Plant Hardiness Zones
The classification of hardiness according to climatic zones of the USDA,includes a value for the coldest climate zone and a value for the warmest climate zone. For example, if the numbers Z4-Z9 are given, the coldest (Z4) and the warmest climate zone (Z9) are indicated. The plant is survivable in zones 4 to 9, but probably not in Z3, because of the strong cold in wintertimes, and also not in Z10 because the climatic conditions are unsuitable in summer. Other factors like soil and microclimates can influence winter hardiness to some degree.Please refer to the USDA Plant Hardiness Zone map to determine in which zone you are located.
Source: RHS Index of Garden Plants, USDA Online Plant Database, A. Armitage - Herbaceous Perennial Plants.
Range of average annual minimum temperature for each climatic zone
|1||< -45.5||< -50|
|2||-45.5 to -40.1||-50 to -40|
|3||-40.0 to -34.5||-40 to -30|
|4||-34.4 to -28.9||-30 to -20|
|5||-28.8 to -23.4||-20 to -10|
|6||-23.3 to -17.8||-10 to 0|
|7||-17.7 to -12.3||0 to +10|
|8||-12.2 to -6.7||+10 to +20|
|9||-6.6 to -1.2||+20 to +30|
|10||-1.1 to +4.4||+30 to +40|