Sometimes we have a particularly cold winter, and plants that were meant to survive do not. Other times, we have a mild winter, and plants that are considered annuals may surprise us when they reappear in the spring. So then, what’s the point of paying attention to hardiness zones if nothing is a real guarantee?

To answer that, let’s take a step back and look at how the hardiness zone system works.

Canada is divided into hardiness zones numbered from 0 to 8. Generally speaking, the lower the number is, the colder the climate will be. For reference’s sake, zone 0 is Alert, Nunavut, and zone 8 is the southern tip of Vancouver Island. As you can see in the graphic (courtesy of Natural Resources Canada, click to zoom), London is on the border between 6a and 6b.

It isn’t all about temperature, however – the hardiness zones also take into account average rainfall and snowfall amounts for a region; these can have a direct effect on how well plants can winter over. Places with more precipitation (especially snow) can, to a certain degree, offset cold winter temperatures to allow for less hardy plants to survive. Snow cover acts as an insulator, keeping the roots of our plants protected from the coldest temperatures.

It’s important to note: since hardiness zones are based on the average annual precipitation as well as the average minimum winter temperatures a region receives, we will always have years that are above and below average. It’s the nature of Mother Nature – our weather and climate will never neatly fit into the slots we’ve created for it!

Of course, there are plenty of safe bets. London is Zone 6, and it’s a safe bet a Zone 5 plant will do perfectly well here. It would take a cold winter of epic magnitude to completely do in a Zone 4 or 5 plant. Established plants are well-equipped to survive – you might see some dieback after a cold winter, but most plants that aren’t killed outright will eventually bounce back. It’s also a safe bet that a Zone 8 plant won’t winter over successfully – and even if it does, it may not flower.

Another consideration is that of microclimates. Microclimates are tiny areas of our yards where conditions are more or less favourable to plants than others. A sheltered corner on the south-east side of a building might be a warm enough microclimate to allow otherwise tender plants to survive. Conversely, an exposed area on top of a hill could do in a plant you would otherwise have expected to survive. There’s no tried and true method to determining what parts of your yard may have such microclimates, but for some guesswork and plenty of trial and error (oh, the joys of gardening!).

So what can we do to prepare our plants for a colder than expected winter? All we can do is make sure we given our gardens all of the attention they need during the growing season. Better-established, healthier plants are better able to winter over than plants that have been neglected or allowed to flounder in drought conditions. Water, fertilize, deadhead, and repeat.

For more tips on preparing your plants for fall and winter, head to our Garden Maintenance page!