Zinnia Oklahoma Series

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  The RHS Award of Garden Merit (AGM) helps gardeners choose the best plants for their garden.

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)