Heat zones and hardiness zones

This was the week that temperatures on the balcony dropped to -3°C (26°F) and we had 40 cms (about 16 inches) of snow within 36 hours. That's more than I've seen in Milan for about 20 years - and the first time I've ever seen snow heaped up on the balcony. It is however well within the limits of our hardiness zone.

It seems that every US gardener knows what hardiness zone they're in. Yet the idea is much less common in Europe. It is possible to find out though, and backyardgardener.com links to maps showing not only the US, but also Europe ( this one from gardenweb.com is particularly good - click on the different countries to get an enlarged view of wherever you are in Europe, Australia, China and the States.

Knowing your hardiness zone isn't always much help though. First of all, because the categories seem to me to be extremely wide. We're in zone 8, which means that temperatures might go as low as -12°C. Well, yes I suppose they might. But they almost never do. If we have a week below freezing with a couple of days at -5 or -6, that's an exceptionally cold winter. Most years we never see temperatures drop far below freezing at all. Even this week we've mostly been around 0-2°.

One reason for that however is that we're in the middle of a city - where temperatures are always 2-3°C higher than the surrounding countryside. And that's another problem with hardiness zones - you need to take into account the individual factors of your garden : how exposed is it? If you're south facing, if you have nice sheltering walls, or if like me you garden on a balcony that receives warmth from the house, the official temperatures and those which your plants experience may be quite different. I've said before that I can get so-called annuals like petunias to survive the winter on the balcony just by covering them and placing them against the walls of the house. The conditions in the micro-climate of your own garden may be quite different from the "official" conditions of your zone.

However, the other problem is that hardiness zones only take into consideration the low, winter temperatures. I've said we're in zone 8 here - but so is much of Britain, and the type of plants that grow well here and in Britain are often quite different. We may have similar winter conditions, but the summer heat is another story.

In the US, it's also possible to find out your heat zone. The zones are based on how many days the temperature rises above 30°C / 86°F, and you can see how the zones are labelled at the site of the American Horticultural Society. But I can find nothing for any other part of the world. And as I lose far more plants to heat than cold, I'd like to know.

But for now, we're still coping with the snow.