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Hardiness ratings refer to the UK growing conditions. Minimum temperature ranges are shown in brackets.

H1a - Under glass all year (>15ºC)

H1b - Can be grown outside in the summer (10 - 15ºC)

H1c - Can be grown outside in the summer (5 - 10ºC)

H2 - Tolerant of low temperatures, but not surviving being frozen (1 to 5ºC)

H3 - Hardy in coastal and relatively mild parts of the UK (-5 to 1ºC) 

H4 - Hardy through most of the UK (-10 to -5ºC)

H5 - Hardy in most places throughout the UK even in severe winters (-15 to -10ºC)

H6 - Hardy in all of UK and northern Europe (-20 to -15ºC)

H7 - Hardy in the severest European continental climates (< -20ºC)

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