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Days Until First Frost

Zoning

By Elizabeth Spence

What zone are we in?  What is a zone anyway?  Why do we need zones?

These are some of the questions we have been asked over the years.  And good questions they are.

Simply put, a “zone” in gardening is the rating system for tree, shrub and perennial plant hardiness. You can check to see if  the number of the zone given for a plant is the same as that of your location. If it is, then the plant will probably survive the winter and come back next year.  If it isn’t, then you have to rethink what you want the plant to do for you, and whether you want to risk growing it at all.

In Canada, the climate is highly variable.  Many plants that grow in Vancouver, won’t grow in Yellowknife or Pugwash.  This is why a Government of Canada Plant Hardiness Zone Map is a useful initial tool. It tells you what your zone is.

It divides the country into different hardiness sections: 0 being the coldest (up north), and 9 being the warmest (Vancouver Island).  There are sub-categories of “a” (slightly cooler) and “b” (slightly warmer).

Canadian hardiness zones are based on data collected over 30 years, and include factors such as elevation, length of frost-free period, summer rainfall amount, humidity, minimum and maximum temperatures, snow cover, and maximum wind speed.

Government of Canada's Plant Hardiness Map for Our Area

As you see, here in Nova Scotia, our general categorization is zone 5 to 6.  Along the Atlantic coastline and from Halifax  south, you will find zone 6a and b, but the rest of the province is 5a or 5b.  That means that for plants to succeed here on the North Shore, the zoning number should be 5 or lower. 

So, lucky us: we can grow an array of arctic plants.  In my previous garden I had a zone 2 Silver Buffalo Berry bush (Shepherdia argentea) which thrived like anything. It was gorgeous.

If we grow plants marked between zones 6 and 9, they are less likely to survive the winter here.  In fact, don’t even think about Zones 7, 8 or 9, unless you want to treat the plant as an annual, where it will only live until winter comes. The cold will kill it.

Cold Shock in a Plant. It's a Goner.

 

But then you ask: “Why did my zone 5 plant fail in my garden?” “Why did a zone 6 plant survive this winter?”

A lot of it has to do with micro-climates.  Some sections of our gardens might be exposed to vicious nor’-easters, some are protected by walls or fences or trees, some are next to a building facing south, some are on a slope, some are in soggy ground or next to water or near the sea. 

Fundamentally, micro-climates have to do with temperature, moisture and sunlight, and they can vary significantly within one garden. 

So you might have pockets of zone 5a, 5b, 6a, and 6b in the same garden,  Perhaps a bit more, perhaps a bit less.

Being aware of where you place your flower bed or vegetable patch in the landscape is essential for understanding how your plants are likely to perform.

Microclimates are so distinct that you have to work them out for yourself by trial and error and observation.  It’s experience as well. No zoning map can provide that.

It is such fun to create your own little microclimates too: build a little wall here, put a pond in there; create a slope somewhere else, plant some bushes, install a raised bed, build a greenhouse even.

Artist's Impression of Microclimate Features

 

Can I trust Zoning Numbers?       Answer:  Well, sort of, but not always.

There a few more factors at play here that make determining your zone a bit trickier.

First of all, there is the USDA Plant Hardiness Zone map which uses another system.  

It is based only on the average annual minimum temperatures, nothing else, and the zoning numbers are therefore a bit different.

The general rule of thumb is that you add one to get your Canadian zone number.

This becomes important when you’re looking at US websites, catalogues or magazines.

 

One of the huge problems with zoning information comes when you are looking at plant tags.

Many Canadian nurseries do use the US zoning system on their plant tags, which means that if we think a US Zone 6 plant is Canadian Zone 6, it’s not.  It’s nearer Canadian zone 7. The cold hardiness of the plant is inaccurate for us.

A lot of Canadian nursery plants with their tags originate in the US, so beware. It’s essential to check the tag to see if there is any information about where the plant was grown or where the label came from.

Sometime there is no information.  You just have to ask your suppliers.  They should know.  It’s not that the difference between the two zoning methods is that much different, but sometimes it can result in disappointment.

Jennifer has been on the warpath with growers themselves several times on this very point

 

Note the zone is marked USDA Zone 3-8

 

There are also some notorious stories of plants being completely mis-assigned zones.

Take Coreopsis, ‘Limerock Ruby’, for instance.  It was stated to be hardy in US zones 4-9 (and still is on some sites).  But it wasn’t.  It turned out to be hardy in US zones 7-11.

The problem was that since most Coreopsis are indeed hardy to zones 4-9, no attempt was made to see if this cultivar was any different.  It is the rush to get things to market that is apparently behind this sort of thing. Time constraints put a stop to doing the proper research. 

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Coreopsis "Limerock Ruby"

 

Gardeners, like everyone else, rely on accurate information, and so it’s good to know that the Canadian Plant Hardiness map is being updated this year.  The last time that was done was 10 years ago.

The government tells us that in Canada there are some places where there have been no zoning changes. But overall there has been an increase of about a zone to a zone-and-a-half since the last map was drawn up.

Except along the East Coast.  That’s us.  We can expect to see an increase of half-a-zone if we’re lucky, so it’s probably not going to make much difference. All to do with the Atlantic Ocean and icebergs, apparently.  So zone 5 is probably still the best if we want to play it safe.

But who knows?  Perhaps here on the North Shore,  before too long, we may be in the enviable position of growing zone 6 plants, or even – holy mackerel – zone 7 plants with impunity. 

If you can trust the label, that is.

 

Copyright © 2024 Elizabeth Spence

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