The USDA (United States Department of Agriculture) zone system is based on the average  yearly recorded minimum temperatures in a certain zone. The latest edition of the British Royal Horticultural Society's "Dictionary of Gardening"  gives a zone rating for every plant that it describes, based on the conditions in the native habitat of the plant. It should be noted that for the winter survival of a certain plant in any given location, outside of its native habitat,  many other factors should also be considered (local humidity, local temperature fluctuations, amount of wind and sunshine, latitude, microclimate etc.)  I think the zone system is a very good way to obtain a first idea of a certain plant's hardiness - all over the world - and, for that purpose,  I have found it a perfect system. But you should also consider your local situation and those other factors that I mentioned.