Hardiness zones

Every perennial on this site shows a zone range. This page is what that range means, and what it does not.

What a zone number is for

A hardiness zone answers one question: will this plant live through winter where you are. That is it. It says nothing about how well it will flower, whether your soil suits it, or how long your summer is.

The numbers run from 0 to 9. Higher is milder, so a zone 8 garden on Vancouver Island has an easier winter than a zone 2 garden further north. Each zone splits into a and b, and b is the slightly warmer half of the pair.

Canadian zones and American zones are not the same thing

This trips people up constantly, because both maps use plain numbers and the numbers look interchangeable. They are not.

The American map measures one thing: the average coldest winter temperature. The Canadian map measures seven things. Along with the coldest month it takes in the length of the frost free period, rainfall from June to November, how warm the warmest month gets, a winter index built from January rainfall, how deep the snow usually gets, and the strongest wind gust in thirty years.

Snow depth is in there because snow is a blanket. A plant under half a metre of it is having a much gentler winter than the air temperature suggests, and a bare windy garden at the same temperature is not.

Because the two maps are built from different measurements, you cannot convert between them. Natural Resources Canada laid the two over each other and found Canadian zone 4a alone spread across nine different American zones, from 3a to 7a. Places with very different winter cold land in the same Canadian zone, because the other six measurements pull them back together.

So the rule of thumb you will see on garden sites, that the two systems differ by about one zone, does not hold. When a seed packet or a plant label says zone 5, it is worth knowing which map it came off.

Finding yours

Natural Resources Canada keeps the Canadian maps at planthardiness.gc.ca, where you can look up your own spot by town. They publish an American-style minimum temperature map of Canada on the same site, so you can get both numbers in one place. The maps were refreshed for the first time in over a decade, using the last thirty years of climate records, so the zone your parents gardened in may not be the one you are gardening in now.

Annuals do not have a zone

A zone number is about surviving winter, and an annual never tries to. It grows, flowers, sets seed and dies in one season, and frost ends it wherever you live.

So for an annual, the number that matters is not your zone. It is your last frost in spring, your first frost in autumn, and how many days the plant needs between the two. That is why the listings give days to maturity and count sowing back from your own last frost, rather than printing a date that would only be right for one garden.

A zone range is a starting point

Your garden is not a flat square of map. There is a sheltered spot against a south wall that runs a zone warmer, and a low corner where cold air settles and everything dies first. Drainage matters as much as cold does, and a great many perennials that are said to fail in a hard winter actually drowned in a wet spring.

So treat the range on a listing as the honest published figure it is, and then use what you know about your own ground. You know it better than a map does, and better than I do.