Phormium tenax 'All Black'

Hardiness ratings

All ratings refer to the UK growing conditions unless otherwise stated. Minimum temperature ranges (in degrees C) are shown in brackets

• H1a: under glass all year (>15C)
• H1b: can be grown outside in the summer (10 - 15)
• H1c: can be grown outside in the summer (5 - 10)
• H2: tolerant of low temperatures, but not surviving being frozen (1 to 5)
• H3: hardy in coastal and relatively mild parts of the UK (-5 to 1)
• H4: hardy through most of the UK (-10 to -5)
• H5: hardy in most places throughout the UK even in severe winters (-15 to -10)
• H6: hardy in all of UK and northern Europe (-20 to -15)
• H7: hardy in the severest European continental climates (< -20)