Plant Hardiness Zones: USA and UK
Information on your area’s minimum annual temperatures is essential for proper plant selection. So-called plant hardiness zones or ratings give you some indication as to how hardy plants must be to survive the coldest of winter temperatures in a particular area. Of course, a plant’s water requirements (rainfall) and sun / shade requirements are also important for plant selection and health.
The United States is one of the leaders in the development and dissemination of plant hardiness data.
USA Plant Hardiness Zones
For the United States, the Agricultural Research Service (ARS) of the U.S. Department of Agriculture (USDA) in cooperation with the Oregon State University (OSU), have developed the official USDA Plant Hardiness Zones.
Source: the U.S. Department of Agriculture, Agricultural Research Service (ARS) in cooperation with the Oregon State University.
There are 26 zones in the USDA model, which takes into account the average annual minimum temperature in a particular geographic area. If you live in Madison, Wisconsin, for instance, you are in USDA zone 5a and the plants you select for your garden must be able to withstand temperatures as low as -20°F to -15°F (-28.9°C to -26.1°C).
The USDA also has an interactive ZIP code-based look-up tool on its website that will enable a more accurate determination of the correct hardiness zone – especially handy if you live close to the border between multiple hardiness zones. USDA-Zip Code Interactive Map
UK Plant Hardiness Zones
In the UK, the Royal Horticultural Society (RHS) has developed a plant hardiness model based largely on USDA hardiness zones. The RHS model, however, uses absolute minimum recorded temperature not the long-term average minimum temperature used by the USDA model.
Source: rhs.org.uk.
There are nine hardiness ratings in the RHS model. The RHS recommends that to be confident of complete hardiness, UK residents should opt for plants with ratings of H5 to H7 (-20°C to -10°C), which it equates to USDA Zones 1-8a.
The RHS website has a brilliant plant selection search tool that allows users to find plants that satisfy a variety of attributes of both your garden (e.g., sunlight and soil) and the plants themselves (e.g., foliage colour, season colour, height, and hardiness). RHS Plant Search Tool
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