Understanding Hardiness Zones

It’s amazing how confused some gardeners are about hardiness zones. Yet they’re really quite simple. They’re designed to tell you how cold it’s likely to get in winter in a given region and, since cold is a major limiting factor in plant survival, to determine which plants are likely to survive in that region. The first modern hardiness zone map was developed by the United States Department of Agriculture (USDA) in 1960 (there were precursors dating back to 1938) and others have been developed for other countries over the years.

Agriculture Canada, together with Natural Resources Canada, developed a hardiness zone map for Canada, and, of course, the USDA and AgCan maps don’t fully correspond. Would you expect otherwise? There’s about a one zone difference in the colder zones, largely because the Canadian zone map starts at zone 0, while the US one starts at 1, but also, the Canadian map takes into account other factors in determining its zones than simply the lowest winter temperature, such as snow cover and wind.

As a result, if you live in the US, you should follow the US zone map and if in Canada, the Canadian one.

Once You’ve Found Your Zone

Once you’re determined your zone (click on the appropriate map links above), memorize it. It’s something you’ll need regularly as you garden. I, for example, am a proud AgCan zone 4b (USDA zone 3b).

How Hardiness Zones Work

You will have noticed that there are twelve zones on the USDA zone map (ten on the Canadian one), all numbered. The smaller the number, the colder the winter; the higher the number, the milder the winter. In USDA zone 1 (AgCan zone 0), for example, it’s so cold the ground never fully thaws out, so no horticulture is really possible. At the opposite extreme, zone 12 is the hot tropics where it never gets cold and you can grow coconut palms (Cocos nucifera). (There is no zone 12 on the Canadian map: it’s not needed, as there is no tropical climate in Canada.)

Each zone is further divided into two subzones: a and b. Thus, your zone could be zone 3a or 3b; 5a or 5b, etc. As with the actual zones, where the smaller the number, the colder the winter, the same is true for the two letters, subdivision “a” being colder than subdivision “b”.

Not so difficult, is it?

How to Use Hardiness Zones

Now that you know and understand your zone, you can start to match it to the plants you can grow, as plants too have been classified as to the coldest zone where they will survive the winter. So you need to look for plants that will grow in your zone and any colder zone.

If you live in zone 3b, you need plants hardy to zones 1a, 1b, 2a, 2b, 3a or 3b, but should avoid plants in zones 4, 5 or more, as they’re unlikely to be hardy in your area.

If you live in zone 5b, you get more of a choice. You can choose from plants in zones 1a, 1b, 2a, 2b, 3a, 3b, 4a, 4b, 5a or 5b, but should avoid plants of zones 6, 7, 8, etc.

These days, most nurseries in the US and Canada use hardiness zones to help gardeners tell if a given plant is likely to be hardy where they live. The hardiness zone is printed on the labels of most hardy plants (perennials, shrubs, trees, conifers, etc.), so you can check at a glance at the time of purchase. If the zone is missing, you can usually find it on the web or in a book.

Again, always look for plants hardy to your zone or to any zone with a lower number.

Heat Zones

Note that hardiness zones were never designed to determine how much heat plants can take. So when you see books, web sites or plant labels that give a zone range for a plant, say zone 4 to 8, the latter figure should be taken with a grain of salt. There is an American heat zone map, developed by the American Horticultural Society, but it has never really caught on, largely because it is proprietary and the information has never been widely circulated. It would mainly be useful in areas with very hot summers, as many cold-hardy plants won’t tolerate extreme summer heat and it would be nice to know which ones.

In Europe

Yes, there are hardiness zones maps for Europe, but they are much less used there, largely because the European climate is much more homogeneous than the North American one … and much milder. In Great Britain, for example, almost the entire island lies in hardiness zones 8a and 8b, with only the southwest coast in the warmer zone 9 and a few mountains and part of the north in cooler zone 7. (Compare that to the USA with a 12-zone range!) The differences are so slight that nurseries just tend to mention that plant X would need winter protection in cold areas … and for the Brits, zone 7, which most Americans and Canadians would consider as having mild winters, is bone-shatteringly cold!

Hardiness zones: a tool so easy to use … when you understand them!