Hardiness Zone (Z)
The USDA Hardiness Value is given in Zx format on the Notice of each Plant Species
(Z for Hardiness Zone, x for one of the USDA Hardiness Scale values from 0 to 14)
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What is Hardiness Zone?
Each plant species thrives in a given temperature range
(adapted to the climatic environment in which it evolved)
To help gardeners choose the species they grow, the United States Department of Agriculture (USDA) established the Contemporary Hardiness Zone Standard(1) in the 1960s(2) (US National Arboretum, DC, USA)
It maps geographical areas of a country with winter minimum temperature (long-term average)(3), allowing Gardeners to know where a plant species will likely survive to winter outdoors
The scale used is relatively arbitrary - from 0 to 14(4) in steps of 10 Fahrenheit (from -70 to + 70), which translates as follows in °C:
| 0 | [ -57: -51 [ |
| 1 | [ -51: -46 [ |
| 2 | [ -46: -40 [ |
| 3 | [ -40: -34 [ |
| 4 | [ -34: -29 [ |
| 5 | [ -29: -23 [ |
| 6 | [ -23: -18 [ |
| 7 | [ -18: -12 [ |
| 8 | [ -12: -07 [ |
| 9 | [ -07: -01 [ |
| 10 | [ -01: +04 [ |
| 11 | [ +04: +10 [ |
| 12 | [ +10: +16 [ |
| 13 | [ +16: +21 [ |
| 14 | [ > +21 ] |
Find your USDA Hardiness Zone
Interactive map of another country
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Remarks
- Other hardiness zone systems exist
- The USDA Hardiness Zone is a Probability Indicator, nothing more. Indeed:
- local environment can modify the temperature significantly
(example: north vs. south exposure, sheltered or not, presence of underground springs, snow cover, proximity to a lake, river, forest, etc.) - the actual temperature fluctuates in peaks while the Z index corresponds to average values
(example: a Z8 plant will be less likely to survive winter if there are several nights at -15°C, although the geographical area is in Z8) - parameters other than temperature influence the survival of plants in winter
(example: physiological state of a plant. An Echinopsis kept dry since October will resist -10°C in January, but will die at -1°C if it was watered; A dormant plant will bear frost without problem but may die from a late frost if it had already started growing; etc.) - A species able to live in a given hardiness zone can also live in geographical areas with warmer winters, yet falling in the same climatic category
- Additional Factors to Hardiness Zone define the possibility of growing a plant in your garden
(soil type, rainfall, maximum heat in summer, etc.) - Hardiness Zones are likely to change with global warming
(1) Different from Climatic Zones - see for example the Köppen Classification
(2) The first Hardiness Zone system was developed in the 1920s at the Arnold Arboretum (MA, USA)
(3) Currently (2024) a 30-year average (1991-2020)
(4) Refined by a subdivision into 2 subcategories a and b in steps of 5 Fahrenheit