The USDA climate zones are based solely on the average minimum low winter temperature for any given area, as follows:

Zone 1 Below -50 degrees F (-45 C)
Zone 2 -50 F to -40 F (-45 C to -40 C)
Zone 3 -40 F to -30 F (-40 C to -34 C)
Zone 4 -30 F to -20 F (-34 C to -29 C)
Zone 5 -20 F to -10 F (-29 C to -23 C)
Zone 6 -10 F to 0 F (-23 C to -18 C)
Zone 7 0 F to 10 F (-18 C to -12 C)
Zone 8 10 F to 20 F (-12 C to -7 C)
Zone 9 20 F to 30 F (-7 C to -1 C)
Zone 10 30 F to 40 F (-1 C to 4 C)

These may also be denoted as a or b segments, such as Zone 7a or Zone 8b. Zone 7a would be from 0 F to 5 F, and Zone 8b would be from 15 F to 20 F.

If you know the average or expected minimum winter temperature for an area, this should make it easy emough to determine its equivalent USDA climate zone.

If it's of any more help, I seem to recall that one of the rose books written by Peter Beale showed the USDA climate zones on a map of Great Britain. I'm sorry that I can't say which book, but I don't have any of them nearby at the moment. Perhaps the RHS can give you that information.

In which case most of Britain will fall in zone 8 (minimum temperatures range from -12oC to -6.5oC). Southern England, tends to be warmer than the northern counties (in the upper minimum temperatures in the winter) so probably justifies an 8b. Thanks for the info.

This site has a map with rough approximations of the USDA zones
http://www.b-and-t-world-seeds.com/European.htm

But I find that so many other climatic factors are important apart from average minimum winter temperatures that the USDA zones are almost useless in Europe.

Verica - that's exactly right, with the South West stretching it into Zone 9. It makes it hard for some in the Southern states to acccept that we can grow teas and noisettes when we are geograhically so far north.

Marianne - I think the USDA zones are useful if used merely to signify areas where different plants will survive a winter. To use them for anything else would be as useless within the US as it would be in Europe.

One unfortunate side effect of this "zoning" has been to deter individuals and organisations from trying "borderline - hardy" plantings of many species, meaning that soon there will be a greatly reduced diversity within zones both in gardens and in the managed environment.

Jon

It is even harder for some in the Northern states, and most of all for Alaskans, to accept that although I am almost exactly on the 59th parallel my USDA zone is 6 because of influence from the Gulf Stream. I grow two noisettes, 'Gloire de Dijon' and 'Meteor' and one china, 'Gräfin Esterhazy'. I find, however, that although minimum winter temps may be higher in my part of the world, our cold spells often last longer than in southern Ontario and Wisconsin, the parts of North America I am familiar with. Our autumns are shorter which means there is the risk that more tender roses don't go dormant in time and freeze because of this although temperatures may not be so terribly low.

Slightly better versions of the EU USDA hardiness map at:

http://www.naturenode.com/seeds/zoneeurope.html
and
http://www.marblenet.es/pjse/mapuse.htm