The United States Department for Agriculture (USDA) plant hardiness zone map, 1990
... Zones - a You Ask, We Answer GuideIf you blog about gardening here in Britain, it's not too long before you encounter your American and Canadian cousins who all talk about which zone they garden in as if it was as natural as breathing. Strike up a conversation with any of these gardeners and it's not long before you're asked what zone is your garden? Luckily when I was first asked this last year, I had just been looking at the above map in The Essential Design Workbook by Rosemary Alexander.
The USDA map is based on winter low temperature records and divides the continent into 11 hardiness zones - zone 1 is the coldest and zone 11 the warmest (plants surviving lows of -45.6 degrees centigrade or below, and lows of just 4.5 degrees centigrade respectively).
Whilst knowing their garden's zone is a useful tool for your American and Canadian friends to understand what they can grow in their gardens, there can be differences based on altitude, soil type, rainfall, day length, rural or city location etc etc. For example, Austin (Texas) and Portland (Oregon) are both nominally in Zone 8, but their local climates are very different. This also holds true when trying to place a British garden into these zones. Based on latitude alone, we should be gardening in relatively chilly zones 2 to 4 like they do in Canada. However, our maritime climate and proximity to the warming Gulf Stream puts us in the warmer zones 7 to 9.
The limitations of the USDA classification are recognised and consequently a couple of other maps were developed. The American Horticultural Society (AHS) uses highest summer temperatures for their Plant-Heat-Zone map and for western US gardeners there's the 24 zone Sunset map, named after the magazine in which it was first published. This factors in the length of growing season, humidity and rainfall patterns in addition to the winter lows and the summer highs used by the USDA and AHS. I believe this information is used for plant labelling and by plant suppliers - perhaps one of my American or Canadian readers can confirm this? I haven't been able to find AHS or Sunset equivalent maps for outside North America, but this link shows the USDA equivalents for Europe, if you're interested.
I've put my garden into zone 8a (temperatures down to -12 degrees centigrade) and whilst I'm happy with my conclusion, I'm still a bit cautious in quoting it too extensively. My garden rarely goes below -10 degrees centigrade (even this year!), but my south-western England winters do tend to be wet and I also garden on clay, a relatively cold soil. In my experience this combination is lethal for both tender plants and cold tolerant alpines, neither of which like a wet winter. Therefore I grow these kinds of plants much closer to the house - at the top of the slope (drier) and by my garden or house walls (warmer). Thus I'm using my garden's microclimates to ensure I can grow a wider variety of plants without having to give them much in the way of special treatment. That said, the recent weeks of very cold weather will surely test the antarctica portion of my tree fern's latin name!
Of course we don't have the USDA, AHS or Sunset information on our plant labels or at the garden centre. Here in Britain we're more used to the terms tender or half-hardy (protect from frost), hardy and fully hardy (tough as old boots) when choosing which plants to buy and I suspect many of us make some allowances when buying. For example, I shouldn't be able to overwinter half-hardy plants in my garden, but I know that by planting half-hardy fuchsias in the walled beds or keeping them by the house, most of them will survive. I also know this doesn't apply to my front garden as it's north facing. The RHS has devised a more detailed hardiness categorisation for the UK, which can be found here and you'll see it quoted in the RHS Plant Finder and on some plant labels.
I'm off now to have a look at my tree fern to see if I was a bit optimistic in not wrapping it up for the winter. Fingers crossed my dahlia duvet has worked too...
Do check out the ABC Wednesday blog for more entries on the theme of Z. I hope this piece hasn't been too much of a Zzzzzzzzzz for you!