Can Gardeners Trust Hardiness Zones?

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.

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.

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!