Plant hardiness ratings explained

You may have seen plant descriptions followed by a coding of numbers and letters. Here's an explanation of the plant hardiness systems used here and in the USA.

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When we buy a plant, there’s always the temptation to make an impulse purchase. Yet the first question that should pop into a gardener’s head is ‘will it actually thrive in my garden?’

In a climate as varied as the British Isles, one of the main factors used to work this out is whether it can survive the winter outdoors unaided – what we call ‘hardiness’.

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 alternating 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, while many plants are in-between. 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.

 

USDA and RHS scale (US and UK systems)

 

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 now 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 (see below) and plants have been assigned according to the coldest zone they can grow in.

USDA zones relating to USA and also applied to Europe. The scale above shows you waht temperature range each colour relates to

 

The Royal Horticultural Society (RHS) has recently updated its more descriptive system of hardiness ratings to reflect the need for the greater precision gardeners require to make choices about plants for their gardens. Like the USDA system, it has adopted a scale based on winter minimum temperatures but is in 5°C steps (see scale illustrated above).

However, reflecting the variable nature of our winters, each rating also has a description for the garden conditions. The scale runs in the opposite direction to the USDA system, but there are broad equivalents between the two. Even so, for the reasons explained, just because a plant is rated USDA zone 8a, it doesn’t mean that it will thrive in a similarly rated area in the UK; nor that RHS hardiness rating 4 will be the same as USDA zone 8.

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. This is because the climate gardeners experience in their gardens here can vary significantly within a region. If you are keen to get the most from your garden, it is essential to understand the local conditions for your garden and, indeed, of the marked variations within it – the latter being known as the microclimate.

 

A little bit more on microclimate

For those keen to grow tender plants, a south-facing aspect is desirable to maximise exposure to the sun’s heat. Likewise, shelter is critical, either to protect plants from cold winds or from cold air flowing down hillsides.

Gardeners will talk about frost pockets or frost hollows. These are where cold air drains off exposed slopes, as cold air is heavier than warm air, and collects in valleys or in sheltered areas where it cannot escape. This leads to cooler overnight temperatures and more frosts.

Where possible, lay out your garden to avoid slowing or trapping cold as it filters down slopes. Hedges, fences and walls can be strategically placed to protect plants or to provide sheltered nooks where you can grow plants that would not flourish in the open. Walls provide an additional benefit because they absorb heat during the day, which they then give off again at night. Cleverly, this keeps the surrounding air several degrees warmer, which can make all the difference to your plants on a frosty day.

Bear in mind, though, that structures such as walls, fences and hedges also cast shadow. This can mean that after an overnight frost, even though the daytime temperature rises above freezing, the ground remains frozen when in permanent shade.

 

Useful Information

RHS hardiness ratings rhs.org.uk
USDA hardiness maps planthardiness.ars.usda.gov

Books

The Gardener’s Guide to Weather and Climate by Michael Allaby (Timber Press, 2015). Weather in the Garden by Jane Taylor (John Murray Publishers, 1996). 

 

• Taken from a longer feature in The Plant Issue (229), a special edition of Gardens Illustrated

 

Words Dr John David, Head of Horticultural Taxonomy at RHS Wisley and Leigh Hunt, Principal Horticultural Advisor at RHS Wisley. 

Illustrations Liam McAuley and Chris Jones

 

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