What are hardiness ratings?

How to interpret the hardiness ratings found in the magazine

Throughout Gardens Illustrated there are references to a plant’s hardiness described using two separate scales: one from the Royal Horticultural Society (RHS) and the other from the United States Department of Agriculture (USDA). Both are based on accumulated experience of 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 adapt 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.

The USDA scale was developed in the 1960s for North America but it is now widely used in other countries around the world. It’s based on minimum average temperatures in a range of zones – as shown in the two illustration below – which increase in steps of 10°F. 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 we have done here, a range of zones in which the plant is known to grow. These range from 1 (very hardy) to 13 (least hardy), with each divided into two 5°F subzones ‘a’ and ‘ b’. This USDA system has been applied to the UK and Europe and gardeners here can use the mapped zones as a general guide.

The RHS scale, introduced in 2012, is a more descriptive system of hardiness ratings. Like the USDA system, it uses a scale (this time in 5°C steps) from H1 to H7, based on minimum winter temperatures. However, it has two major differences. The first is that it runs in the opposite direction to the USDA system – 1 is very tender, 7 is very hardy. It also offers a description for the garden conditions to help reflect the variable nature of UK winters. These are summarised in the scale below, but you can find more detailed descriptions on the RHS website. The most tender rating, H1, has been subdivided into three categories A, B and C.

Perhaps, though, the more important difference is that the RHS scale is a rating of the plant’s hardiness and hasn’t been translated into mapped zones for the UK.

All rating given in the magazine are accurate to the best of our knowledge. Not every plant has been given a rating by the RHS, but where none is available our contributors will sometimes rate the plant based on their own trials. USDA scales are taken from the website of the Missouri Botanical Garden (missouribotanicalgarden.org) or based on growers’ experiences in the USA. Both are included as a guide only to help you predict what you should be able to grow in your garden.

This guide is based on a longer feature that appeared in issue 229 of Gardens Illustrated by Dr John David, head of horticultural taxonomy, and Leigh Hunt, principal horticultural advisor, at RHS Wisley. Find out more about the scales at: RHS hardiness ratings rhs.org.uk/plants/trials-awards/awardof-garden-merit/rhs-hardiness-rating USDA hardiness maps planthardiness.ars.usda.gov/PHZMWeb