Plant Hardiness

Find your zone!

It’s fun to plant native trees or grow ornamental grasses with unpronounceable names, but we should never lose sight of the fact that we’ve all been there: the novice gardener. Not knowing what a petunia looks like (been there, done that). And not understanding hardiness zones!

We remember the moment when, full of enthusiasm, we walked into a real garden centre, ready to buy shrubs and perennials to brighten up the edges of a patio. We explained our great project to a garden centre specialist. Then came the disconcerting question: “What’s your zone?”

Your zone? Your comfort zone? Your industrial zone? Your offensive zone in your garage hockey league? “Are you 4b or 5a?” asked the specialist.

The garden centre expert is actually talking about hardiness zones. There are two systems, one for the United States (we call them USDA zones) and one for Canada. Today I am going to talk about the Canadian system. To explain it simply, let’s say that Canada is divided into zones, each with a number, according to the severity of the climate. The colder it is, the lower the number. This zone allows you to know which plants will grow well in which location. Just like sunlight or soil type, the hardiness zone is another factor to consider when choosing plants for your garden.

Hardiness

Firstly, hardiness is the ability of a plant to survive the cold and various winter conditions. All plants that survive the winter in a region without damage are said to be hardy in that region. Hardiness mainly applies to trees, shrubs, conifers and perennials. Annuals are generally not hardy, and it does not matter anyway as they are replaced every year. Hardiness is not a gift from heaven. Plants have developed a number of mechanisms to survive winter.

First, the Plants Prepare for Winter

The composition of plants differs between summer and winter. If there was a terrible frost in mid-July, many so-called hardy plants would suffer because they are not prepared. This process of preparing for winter begins in August and is called hardening.

Shorter days mark the beginning of hardening. Growth slows down. Trees begin to turn soft shoots into hard wood. Then, as temperatures cool, plants begin to build up reserves in their roots. Deciduous plants shed their leaves. Perennials drop their aerial parts. Some plants even have a kind of antifreeze, like the one you put in your car, that allows the water in their cells to withstand temperatures as low as -40°C.

In this way, each plant prepares itself to withstand a certain degree of cold. We have therefore identified for each plant its limit, the northernmost zone in which it can grow. For example, a larch (Larix laricina) can grow in zone 1a, which means it is happy in Labrador City. But it is also happy in zones 2, 3, 4 and 5. On the other hand, the magnificent perennial hibiscus of the Luna series, which is hardy in Zone 4, is a little ‘restricted’ in its zone. It is unlikely to survive winters in zones 3, 2 or 1. It is also good to note that in the USA, hardiness zones are usually given as a range, for example USDA zone 3 to 9, as the southern heat can also be a limiting factor for plant survival. Some plants will not survive prolonged periods of heat, or may simply need a dormant period to stay alive.

Already in the fall, most plants have long since begun a process of preparing for winter, which is called hardening. Photo: Julie Boudreau

The Map… or Maps

The first hardiness map for Canada was published in 1967. Since then, climate information has multiplied and we now have a more accurate map. The zones are determined by the average temperature of the coldest month, the number of frost-free days, precipitation, the temperature of the warmest month, January rainfall, snow depth and wind gusts. By combining all these factors, we obtain the following map .

Exceptions to the Rule

Although the map is a good starting point for identifying your zone, it can vary greatly within a simple 15 km radius. For example, a plot of land surrounded by a large cedar hedge has a microclimate and can grow plants that are one zone higher (zone 5 plants in a zone 4 garden). Also, there are many testimonies of people who grow lavender in zone 3, when the plant is zone 4. The same lavender will not be happy in Montreal, which is zone 6. The accumulation of snow and the heat of the summer have a lot to do with this. The owner of a plot of land exposed to strong winds should choose plants that are one zone below the prescribed zone.

The important thing to remember about hardiness zones is that they are good indicators to start with, but if you know your garden well, you will eventually know what will grow there and what will not. So the next time you are asked THE question, you can say, “Well, yes, my zone is 4b, but I have a microclimate…”.

Find your zone! The following list indicates the hardiness zone for most major cities in Canada.

By growing plants that are hardy enough for our area, we avoid the hassle of having to use winter protection. If we choose hardy plants, winter protection is unnecessary! Photo: Julie Boudreau

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Julie Boudreau is a horticulturist who trained at the Institut de technologie agroalimentaire in Saint-Hyacinthe, Québec. She’s been working with plants for more than 25 years. She has published many gardening books and hosted various radio and television shows. She now teaches horticulture at the Centre de formation horticole of Laval. A great gardening enthusiast, she’s devoted to promoting gardening, garden design, botany and ecology in every form. Born a fan of organic gardening, she’s curious and cultivates a passion for all that can be eaten. Julie Boudreau is “epicurious” and also fascinated by Latin names.

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