Understanding Hardiness Zones

Standard
20170704C.jpg

This is the print version of the USDA hardiness zone map. To find your zone more precisely, click on the interactive USDA hardiness zone map. Illus.: USDA

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.

20170704A.jpg

The AgCan hardiness zone map: click on the plus sign (+) and move cursor for a closer view. Illus.: Natural Resources Canada

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

20170704B.jpgNow 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

20170704E.gif

American Horticultural Society plant heat-zone map. Illus.: AHS

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

20170704E Fred L'Apiculteur.jpg

European hardiness zone map. Illus.: source unknown

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!20170704C

Advertisements

Canadian Gardeners: Beware of US Hardiness Zones!

Standard

Sorry, American gardeners! I know that this blog is mostly read by Americans, but today I have a special message to share with my fellow Canadians. So the information in this blog doesn’t really concern you. However, if you want to understand a problem that causes much frustration to gardeners north of your border (or east if you’re from Alaska), read on!

20150412AEnglishMost Canadian gardeners know their hardiness zone and know that, when it comes to choosing a hardy plant (perennials, trees, shrubs, etc.), it’s best to choose one adapted to their zone or any colder zone. For example, if you live in zone 6, you should choose plants from zone 6 or even colder zones: 1, 2, 3, 4, or 5. If you live in zone 3, you’d choose plants from zones 1, 2 and 3. It’s not really that complicated.

(To find your Canadian hardiness zone, go here).

20150412BWhat is less known is that the Agriculture Canada hardiness zone system does not quite match the one used in the United States, that of the USDA (United States Department of Agriculture). Among others, the American system only considers the minimum temperature in a given region, not the duration of the minimum temperature. But when temperatures drop to -15˚C (5˚F) for one night, it doesn’t affect a plant nearly as much as when the temperature stays at -15˚C (5˚F) for a week. The Canadian hardiness zone map reflects this detail and also other factors (snow cover, etc.), giving a more realistic picture of local conditions.

The result of these differences is that there is about a one zone difference between the two systems. For example, a plant labelled with the USDA hardiness zone 5 is actually a zone 6 in the Canadian system. Here is a chart that compares the two systems:

Comparing the USDA and Agriculture Canada Hardiness Zones

USDA Zone
0
1
2
3
4
5
6
7
Canadian Zone
1
2
3
4
5
6
7
8

So if you read an American gardening magazine or book or visit a gardening website as Dave’s Garden, just mentally add a number to any zone that is indicated. I must admit my own books include both zones, but I believe that I’m pretty much the only author who does that.

When Nurseries Lie to Us

Crossed fingersIf it were only a question of mentally correcting the zones while reading, the situation wouldn’t be so bad, but unfortunately, many Canadian nurseries use, without ever stating it, the American hardiness zone system. I can’t say whether they do this on purpose or if they simply don’t know they’re making a mistake, but you have to admit it is definitely more advantageous for them to exaggerate the hardiness of a plant a little. The result is that many plants labels seen in Canadian garden centres are misleading: they exaggerate the cold resistance of the plant they identify. And that causes much confusion among Canadian gardeners who rely on the plant’s hardiness zone to help them purchase plants hardy enough for their gardens.

This is not a minor problem: each year, due to misleading information, millions of plants are planted in areas where they are not hardy and therefore either die over the winter or are severely damaged by the cold. Indeed, some plants in Canada are almost always sold with the wrong zone. Here are a few examples:

Hardy Orange (Poncirus trifoliatus)
Zone usually indicated: 6
Actual Canadian hardiness zone: 7

Butterfly Bush (Buddelia davidii)
Zone usually indicated: 5b
Actual Canadian hardiness zone: 6b

Rose of Sharon (Hibiscus syriacus)
Zone usually indicated: 5b
Actual Canadian hardiness zone: 6b

Kousa Dogwood (Cornus Kousa)
Zone usually indicated: 5b
Actual Canadian hardiness zone: 6b

So be careful when buying supposedly hardy plants in Canada: some nurserymen have a marked tendency to exaggerate hardiness zones… in their favor! And you’re the one who loses out!

Caveat emptor!