Garden Blooms

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RHS Hardiness Ratings

In 2013 the RHS introduced a new system of hardiness ratings for the UK, these are explained below.

H1a Warmer than 15°C Heated glasshouse -tropical. Usually houseplants
H1b 10 to 15°C Heated Glasshouse-subtropical. Houseplants or outdoors during the warmest part of summer
H1c 5 to 10°C Heated glasshouse-warm temperate. Can be grown outside in summer.
H2 1 to 5°C Tender-cool or frost free glasshouse. Tolerant of low temperature but not being frozen.
H3 -5 to 1°C Half hardy-unheated glasshouse/mild winter.
H4 -10 to -5°C Hardy-average winter
H5 -15 to -10°C Hardy-cold winter
H6 -20 to -15°C Hardy-very cold winter
H7 colder than -20° Very hardy


Remember, hardiness is not purly about temperature,
 If the ground is unsuitable in terms of drainage the plant may still not may survive the winter.