Gardening Plant Hardiness

Can Gardeners Trust Hardiness Zones Anymore?

Think twice about trusting the hardiness zones printed on plant labels these days! They’re often way off! Ill.: Clipart Panda, berserkon.com, PinClipart.com & PikPng, montage aidbackgardener.blog

I’m a big fan of truth in advertising in the gardening world. I mean, a home gardener should be able to get what they pay for, don’t you think? 

Well, if you’re counting on finding accurate information on the label that accompanies new plants, you’re probably in for a disappointment, especially when it comes to the hardiness zones listed. Yes, there are serious nurseries who label their plants correctly as to how much winter cold they can take, but there’s a whole slew of them who, to be frank, are just out for your money and are consciously lying to you.

How the Dirty Little Secret Came Out

Several plants of coreopsis limerock ruby with small, dark red flowers in a flower bed.

 Coreopsis ‘Limerock Ruby’: the plant that taught many gardeners not to trust plant hardiness zones on plant labels. Photo: geraniumrozanne.com

In 2002, I, like thousands of other gardeners across North America, bought a beautiful coreopsis with red flowers, ‘Limerock Ruby’ (Coreopsis ‘Limerock Ruby’). It was the first time I’d ever seen a truly 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 in zones 4 to 9 and I live in zone 4, I expected it to reappear the following spring. But it didn’t grow back. Well, I thought, maybe that the spot where I planted it was a little too humid or maybe it wasn’t quite sunny enough … or maybe I should have cut it back in September to force to rest after such a long season of bloom. 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 it wasn’t just me: the vast majority of gardeners had lost their plants of ‘Limerock Ruby’! 

The truth came out pretty quickly: no one had bothered checking the hardiness of this plant before launching it. The suppliers had simply assumed that it was a zone 4 plant because the hybrid was largely derived from the species C. rosea, considered hardy to about that: zone 4.

Today we know that Coreopsis ‘Limerock Ruby’ was actually developed from a very frost-tender strain of the species, one from the extreme south of its range, and is only hardy in zones 8 to 10. Yes, it’s practically subtropical and it can only be used as an annual in most temperate climates. 

Would you believe, though, that, although some nurseries now do give the right hardiness zones for this plant, others haven’t bothered updating their information and still offer it as a zone 4 to 9 plant. That’s nearly 20 years after the news broke!

This one incident taught a lot of gardeners they can no longer trust the hardiness zones listed on plant labels … but that was a long time. Not only is there a whole new generation of people gardening today, but a lot of us older folks have had time to forget.

Plants Released With No Hardiness Testing Whatsoever

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

The Way Things Were

Black and white photo of flower garden in the 1920s

Not so long ago, plants were only released after they had been thoroughly tested. Photo from The Complete Garden by Albert David Taylor

It hasn’t always been that way.

First, there was a sort of hybridizer’s code of ethics that had been applied for almost since hybridizing new plants first became big business in the mid-19th century.

It called for plants to be thoroughly vetted before they were released. How long to trial them depended on the plant, but often 4 or 5 years for fairly fast-growing perennials and 10 years or more for slower growing perennials, trees and shrubs. During that time, the plant had to prove it was stable and not given to reversions and mutations. And that also left enough time to test it in other gardens under various climatic conditions, as typically nurseries all over the world would trial the plants on their grounds as they built up stock. So, when the plant was finally launched, all its main characteristics, including its cold hardiness, were pretty well known.

This wasn’t much of a constraint for hybridizers: they were used to long waits. At the time, no one expected a new plant to reach the market for years at any rate, because multiplying large quantities of them was such a slow process. At the time, new hybrids of hardy ornamental plants (perennials, shrubs, trees, conifers, etc.) could only be produced by division, cuttings or grafting. Perhaps you could make 10 cuttings from a plant one year, then take cuttings from cuttings to develop 100 the next. And so on. Since thousands were needed for a proper launch, years passed. With divisions, usually fewer per plant, the process was even slower. 

That meant years went by between the moment the new plant was judged interesting enough to be produced commercially 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 original development and it becoming widely available. 

The Situation Today

Tissue culture laboratory (4 photos) showing plants in glass beakers.

Most new introductions these days are produced by the thousands by tissue culture laboratories: one plant can even become millions of plants in just a few short months! Photo: Красноштан Василь Ігорович, Wikimedia Commons

Nowadays, most new plants are produced by tissue culture, in which tens of thousands, even millions, of identical plants can be produced from a single cell in only a few months. It’s now therefore 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.

Logically, in such a situation, you’d thoroughly vet the plant first, then multiply it. That does happen to a certain extent, but there are a lot of nurseries that don’t bother. They create a new plant through hybridizing or selection. It looks good. It holds up well in a pot. If so, it’s good to go. Wham, bam: within two years, there are 10,000 clones in garden centers. 

But what about its garden performance? Well, as long as it sells well, is that really important? At least, that seems to be the attitude.

What About Trial Gardens?

Trial garden with multiple splotches of colourful leaves and flowers.
Trial gardens these days tend to showcase plants, not really trial them. Photo: Tom Miller, Flickr

Of course, there are still trial gardens, indeed all over North America and Europe. Surely they’re testing for hardiness? But no, or only very rarely

Today’s trial gardens seem to see their main role is to feature the new plants that will be launched the following season, a sort of outdoor, summer long garden show highlighting upcoming varieties. The new plants are simply popped into the trial garden in the spring, displayed, enjoyed, compared and commented on, then removed before winter. 

That’s fine for annuals, designed for one-season use anyway, but what about plants that are supposed to live for years in the garden? You’d think a 3-year trial would be a minimum, because surviving winter is a challenge for plants and after 3 different winters in trial gardens everywhere, there’d be at least idea of its resistance to cold. But that’s not what happens. The only aspect of interest today seems to be how the plants looks when grown for one single season, as if it were an annual. 

Perennial trial featuring echinaceas.

There are a few trial gardens that do carrying on multi-year trials of hardy plants… but usually ones already on the market. Photo: Rick J. Lewandowski, Mt. Cuba Center

The few trial gardens that still test perennials and shrubs for more than 1 season generally spotlight plants already on the market, when the horse is already out of the barn, so to speak. How nice to learn from a report from such a trial garden that the plant you bought was a total dud … two years after you had already discovered that from personal experience!

Today, a new hosta, echinacea or spirea often reaches to market with only 2 years of experience behind it. Usually straight from the hybridizer’s nursery, and thus only tested, at best, in one zone. However, many have never spent even a single winter outdoors in any climate, cold or not, but have lived their entire short existence within the protective enclosure of a greenhouse! Only a very few of the new plant introductions that we see show up each spring in garden centers have been tested adequately, especially when it comes to their ability to survive tough winter conditions. 

The Rush to Release

Drawing of man stealing plants.

Industrial spying and plant thievery are becoming the rule rather than the exception in the horticulture industry. Victor Kerlow, choppedintwo.blogspot.ca

The pressure to launch new plant products is intense these days. There’s big money to be made if you have an attractive and novel plant you think the gardening public will like, yet if a nursery lingers too long, competing nurseries, who carefully eye each other’s progress, can copy the cross, and, with tissue culture to help them out, quickly bring out their own version. There have even been cases where plants or cuttings of new hybrids have been stolen and the thief launched the plant under a different name before the more cautious originator was even able to react. (The world of new plant introductions is a dog eat dog one!) A lot of old-time hybridizers have simply left the business, discouraged by the backstabbing and unscrupulousness of today’s gardening world. 

Plant hardiness? Who Cares?

Because of this, many wholesale nurseries now simply wipe their hands of hardiness issues. They can’t be bothered to even consider it. To protect themselves from any criticism, they simply slap “Zones 5 to 8” on all the new plants they produce they think ought to be hardy, a sort of middle-of-the-road hardiness zone range that most hardy plants would be likely to grow in. And if they suspect the plant might be a bit iffy in zone 5, they’ll simply label it zones 6 to 8. 

This is very discouraging for gardeners like myself who live in regions colder than zone 5, because if you were to trust what the label says, almost no new introductions would appear to be hardy where I live (zone 4). 

Inertia Reigns

Even when a plant has been on the market for 4 or 5 years, long enough for there now to be at least a decent idea of its hardiness, its label will probably 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. 

Yes, many of the untested plants labeled zone 5 or 6 turn out to be much hardier than that: hardy to zones 2 or 3 in some cases. Yet once a plant is released with a false hardiness zone rating, that misinformation unlikely to ever be corrected. 

This is mostly due to inertia: someone at the wholesale nursery that supplies the plant has to really feel this detail is important enough to be worth doing something about it. The new info has to be added to the computer, new labels have to be printed, clients have to be advised to change their own labeling, etc. 

Plant label for a garden phlox sowing it to be hardy to only zone 5, while it is actually hardy to zone 3.

This garden phlox has proven itself hardy well into zone 3, but the label still says zone 5. Photo: Emerald Coast Growers

But in reality, who in the industry gives a hoot about catering to the small number of gardeners who live beyond zone 5? So what if the label says zones 5 to 8 and the plant is really hardy to zone 3? That’s only of interest to maybe one gardener in 50, likely from some climatically challenged region in Canada or Siberia of limited importance to suppliers! It’s so much easier just to leave the label alone.

Pots with coloured plant labels.
Changing a plant label can be expensive, but is it legitimate to leaf to continue to sell plants under false pretences? Photo: okishop.co.uk

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 clearly hardier than previously thought, the label usually continues to underestimate its hardiness.

Drawing comparing given height of diabolo nine bark with its true height, with woman's figure as a comparison.

The dimensions of ninebark ‘Diabolo’ (Physocarpus opulifolius Monlo) as printed on the label (left). Its true dimensions (right), are twice that. Ill.: Hydro-Québec, montage: laidbackgardener.blog

And it’s not only the plant’s hardiness that is often incorrect on the label, by the way. 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 mature dimensions of 5 feet × 5 feet (1.5 m × 1.5 m)? Yet any home gardener knows from experience that it easily reaches twice that: 10 feet × 10 feet (3 m × 3 m)! 

Do you honestly think that suppliers don’t know that? They do and have known it for 20 years now (‘Diabolo’ was launched in 2000), but … changing the label would be inconvenient. Some growers have updated their labels (thank you!), but many haven’t. 

That’s horticultural inertia at work!

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 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 trialling new plants.

Personally, when I shop for new plants, I have learned to ignore the zone on the label and trust my gut instinct … 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 labeling, but caveat emptor!

Garden Communicators Also at Fault

It pains me to also point out that garden communicators (writers, lecturers, bloggers, etc.) are also at fault. I’m one, so I should know. 

Larry Hodgson, the laidback gardener, signing a book.

I try hard to get correct information to home gardeners. Not all garden communicators do. Photo: laidbackgardener.blog

When writing about a plant, they should do due diligence: going to more than one source for information and doing a bit of digging. They should not just be parroting what the nursery selling the plant has been pushing as pertinent information. After all, that nursery has everything to gain from making the plant seem as desirable as possible, even if that means lying about it. There are almost certainly people in the horticultural industry who do have pertinent information about the plant, often something from personal experience. All the garden communicator has to do is check around … but so many don’t.

When I write about a plant, I do a lot of digging into its behavior and talk to a lot of people. True enough, I admit I have a special interest in making sure I give the appropriate plant hardiness zone, given that I live on the outer edge of what many conceive to be the gardenable world and share information with gardeners in even colder climates. That makes for a lot of research, but, I dare to hope, a more honest piece of writing. Yet, I see a lot of garden communicators who just repeat the supplier’s information as it were the word of God. Shame on them!

And shame on the entire industry of horticulture for not being honest with a major end client, the home gardener. They’ve taken a valuable piece of gardening information—the hardiness zone—the degree to which a plant can adapt to cold—and made it essentially useless. 

Such sad situation!

Text based on an article originally published on December 28, 2015.

Larry Hodgson is one of Canada’s best-known garden communicators. He has notably been editor-in-chief of HousePlant Magazine, Fleurs, Plantes et Jardins, À Fleur de Pot and Houseplant Forum magazines and is currently the garden correspondent for Le Soleil and radio garden commentator for CKIA-FM Radio. He has written for many garden publications in both the United States and Canada, including Canadian Gardening, Harrowsmith, Horticulture, Fine Gardening and Organic Gardening. He also speaks frequently to horticultural groups throughout Canada and the U.S. His book credits include The Garden Lover’s Guide to Canada, Complete Guide to Houseplants, Making the Most of Shade, Perennials for Every Purpose, Annuals for Every Purpose, and Houseplants for Dummies, as well as nearly 60 other titles in English and French. He is a past president of the Garden Writers Association (now Garden Communicators International) and the winner of the prestigious 2006 Garden Media Promoter Award offered by the Perennial Plant Association. He resides in Quebec City, Quebec, Canada.

16 comments on “Can Gardeners Trust Hardiness Zones Anymore?

  1. Good info………… but you should also perhaps talk about how the geographic range of the hardiness zones has shifted over the last few decades. with climate change many places are just enough warmer to warrant a new zone. AgCanada has been updating. A while back, I found a handy chart from Alberta Agriculture, dated 1997, that listed the percent likelihood of the last and first frost day for the prov, including my town of Edmonton. It mainly confirmed the long tradition, plant on Victoria Day. But I knew I had been successful planting much earlier, so I started making a spreadsheet of actual dates (when night was zero or less) going back about 30 years to (then) 2018, By my calcs, the date should be closer to May 12 these days. Adapt to the reality of the times.

    • It gets complicated, though. Where I live, we now have longer, colder springs than in the past: I’ve lost about 2 to 3 weeks spring gardening time. I used to plant out on Victoria day 40 years back, but now I have to wait until about mid-June. However, our falls are longer and warmer (well, most years). And no sign of significantly warmer winters here, only more irregular temperatures. Hard to explain.

      • yes indeed. that’s why I prefer climate change to global warming. there are winners and losers. Here, some migrating birds lack food because their normal arrival time means insects have already come and gone. with you they likely have to wait awhile.
        Aside: 40 years ago I wasn’t gardening but I did do some tissue culture experiments while studying biology at the universite du montreal. I thought it would be a great subject for a masters, but the prof said “you can’t do a thesis on a technique”!! so I eventually moved back to edmonton and shifted to a new field.

  2. Very informative post. I was tired of yellow Coreopsis so moved them to the property border and bought number red ones. I loved them and couldn’t wait for them to pop up the second year. Not so much. I never saw them again but some of the yellow ones came back. Nice to know I wasn’t alone. 🙂

  3. Christine Lemieux

    Me too! I lost a Limerock Ruby and thought perhaps my soil was too wet. Great article. I have often tried to figure out if tags are using US or Can zones. I am a Can Zone 5. Guess I don’t have to think about that too much, as they are often just guesswork anyway! BTW, I too have lost gardening time in spring, but tomatoes were still ripening the end of last October!

  4. Wow, who knew! Very informative as usual!

  5. SO TRUE! Inaccuracies in regard to climate zones are not much of problem here just because the climate is so accommodating to just about anything we plant. However, I do notice many issues with other aspects of descriptions, particularly the resiliency of new and improved cultivars that are obviously weak. I am convinced though, that many new and improved cultivars are designed to be weak so that they need to be replaced by more new and improved cultivars. What makes it worse is that so many exploit the ‘sustainability’ craze.

  6. Nice! Informative article. I think you also consider the changes in climatic conditions over the past few years. Due to this there is also change in hardiness zones. A few years back, I bought green mountain boxwood hedges and they are growing well. I discussed the zones and climatic conditions with the people from where I have purchased boxwood.

    • I do write about climatic change as it concerns gardening every now and then, but it’s a very complicated issue and there is no clear one-way movement. In my area, for example, we have cooler springs than 40 years ago (about a 2 to 3 week loss), but longer, warmer falls. And our winters are not any warmer (-41 last year: the second coldest temperature ever recorded). I suspect that what a lot of us are experiencing in winter is not really a warmer zone, but more irregular weather, going from mild to bitterly cold more frequently.

  7. Agree with all the above comments, but also am finding I need to consider summer dryness tolerance these days, and whether (particularly with vegetables and annuals) plants require a number of days over 80F to do their stuff. And while my winter lows have certainly improved, my frost free season isn’t really predictably longer despite global warming. So I tend to research what climate the plant species the plant species originated in, and then try it in case. Further, some nurseries (Tony Avens) have changed likely growing zones based on information from clients. And Dave’s Garden allows people to add where they are successfully growning a plant. Those of us with northern challenges become very creative about sourcing plants. Now, if I could only find someone to fix the four hooved rat problem…

  8. I had no idea – so interesting, thanks!

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