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RHS Hardiness Ratings

 

The Royal Horticultural Society has devised a system of hardiness ratings to enable gardeners to assess the hardiness of garden plants in the United Kingdom


The hardiness rating serves as a general guide to growing conditions, and should be interpreted as follows:

 

Rating

Temperature ranges C (F)

Category

Definition

H1a

warmer than 15 (>59)

Heated glasshouse - tropical

Needs to be grown as a house plant or under glass all year round.

H1b

10 to 15
(50 to 59)

Heated glasshouse - subtropical

Can be grown outdoors in summer in sunny and sheltered locations but generally performs best as a house plant or under glass all year round.

H1c

5 to 10
(41 to 50)

Heated glasshouse - warm temperate

Can be grown outdoors in summer throughout most of the UK while daytime temperatures are high enough to promote growth.

H2

1 to 5 (34 to 41)

Tender - cool or frost-free glasshouse

Tolerant of low temperatures but will not survive being frozen. Except in frost-free inner-city areas or coastal extremities requires glasshouse conditions in winter, but can be grown outdoors once risk of frost is over

H3

-5 to 1
(23 to 34)

Half-hardy - unheated glasshouse / mild winter

Hardy in coastal / mild areas except in hard winters and at risk from sudden (early) frosts. May be hardy elsewhere with wall shelter or good microclimate. Can survive with artificial winter protection.

H4

-10 to -5
(14 to 23)

Hardy - average winter

Hardy through most of the UK apart from inland valleys, at altitude and central / northerly locations. May suffer foliage damage and stem dieback in harsh winters in cold gardens. Plants in pots are more vulnerable.

H5

-15 to -10
(5 to 14)

Hardy - cold winter

Hardy through most of the UK even in severe winters. May not withstand open or exposed sites or central / northerly locations. Many evergreens suffer foliage damage and plants in pots will be at increased risk.

H6

-20 to -15
(-4 to 5)

Hardy - very cold winter

Hardy throughout the UK and northern Europe. Many plants grown in containers will be damaged unless given protection.

H7

colder than -20 (< -4)

Very hardy

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

This hardiness system was introduced by the RHS in 2012, and supersedes the H1-H4 ratings.