If you define hardiness as “a plant’s ability to survive in adverse conditions” then the lowest temperature of an area - traditionally used as a bench-mark in the UK - is not the only factor which needs to be taken into account .
There are a host of other influences that must be considered, especially as the climate changes:
* Recording the number of days when high summer temperatures occur is a key factor affecting whether a plant can survive and will become even more crucial if the planet heats up as predicted. A Heat Zone system has been devised by the American Horticultural Association based on the number of days an area has when the temperatures reaches at least 86F (30c) http://www.ahs.org/publications/heat_zone_map.htm
* The type of soil, its ability or inability to release nutrients, or to drain, or to allow roots to penetrate to sufficient depth.
*The level of rainfall, and when it falls, is vital. In the US, drought is a problem in hotter areas in particular. In Northern Europe, it is often not the lack of water but an excess, especially if it is accompanied by cold spells, that affect a plant’s survival.
*Shelter, in a larger area as provided by hills and mountains or more locally by buildings, walls and trees, can affect what can be grown. Towns and cities are usually a few degrees warmer than rural areas. Frosts can be preceeded by snow which provides protective insulation for plants; frost before snow, especially if it is accompanied by drying winds, can damage plants. An overnight frost on a calm night can be harmless; a prolonged frost can prove fatal. Northern and eastern winds, or salt-laden winds from the sea, can be disastrous.
UK hardiness zones will most certainly need to be re-evaluated in the future. Maybe they will have to follow the States and incorporate the fact that the UK now experiences many more summer days when the temperature rises above 30c ? Or winter days like in 2010/11 when temperatures dropped to below -12 in some parts of the coutry for many days.
Does anyone know who decides – is it the RHS or the Met Office?