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Plants need certain temperatures to survive. While numerous plants cannot cope with the hot midday sun in summer, there is a risk of frostbite for outdoor plants in winter. Plants that can withstand sub-zero temperatures are therefore referred to as frost or winter hardy. This does not mean that they can, in principle, also tolerate extremely sub-zero temperatures, as they occur in various regions of Germany. That is why there are winter hardiness zones, which serve as ideal orientation for the right choice of plants in the home garden.
hardiness zones
The term hardiness zones is used in botany. This is a division of areas, which are determined by their average lowest minus temperature. Experts also speak of area climate classifications according to international standards. However, the expression USDA climate zones is also used. USDA is an abbreviation for the American US Department of Agriculture. Because this is the publisher of the climate zones. Since the climate is constantly changing worldwide, temperature values are regularly adjusted. The last revision was made in 2012 and refers to the data collection between 1975 and 2005.
notice: The winter hardiness zone is often abbreviated to WHZ. The simple letter "Z" is also often used for "zone". However, that means the same thing.
climate zones and plants
Hardy and hardy plants differ in their cold tolerance. The properties hardy and hardy consequently only say that they can winter outdoors in sub-zero temperatures, but not where the maximum limit is. For example, less cold-tolerant plants can stay outside in milder regions of Germany without any problems, but would not be able to survive where the average minus temperature is significantly lower. That is why there are hardiness zones. They provide information about the areas in which a plant can survive the winter or where the chances of surviving are best.
tip: The next time you buy "hardy" plants, better check the hardiness zone. However, this is often not specified. Then simply ask the retailer from the specialist garden trade.
zone numbering
Hardiness zones are sorted numerically from 1 to 13. In the meantime, each digit is supplemented by the letters a and b. This "refines" the temperature classification again in equal steps of 5 Fahrenheit (American temperature measurement unit).
Number 1a includes areas with the world's lowest average sub-zero temperatures. These can only be found in polar regions. This is followed by hardiness zone 1b. It continues with zones 2a, 2b, 3a, and so on. Climate zone 13b represents the warmest area.
temperatures of the winter hardiness zones
| Zone | Temperature range in °C |
| 1a | -51.1°C to -48.3°C |
| 1b | -48.3ºC to -45.6ºC |
| 2a | -45.5ºC to -42.8ºC |
| 2 B | -42.7ºC to -40ºC |
| 3a | -39.9ºC to -37.3ºC |
| 3b | -37.2ºC to -34.5ºC |
| 4a | -34.4°C to -31.7°C |
| 4b | -31.6ºC to -28.9ºC |
| 5a | -28.8ºC to -26.2ºC |
| 5b | -26.1°C to -23.4°C |
| 6a | -23.3ºC to -20.5ºC |
| 6b | -20.4ºC to -17.8ºC |
| 7a | -17.7ºC to -15ºC |
| 7b | -14.9ºC to -12.3ºC |
| 8a | -12.2ºC to -9.5ºC |
| 8b | -9.4°C to -6.7°C |
| 9a | -6.6ºC to -3.9ºC |
| 9b | -3.8ºC to -1.2ºC |
| 10a | -1.1°C to +1.6°C |
| 10b | +1.7 °C to +4.4 °C |
| 11a | +4.5 °C to +7.2 °C |
| 11b | +7.2 °C to +10 °C |
| 12a | +10.0 °C to +12.8 °C |
| 12b | +12.8 °C to +15.6 °C |
| 13a | +15.6 °C to +18.3 °C |
| 13b | +18.3 °C to +21.1 °C |
Hardiness zones in Germany
The German-speaking area includes Germany, Austria and Switzerland. In the lowlands, the winter hardiness zones 6 and 7 are predominantly found. The Alpine region, where winter hardiness zones 5 and 6 prevail, is significantly cooler, and in the high Alps even between 4 and 5. Other examples of winter hardiness zones in Germany are as follows :
- Berlin 7b
- Bremen 7b
- chunks 6b
- Dresden 7b
- Frankfurt 7b
- Hamburg 8a
- Helgoland 8b
- Kassel 7a
- Luedenscheid 8a
- Mainz 8a
- Munich 7a
- Munster 7b
- Oberstdorf 6b
- Rostock 7a
- Stuttgart 7a
- Villingen 6a
Cold Protection Recommendations
As previously mentioned, determining a hardiness zone is not a guarantee of outdoor survival. Garden owners and plant lovers should always consider "complications" and prepare themselves or the outdoor plants accordingly, depending on their sensitivity to cold. The following is therefore recommended for the usual winter hardiness zones in German-speaking countries:
- WHZ 5, 6 and 7: protect evergreen plants from frost
- Zone 8: Cover the surface of the earth with leaves, pine needles, straw or twigs
- WHZ 9: Cover plants with fleece or move to frost-free but cool winter quarters
- WHZ 10: allow plants to hibernate frost-free (depending on plant requirements between five and 15 °C)
tip: If outdoor plants are overwintered in pots, it must be taken into account that the cold will hit much more here and will reach down to the roots, so that the potted plant can freeze to death. The next higher (warmer) value of the given winter hardiness zone should therefore be taken into account to be on the safe side.