum
4b yellow-wood                     Cladrastis lutea
4b Scotch elm                      Ulmus glabra
5  Norway maple                    Acer platanoides
5  English oak                     Quercus robur
5a Douglas maple                   Acer glabrum var. douglasii
5b tulip-tree                      Liriodendron tulipifera
5b sycamore                        Platanus occidentalis
5b horsechestnut                   Aesculus hippocastanum
6  European beech                  Fagus sylvatica
6  tree-of-heaven                  Ailanthus altissima
6  redbud                          Cercis canadensis
6  goldenchain tree                Laburnum X watereri
6  yellow-cedar                    Chamaecyparis nootkatensis
6  western redcedar                Thuja plicata
6b eastern flowering dogwood       Cornus florida
7  sweetgum                        Liquidambar styraciflua
7b Douglas-fir                     Pseudotsuga menziesii var. menziesii
7b Garry oak                       Quercus garryana
8  arbutus                         Arbutus menziesii
8  western flowering dogwood       Cornus nuttallii
8  Monkey-puzzle                   Araucaria araucana

Zone hardiness of some indicator shrubs

0  northern Labrador-tea           Ledum decumbens
1  Saskatoon serviceberry          Amelanchier alnifolia
1  hedge cotoneaster               Cotoneaster lucidus
1  silver buffalo-berry            Shepherdia argentea
la Labrador-tea                    Ledum groenlandicum
2  Siberian pea-tree               Caragana arborescens
2  Siberian dogwood                Cornus alba `Sibirica'
2  European cotoneaster            Cotoneaster integerrima
2  silverberry                     Elaeagnus commutata
2b common juniper                  Juniperus communis var. depressa
3  winged euonymus                 Euonymus alatus
3  staghorn sumac                  Rhus typhina
3  smooth sumac                    Rhus glabra
3a Canada yew                      Taxus canadensis
3b peegee hydrangea                Hydrangea paniculata `Grandiflora'
4  bouquet blanc mock-orange       Philadelphus `Bouquet Blanc'
4  Japanese yew                    Taxus cuspidata
4  black chokeberry                Aronia melanocarpa
4a American hazel                  Corylus americana
5  common smoke-tree               Cotinus coggygria
5  early forsythia                 Forsythia ovata
5  fragrant viburnum               Viburnum carlesii
5a Oregon holly-grape              Mahonia aquifolium
5b spicebush                       Lindera benzoin
6  Japanese maple                  Acer palmatum
6  slender deutzia                 Deutzia gracilis
6  showy forsythia                 Forsythia x intermedia `Spectabilis'
6  vine maple                      Acer circinatum
7  box tree                        Buxus sempervirens
7  Hidcote hypericum               Hypericum hookerianum `Hidcote'
7  cherry-laurel                   Prunus laurocerasus
8  Japanese aucuba                 Aucuba japonica
8  Chilean pernettya               Pernettya mucronata
8  laurestinus                     Viburnum tinus

The production of the new Plant Hardiness Zones map was made possible through a collaborative effort by scientists at Natural Resources Canada's Canadian Forest Service, Agriculture and Agri-Food Canada and Natural Resources Canada's National Atlas of Canada.
COPYRIGHT 2003 This material is for informational use. Views are not those of the editorial committee. Reference to commercial products is made with no discrimination intended or endorsement by The Prairie Garden.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 2003 Gale, Cengage Learning. All rights reserved.

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Publication:Prairie Garden
Geographic Code:1CANA
Date:Jan 1, 2003
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