Hardiness zones is a climate mapping concept applied to the continent to give people some guidelines about what plants will survive where.

The North American model looks at variations from the Arctic to the tropical. A number system is used where the smaller the number [1] is the coldest, and larger [10] is the warmest. These ideas are based on the average extreme low winter temperatures. The zones vary greatly as we move around the continent as mountain ranges, oceans, and major climate patterns affect them.

Trees do not grow in zone 1 and somewhere as we head south we pass a line where northern trees, hard wired for deep cold, leaf loss and a dormant period of rest, are not happy. Apples are not grown in southern California and Florida, but oranges, lemons and grapefruits are.

Hardiness zones are not carved in stone, endless variations exist; as I said, they are guidelines. In the prairie west, gardeners are always looking for something new, some variety. As you move through the zones 1 - 10, the available number of plant varieties increases with each numerical increment. People are always going into British Columbia, falling in love with a Japanese maple at a nursery, bringing it home, planting it and in the spring finding that it is stone dead.

The concept also works like this: all zone 2 trees will do well in zone 3, all zone 3 trees will do well in zone 4, and on through the zones, limited only as I described, by the loss of a proper dormant period as we approach the tropics. The reverse, bringing a plant rated for a higher numbered zone to a lower is described above with the story of the zone 5 Japanese maple brought to zone 3.

I am a zone 3 arborist, from the chinook belt in Alberta. Chinooks are a local experience known only to the western areas adjacent to the Rocky Mountain front. A chinook is a period of extreme warm air and high winds that can change the temperature from -25°C overnight to +15°C in the following afternoon. The chinook can blow for a few days and be gone as quickly as it came, leaving the trees shaking their heads in puzzlement, and not a little stressed. All my experience comes from there. Zone 3 is a little unusual in its great breadth and most provinces of Canada from east to west have some area in zone 3, sometimes only a thin ribbon as seen on the hardiness map.

As for all models of classification, there are those who like to clump and those who like to split. There are models where zone 3 has its A, B, and C. This is fine, but when choosing your trees, always go with hardier rather than a variety yet unproven. You may live in zone 3, but in the chinook belt, when joking about our hardiness rating I say zone 3 slash 1.

The more severe your location, regardless of its zone rating, always choose the hardier varieties. Perhaps this is boring, but a dead tree, unable to withstand a severe winter, is also very boring. Plant rating can be useful for choosing appropriate plant material to survive in your zone, but be careful. Let some years go by, and keep track of how they do; the plant’s real hardiness test is a series of quirky winters.

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