The Gardeners Almanac

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 panel studied the American Winter Hardiness Zone Map that was 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, they were 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 due to;

• 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, these covered seven main categories ranging from H1 for glasshouse plants to H7 for plants that are fully hardy.(see chart below)

The panel also carried out a review in conjunction with The Garden Centre Association, The Horticultural Trades Association, Label manufacturers, and The Horticultural Press, on all plants that have been given the Award of Garden Merit (AGM), to ensure that their hardiness rating were reliable enough for UK gardeners.

The following hardiness system was introduced in 2012, and supersedes the H1-H4 ratings which previously accompanied the award.

RHS Rating	USDA equivalent	Temperature Range	Category	Definition
H1 a	13	Warmer than 15°C (>59°F)	Heated glasshouse - Tropical	Requires to be grown under glass or as a house plant all the year round
H1 b	12	10°C - 15°C (50°F - 59°F )	Heated glasshouse - Sub-tropical	Can be grown outdoors in summer in a sunny sheltered location, generally performs better as a houseplant or under glass all the year round
H1 c	11	5°C - 10°C (41°F - 50°F )	Heated glasshouse - Warm temperate	Can be grown outdoors in summer throughout most of the UK, providing temperatures are sufficiently high enough to promote growth.
H2	10b	1°C - 5°C (34°F - 41°F)	Tender-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.
H3	9b / 10a	-5°C - 0°C (23°F - 34°F )	Half hardy - 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.
H4	8b / 9a	-10°C to -5°C (14°F - 23°F)	Hardy - Average UK Winter	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.
H5	7b / 8a	-15°C to 10°C (5°F to 14°F )	Hardy - Cold Winter	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.
H6	6b / 7a	-20°C to -15°C (-4°F to 5°F)	Hardy - very cold winter	Hardy across the UK and Northern Europe. Containerised plants will require protection.
H7	6a	Colder than -20°C (<-4°F)	Very Hardy	Hardy in the severest European continental climates including exposed upland situations in the UK.
Related Links:	Frost Zones		https://www.rhs.org.uk/	For more information about the hardiness of plants