• AGM plants

    AGM plants have been through a rigorous trial and assessment programme. They are:

    • Excellent for ordinary use in appropriate conditions
    • Available to buy
    • Of good constitution
    • Essentially stable in form & colour
    • Reasonably resistant to pests & diseases


Hardiness ratings are provided for all AGM plants, and also for plants included in the RHS Plant Search.

Download the hardiness ratings (510kB pdf)

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

 
Rating Temperature ranges ºC (ºF) 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 which previously accompanied the award.


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