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


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