Lectures and book signing
Another fake plant picture

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.