The measurement of plant hardiness has always been a bit of a black art, with a variety of different measures being used in different publications. The basic principle of most of them is to measure the lowest temperature which the plant in question can survive. While many different methods are still in use, we have chosen to use two different ones; the European Garden Flora (EGF) zones and the United States Department of Agriculture (USDA) zones. These are shown below, along with their alignment to the more traditional Royal Horticultural Society zones:

Hardiness zones chart

Whichever system you use to guide you, you should always remember that hardiness is just one of many factors which will influence the survival of your plants. Other considerations should include drainage, shelter, soil depth, nutrition, rainfall and daylength, plus of course the skill of the gardener!