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

Can Gardeners Trust Hardiness Zones?

Standard
20151228A

Coreopsis ‘Limerock Ruby’

In 2002, I bought a beautiful coreopsis with red flowers, ‘Limerock Ruby’ (Coreopsis ‘Limerock Ruby’). It was the first time I’d ever seen a red coreopsis (yellow is the usual color for the genus) and I was very satisfied with the results throughout the summer: the plant didn’t stop blooming, producing a cloud of dark red flowers from July to September. Since the plant bore a label indicating it was hardy to zone 4 and I live in AgCan zone 4, I expected it to reappear the following spring. But no, it didn’t grow back. Well, I thought, maybe that the spot where I planted it was a little too wet or maybe a vole had eaten it. In other words, I blamed myself for losing it.

But ‘Limerock Ruby’ quickly became the horticultural Watergate of the summer of 2003. Negative comments started appearing on websites all over North America. It turned out that the vast majority of gardeners had lost their plants of ‘Limerock Ruby’! The truth came out pretty quickly: no one had checked the hardiness of this plant before launching it. The suppliers had simply assumed that it was a zone 4 plant because most coreopsis are hardy in zone 4. Today we know that Coreopsis ‘Limerock Ruby’ is only hardy zones 7b to 10 and has to be used as an annual elsewhere.

This one incident taught a lot of gardeners they can no longer trust the hardiness zones listed on plant labels… but that was over a decade ago and we’re starting to forget…

Plants Released With No Testing Whatsoever

To be honest, the ‘Limerock Ruby’ incident was rather extreme. Rarely is there so much difference between the hardiness zone listed on a label and the real one. It did however reveal a dirty little secret that the horticultural industry has been trying to hide from home gardeners for years: that cold hardiness of new plants is no longer being tested.

20151228B

In vitro culture means plants can reach the market so quickly they don’t have time to be tested.

Nowadays, thanks to in vitro culture, in which tens of thousands of identical plants can be produced from a single cell in only a few months, it is possible to produce massive quantities of a plant long before anyone understands its true behavior… and that’s quite a change from the old method.

In the past, new hybrids could only be produced by division or cuttings and that meant years went by between the moment the new plant was judged interesting enough to be produced and the moment it first appeared in the average garden center. In the case of a hosta hybrid, for example, it could easily take 20 years between its creation and it becoming widely available. And over 20 years, there is plenty to time to trial the plant. If there was a problem, it would have had time to come out before the plant reached the market.

Today, a new hosta, echinacea or spirea often reaches to market with only 2 years of experience behind it. Very few if any of new plant introductions that we see each spring have been tested adequately as to their hardiness.

Because of this, many wholesale nurseries now simply wipe their hands of hardiness concerns. To protect themselves from criticism, they simply put “Zone 5” on all the new plants they produce they think ought to be hardy (zone 5 is fairly safe, a sort of “middle-ground hardiness zone” that most hardy plants will grow in). And if they suspect it might be a bit iffy in zone 5, they’ll label it zone 6. This is very discouraging for gardeners like me who lives in colder regions than average (I live in USDA zone 3, AgCan zone 4), because if you trust the label, almost no new introductions would appear to be hardy.

Inertia Reigns

Even when a plant has been on the market for 4 or 5 years, long enough to have a at least a decent idea of its hardiness, its label may continue to lie, especially when the plant turns out, as it often does since the ‘Limerock Ruby’ incident, to be hardier than the label says.

This is mostly due to inertia: someone at the wholesale nursery that supplies it has to really feel this detail is important enough to be worth changing. But few are much concerned about catering to the small number of gardeners who live beyond zone 5. So what if the label says zone 5 and the plant is really hardy to zone 3? That’s only of interest to one gardener in 50! It’s so much easier just to leave the label alone.

Of course, changing a label would also be expensive. What to do with the thousands of labels already printed? Thus, even years after nurseries have discovered that a plant they offer is hardier than previously thought, the label often continues to underestimate its hardiness.

And it’s not only the plant’s hardiness that is often incorrect on the label. Any information about the plant that was unknown when it was launched will tend to continue to appear on the label of the plant pretty much forever. For example, how many labels of the oh-so-popular ninebark ‘Diabolo’ (Physocarpus opulifolius Monlo) continue to show a height of 5 feet (1.5 m)? Yet any amateur gardener knows from experience that it easily reaches 10 feet (3 m)! Do you honestly think that suppliers don’t know that? They do and have known it for 15 years now, but… changing the label would be inconvenient. That’s inertia at work!

Canadian Gardeners Beware

American and European readers can skip this next part, but Canadian gardeners have yet another “hardiness zone” consideration to deal with.

2015128D

AgCan hardiness zone map.

Another overlooked factor in plant labeling is that there is difference between the American hardiness zone map (USDA) and the Canadian one (AgCan). If there is a one-zone difference, it’s because the two countries don’t use the same criteria in creating their hardiness zone maps. Thus, a USDA zone 4 plant would actually be a zone 5 plant in the Canadian system. And a USDA zone 5 plant would actually an AgCan zone 6. So when you read an American gardening book, Canadians have to mentally add 1 to the zone listed to obtain the right zone.

  USDA             AgCan

      1                     2

      2                     3

      3                     4

      4                     5

      5                     6

      6                     7

      7                     8

      8                     9

      9                    10

To find the Canadian hardiness zone, add 1 to the American zone.

As long as it’s just a matter of books, the math is easy enough to do: Canadian gardeners need to add 1 to all zones if they are reading an American book; add nothing if they’re reading a Canadian book. But did you know that many Canadian nurseries use American zones on their labels? After all, if they’re dealing was a USDA zone 5 plant, they really should be showing AgCan zone 6 on the label. But in doing so, they would lose a lot of sales because the majority of Canadian gardeners live in AgCan zones 5 and below. If they use the USDA zone for that plant (zone 5), the plant is likely to sell better. So they simply go with the most advantageous zone for their sales. I’m convinced that most Canadian gardeners are not aware of this. So when a plant doesn’t survive the winter, they don’t suspect the fault is not theirs!

The result is that the discriminating Canadian gardener who sees zone 4 or 5 on a label really doesn’t know if he can rely on it or if he should add 1 to get the right zone for his needs.

This would be so simple to fix: all wholesale nurseries selling should be required to mention what hardiness zone they use: USDA or AgCan. But they don’t.

Hardiness Zones: Not Worth the Label They are Printed On

This whole situation is so sad! The very hardiness zones that are meant to help gardeners to make a reasoned choice of plants can no longer be trusted. Instead each gardener becomes a sort of horticultural guinea pig when it comes to trying new plants.

Personally, when I shop for new plants, I have learned to ignore the zone and trust my instincts… but then I’ve been gardening for over 50 years. And I confess to sometimes getting things wrong.

Beginning gardeners, especially those in colder climates, would probably do best to either trust a neighbor with good gardening knowledge or see what other gardeners are saying about new plants on the Internet. What can I say when it comes to plant hardiness labels but caveat emptor!

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!