Plant Hardiness

In 2011 the Royal Horticultural Society (RHS) reviewed plant hardiness ratings for the UK in response to the concerns of many gardeners during the recent cold winters.

A panel of horticulturists looked at the existing RHS hardiness system - which gave plants ratings from HI (heated glass) through H2 (unheated glass) and H3 (hardy in some regions) to H4 (generally hardy) and found it to be inadequate.

During their deliberations the studied the American Winter Hardiness Zone Map, developed by the US Department of Agriculture (USDA) in 1960.

They also looked at the European Garden Flora version which divides most of Europe into seven zones HI to H5, plus Gl and G2.

Under the European system, the British Isles has two zones:

H4 (hardy in mild areas, withstands -5 to -10°C minimum) and H5 (hardy in favourable areas, withstands 0 to -5°c).

It soon become quite clear that neither of these systems really worked in the UK hence the review.

The panel soon become aware of the many variables that can affect plants in the UK, for example;

Physiological condition.

Prevailing climatic conditions.

Growing conditions.

Position in the garden.

Temperature swings, particularly during spring and autumn.

Minimum winter temperatures.

In the end they devised a hardiness ratings chart for plants that is temperature-based.(see below)

This covered seven main categories ranging from H1 for glasshouse plants to H7 for plants that are fully hardy.

The panel also carried out a review of all plants that have been given the Award of Garden Merit (AGM), which have had their hardiness rating assessed in order too make their hardiness ratings as reliable as possible for UK gardeners.

To try and ensure information about The Award of Garden Merit and The new RHS hardiness ratings are well publicised the RHS has been in consultation with its members and:

The Garden Centre Association,

The Horticultural Trades Association,

Label manufacturers

The horticultural press.

For more information about the hardiness of plants

RHS Rating
USDA equivalent
Temperature Range
Lower than -20°C (-4°F)
Very Hardy

Hardy in the severest European continental climates including exposed upland situations in the UK.

6b / 7a
-20°C to -15°C (-4°F to 5°F)
Hardy in very Cold winters

Hardy across the UK and Northern Europe. Containerised plants will require protection.

7b / 8a
-15°C to 10°C
(5°F to 14°F )
Hardy inCold Winters

Hardy in most areas of the UK even in the severest of winters.Some plants in more exposed areas, and contaianerised plants will require protection.Some evergreens may suffer foliage damage.

8b / 9a
-10°C to -5°C
(14°F - 23°F)
Hardy in Average UK Winters

Hardy throughout most of the UK, apart from inland valleys, central and northerly locations and at altitude.Some plants may suffer foliage damage and stem dieback in harsh winters, particuarly in cold gardens.Some normally hardy plants may die in long wet winters, particularly if they are in heavy or poorly drained soil.Containerised plants will require protection.

9b / 10a
-5°C - 0°C
(23°F - 34°F )
Half hardy in unheated glasshouse or mild winters

Generally hardy in coastal and the milder areas of the UK, and in areas with their own micro-climate, e.g. against a house wall. Plants are susceptible to sudden and or early frosts. Containerised plants will require protection.

0°C - 5°C
(34°F - 41°F)
Tender-Requires cool or frost free glasshouse

Tolerant of low temperatures but will not survive freezing!Plants require glasshouse conditions in winter but may be grown outdoors in summer, when all risk of frost has passed.Exceptions to this may be in micro-climates generated by urban and inner city conditions or coastal areas.

5°C - 10°C
(41°F - 50°F )
Requires the warm temperatures of a heated glasshouse

Can be grown outdoors in summer throughout most of the UK, providing temperatures are sufficiently high enough to promote growth.

10°C - 15°C
(50°F - 59°F )
Sub-tropical - requires heated glasshouse

Can be grown outdoors in warm sunny summers in a sheltered location, but they generally perform better under glass or as a houseplant.

Over 15°C
Tropical - requires heated glasshouse

Requires to be grown under glass or as a house plant all the year round

Related Link: Frost Zones

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