Hardiness ratings explained

What the ratings mean

There’s no agreed definition of hardiness but, for our purposes, it is a plant’s ability to endure cold conditions, or in the UK, the ability to withstand changeable periods of freezing and relatively mild, wet weather. Plants are adapted to tolerate cold to varying degrees. Some will take a light frost (temperatures just below freezing) for a couple of hours, others cope with long periods of freezing to remarkably low temperatures.

Based on accumulated experience of a plant’s performance we can establish its hardiness. This is usually presented as a scale, from least hardy to most hardy, normally using winter minimum temperature as the measure. This is exemplified by the United States Department of Agriculture (USDA) scale, developed in the 1960s for North America but now widely used in other countries. The USDA system is a scale of zones from 1 (very hardy) to 13 (least hardy). The zones are defined by 10°F steps, each one being more recently divided into two 5°F subzones indicated ‘a’ and ‘ b’. The US zones have been mapped in remarkable detail and plants can be assigned a rating according to the coldest zone in which they can grow, or as shown in the magazine a range of zones in which the plant is known to grow.

The USDA system has been applied to the UK and Europe and gardeners here can use the mapped zones as a general guide. In addition, the Royal Horticultural Society (RHS) has a more descriptive system of hardiness ratings. Like the USDA system, it uses a scale (in 5°C steps) from H1 (subdivided into a, b and c) to H7, based on winter minimum temperatures but note that it runs in the opposite direction to the USDA system – 1 is very tender, 7 is very hardy. To help reflect the variable nature of UK winters, each rating also has a description for the garden conditions.

The important thing to remember about the RHS system is that it is a rating of the plant’s hardiness and has not been translated into mapped zones for the UK.

In the end, hardiness systems provide a useful guide to predict what gardeners should be able to grow in their gardens. While beginners are likely to depend on the certainty of growing plants that are reliably hardy, enthusiasts will want to push the boundaries and experiment with more borderline plants.

Dr John David is head of horticultural taxonomy and Leigh Hunt is principal horticultural advisor, both at RHS Wisley.

USEFUL INFORMATION RHS hardiness ratings rhs.org.uk/plants/trials-awards/awardof-garden-merit/rhs-hardiness-rating USDA hardiness maps planthardiness.ars.usda.gov/PHZMWeb

A longer version of this article appeared in issue 229.