Zone 4 York Region on the Canadian Plant hardiness zone map.

Perennial Plants and Their Cold Hardiness. "Hardy" isn't always good enough!
All plants started their existence on this planet in a particular type of climate, then evolved over time to survive within the extremes of that climate and adapted to changes over the millennia to survive.

Our garden plants have come to us from all parts of the world, so our job as gardeners is to create conditions that ask all these differing plants to come to a compromise and adjust to OUR climate.    This is how our most popular garden plants became our most popular garden plants!   They're the ones that showed enough flexibility in their site condition demands to be able to grow in conditions beyond those of their native habitat.

A "hardy" plant is named as such if it is adapted to life in a climate with seasonal cycles of warm summers / cold winters.   This is referred to as a “temperate” climate. (Just HOW hardy though, is a further distinction which is discussed later.)     In the case of hardy perennials, from wherever in the world, they have learned how to alter their cell structure through the production of “antifreeze” to live through the dormancy conditions of a winter freeze. (in a perennial, that means the roots. In a shrub, that means their above ground parts, too).   This cold climate plant adaptation means they NEED a period of freeze to regulate when new buds grow and when to flower for seed dispersal. A hardy annual is a plant who’s seed can survive in a temperate climate's winter freeze up, before sprouting.

A further distinction of the word hardy, relates to our Plant Hardiness Zone Map.   The term “hardy” is applied to plants that can survive a zone 6 or colder temperate climate, as indicated by the zone map, where frost enters the ground in winter and air temperatures are below zero.   We are zone 4/5 here in York Region ... the Niagara area is zone 6/7 ... cottage country about an hour north of here is already into zone 3. All these areas are cold climates, but just HOW cold is what the hardiness map tells us. The different numbered areas indicate mimimum average temperatures above ground in winter; how deep the frost level goes below ground; and how many days that are above zero during the growing part of the plants' season before temperatures start to go below zero again.

In other words, unfortunately for us here in regions colder than zone 6, when plant information tags proclaim optimistically that a plant is “hardy”, that doesn’t always mean it will survive for us here in zone 4/5.   In zone 6, the ground freezes only a few inches, and above ground temperature extremes are around –15 in winter.   In zone 4/5, the ground freezes solidly at least a foot deep, and average low temperatures extremes are –20.   That means the “antifreeze” produced by a zone 6 adapted plant isn’t necessarily powerful enough to still work at our lower temperatures.   This problem of interpretation is faced everywhere we research - a web site's plant information that calls such and such a “hardy” perennial, misleads us into thinking we can grow it here when it may be only OK in zone 6 hardiness zone.   We need to look for plants that are hardy to at least zone 5.   Zone 4 ideally, for shrubs in particular.

Half-Hardy   This is a lesser used term in Canada or the US, but is still very much used in any UK or European plant reference sources, especially all the tremendously informative older books.   A plant is said to be “half-hardy” if it is also adapted to a “temperate” climate, whre there's still a cold winter and warm summer climate, but one where the temps never get much below O and the ground doesn’t freeze — more or less zone 7 to 9 on the plant hardiness maps. Here in York Region, a half-hardy perennial can live through autumn cool temps without harm, but will need to be brought indoors before ground freeze. That’s the main difference that makes them only “half” hardy — ground freeze. Their roots never developed the ability to make necessary anti-freeze to survive freezing, which means their cell walls rupture as soon as ice crystals form.   A half-hardy annual’s seed will overwinter and sprout in spring here too, although not as reliably.

Tender    These are plants native to tropical climates - in other words climates based on wet/dry growth seasons rather than cold/warm, where temps never go near freezing point. In perennials, cell structure changes have more to do with protecting against drying out in extreme heat of this climate's dormant season rather than surviving a freeze.    In our climate, these are the plants that collapse at the very hint of autumn frost, whether perennials or annuals. They are completely unfamiliar with how to cope with cold.   (Most of our favourite “annuals” are actually tender perennials, not true annuals. They're called annuals to just keep it simple for consumers, to tell them not to expect them to live over winter.)

We are most concerned with cold, but plants also have a limit to their heat tolerance and have a southern range limit too.   Zero in on exactly the right species of plant you have and properly research it's hardiness limitations.   Here's an example of the various species of Potentilla and their hardiness range of ideal growing conditions.

Potentilla alba Z4-7    If you live in zone 3, a very cold area, this plant won’t survive winter unprotected. The plant also won’t tolerate the extreme heat of zone7 or higher.

Potentilla fragiformis Z5-7    If you’re in zone 4 or colder, or in zone 8 or warmer, this won’t survive without protection. (By mulching or burlaping though, you might get away with it.)

Potentilla nepalensis Z6-8    In zone 5 or colder, or in zone 9 or warmer, this plant won’t survive.
As mentioned in the “fragiformis” description, protecting with mulch or burlap can make a difference of 1 zone or so, but getting a plant past 2 zone barriers is unlikely.

In warm climates some plants don’t survive for totally different reasons than the climate being too cold for them. Lets take the above Potentilla alba as an example. In zone 3 it will die because its too cold and cells rupture in winter. In zone 8 where there's no ground frost to worry about, but the plant still won’t thrive for a different reason—either it needs a frozen dormant period of at least 2 months to stimulate new growth periods, or it just can’t stand the heat of a zone 8 summer and either burns or suffers mildew diseases.

Let good zone hardiness information guide you, but not dictate to you!
Tucked away in the shelter of your mature evergreens are pockets of ground where even the slight lessening of ground frost means that it's a spot where plants only hardy to zone 5 or 6 will survive. You can also mulch perennials thickly over winter to prevent frost going so deep, and carefully remove the mulch bit by bit to catch the warmth of early spring. Not-quite-so-hardy shrubs can be somewhat helped along with a burlap wrapping or a temporary shelter of some collected evergreen boughs. Sandy soil warms quickly and doesn't hold frost so tightly, so if you're blessed with a nice light sandy soil, you have a slight edge over those of us with more clay-ey based soils. These hardiness zone indicators are only guides - find those spots around your grounds where for one reason or the other conditions are more moderate.

                                                        Cheers! _Evelyn
An excerpt from the handouts in the Garden Possibilities "the Language of Plants" lesson. Visit the "classes" page at www.GardenPossibilities.com for more information.

© Evelyn Wolf, 2019 All rights reserved. Please contact for permission to use.

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Zone designations are guidelines, not absolutes. Every region has pockets that are more sheltered or more exposed that can make a difference to which plants just barely survive and which plants thrive.   Even individual Newmarket area properties can have their own “micro-climate” within the larger picture of the regional zone.
For example, a Newmarket backyard with mature cedars sheltering from winter’s north winds and the moderating effect of high density in-town housing, means their garden is essentially a zone5 climate rather than the general region’s zone 4.   However, just a tiny bit north in Queensville, gardens are wide open to northerly winds, with less concrete and brick around, so is a typical zone 4 climate.
Knowing your zone, and then the zone limit of the plant you want to grow, is important. Garden center plant tags often us the American USDA zone map, not our own more detailed Canadian zone map! Some tags do refer to the Canadian map but unless you know the difference, you could be misled into thinking that a plant is OK for our zone 4/5 climate.You might be doing everything absolutely right for a plant, but if it’s just that bit too cold here for it to overwinter without damage, there’s a limit to what you can do to help it.

...an excerpt from the handouts in the Garden Possibilities "Understanding hardiness Zones" lesson. Visit www.GardenPossbilities.com for information.

back to Hardiness Zone index