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How do you know if a plant is hardy?

I’ve been thinking the muddled issue of hardiness zones this week – and muddled they are indeed. These are ratings that classify plants according to the climatic conditions they’ll tolerate – in particular, how much winter cold they’ll take. Maps divide the world zones according to the prevailing conditions.

UK-ZoneMap American gardeners use them all the time, and for their interest here’s a map of Britain (click to enlarge) split up according to the USDA system widely used in the USA. British gardeners will find the, of course, far more complicated US map below.

The problem is this: At a time when plants (and sometimes their coloured labels) move around the world so quickly, when most plant and garden books are published internationally and with everyone looking up plants online wouldn’t it be more helpful to have one single system?

Britain The RHS has a four zone system (some use an extra zone for parts of Scotland) based not on minimum temperatures but on the growing conditions plants require. The RHS also often cites the actual temperatures plants will tolerate or uses a system of symbols.
Europe Here the USDA system is sometimes used but the European Garden Flora uses its own seven zone system based mainly on minimum winter temperatures.
United States There are three systems. The USDA (United States Department of Agriculture) system of eleven winter hardiness zones and the similar National Arbor Day Foundation system are based on minimum winter temperatures. There’s also the completely different 45 zone Sunset system, used almost Usda_map entirely in the west, that also takes summer heat and other factors into account.
Canada Agriculture Canada has created an eight zone system based on minimum winter temperatures, summer rainfall, snow cover and other factors.Australia Here they use a modified version of the USDA system.

I could go on…

So I’m wondering – does it matter? Well, you know what I think. I write online and my books are published all over the place – it would be simple to use just one system.

What do you think? One system for everyone or each to their own? Keep in mind that any system is only going to provide a rough guide.


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One system would indeed be nice, and likely more useful. To get scientists to agree however, is another thing.

In any case, whatever system is used, I like to suggest that they are best used as a "general guideline" only.

Graham Rice

Oh yes of course - none of these systems can be more than a guide. They're not rock solid rules - these ratings are extra information which help us choose. And the more they become complicated as they try to be more precise and more helpful - the more parochial they become and the less helpful to anyone else.


Graham - just to complicate things for the USA further, there's the American Horticultural Society's Heat Zone map too which looks at summer highs!

Trying to get it all into one system I supect would be quite a challenge. I wonder if there's a way of mapping each system onto each other as a starting point?

However, I suspect the result would either be too simple or too complicated to be of use to anyone. Perhaps there needs to be a standard appendix used by publishers of gardening books whose authors 'crossover' continents to show what's applicable where.

Might be a suitable research project for a student somewhere?

Graham Rice

Yes, VP... the heat zone map. I have a large printed copy here. Not up on the wall, though, I have to say. And the Sunset system combines heat and cold.

But, frankly, the only hope of a simple system that could be used, or adapted, across the world is the USDA system. In many countries there might well be a need to subdivide zones but the fact is that it's winter hardiness that 90% of gardeners are worried about (rightly or wrongly). If we can find and popularize a simple universal system for rating winter hardiness - then we can move on to heat.

jodi (bloomingwriter)

I use the USDA system here in Canada, Graham, mostly because it's a universal (well, okay, North American-universal) language that we can relate to whether we're in Maine or Moncton, Massachusetts or Middleton. I actually used to get very exasperated with books published in Britain because they so rarely use a zone reference, but I know what grows here well enough now that I know also what won't grow here, except in occasional microclimates. It would be nice to see some standardization, the way we have with botanical nomenclature.

Graham Rice

Thanks Jodi. That's what I'm all for too - a standard system that everyone around the world can use and understand. It's especially maddening to go to Australian websites and find that what you think is a completely different system is actually almost (but not quite) the USDA system - and with all the zone numbers changed!

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