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Winter Hardiness Zones: A Guide

The winter hardiness of a plant is a key factor for its long-term survival in the garden. Especially in regions with distinct seasons, such as Germany, the adaptability of plants to cold winter temperatures plays an important role. The classification into winter hardiness zones is a valuable guide to help choose the right plants and ensure optimal growing conditions.

What are Winter Hardiness Zones?

Winter hardiness zones are geographical classifications based on the average minimum temperatures in winter. These zones were originally developed by the US Department of Agriculture (USDA) and are divided into eleven main categories. Each zone represents a specific temperature range in which plants can survive the winter. The classification ranges from Zone 1 (extremely cold conditions below -45.5 °C) to Zone 11 (mild winters with temperatures above +4.4 °C).

crop_WHZ_Europa_2020_MDDG_109_full.jpg

Wulff, E. & Bouillon, J. (2024): Erstellung einer neuen Winter-härtezonenkarte für Europa unter Berücksichtigung mesoklimatischer Effekte. Mitt. Deutsch. Dendrol. Ges. 109: 81-98. CC BY-SA 4.0. ©Esri ©USGS ©E-OBS

Average annual minimum temperature

Zone Min. Temperature Max. Temperature
Zone 2a -45.57 °C -42.80 °C
Zone 2b -42.79 °C -40.02 °C
Zone 3a -42.79 °C -40.02 °C
Zone 3b -37.23 °C -34.46 °C
Zone 4a -34.45 °C -31.68 °C
Zone 4b -31.67 °C -28.90 °C
Zone 5a -28.89 °C -26.12 °C
Zone 5b -26.11 °C -23.34 °C
Zone 6a -23.33 °C -20.56 °C
Zone 6b -20.55 °C -17.78 °C
Zone 7a -17.77 °C -15.01 °C
Zone 7b -15.00 °C -12.23 °C
Zone 8a -12.22 °C -9.45 °C
Zone 8b -9.44 °C -6.67 °C
Zone 9a -6.66 °C -3.89 °C
Zone 9b -3.88 °C -1.12 °C
Zone 10a -1.11 °C +1.66 °C
Zone 10b +1.67 °C +4.43 °C
Zone 11a +4.44 °C +7.21 °C
Zone 11b +7.22 °C +9.99 °C


Winter Hardiness Zones Worldwide and Their Relevance to Plant Work

  • Zone 1 to 3: These extremely cold zones are found in the polar regions and some parts of northern Canada and Russia. Only the hardiest plants, such as certain conifers and alpine perennials, can survive here.
  • Zone 4: This zone includes parts of Scandinavia, Canada, and the higher elevations of the Rocky Mountains. Plants must be able to withstand temperatures as low as -28.9°C. This requires careful selection of species that can tolerate such cold.
  • Zone 9 to 11: These warmer zones are found in parts of California, the Mediterranean region, and tropical areas. Plants in these zones are often not frost-resistant and need protection in colder climates.


Winter Hardiness Zones in Germany

Germany is primarily located within winter hardiness zones 6b to 8a, with some regions even extending into 9a. The specific conditions of these zones are crucial when selecting suitable plant species. This is important not only for tree nurseries but also for anyone working with plants, including gardeners, landscape architects, and hobby gardeners.

  • Zone 6b (−20.4 °C to −17.8 °C): This zone is found in the alpine regions in the far south of Germany close to the austrian border. Plants thriving in this zone need to be particularly tolerant in conditions of high elevation. They must be extremely frost-hardy and snow-break tolerant. Gardeners and landscape architects in these areas must select robust plant species that can withstand these extreme conditions.
  • Zone 7a (−17.7 °C to −15.0 °C): This zone covers much of south-east Germany. Reaching from the Bavarian Forest to the Ore Mountains in Saxony until the region of Lower Lusatia around Cottbus in Brandenburg, including some of the Central Uplands like Thuringian Forest and the Harz.
  • Zone 7b (−14.9 °C to −12.3 °C): This zone is found in most parts of central Germany and runs diagonally from the south-west to the north-east including cities like Kassel, Magdeburg, Leipzig and Berlin. It is the main zone in Saxony-Anhalt, Mecklenburg-Western Pomerania and Brandenburg.
  • Zone 8a (−12.2 °C to −9.5 °C): This zone runs from south-west to the north-west of Germany and is found especially along the Rhine Valley, Moselle Valley and around the rivers Ems and Weser, including cities like Mainz, Osnabrück, Bremen and Kiel.
  • Zone 8b (−9.4 °C to −6.7 °C): Based on the updated measurements from 1991 to 2020, zone 8b is now the mildest winter hardiness zone found in Germany. It is located in urban areas around the Ruhr area including the cities of Cologne and Duisburg. The coastal strip of the East Frisian Islands and also the Islands of Sylt, Föhr and Fehmarn are located in Zone 8b.
  • Zone 9a (−6.6 °C to −3.9 °C): Not in Germany! Only at a small region in Belgium and the Netherlands.

Zone 6a (-23.33 °C to -20.56 °C) and Zone 5b (-26.1 °C to -23.4 °C) you can only find in high alpine regions of Bavaria and especially exposed areas around the Allgäu High Alps. In this regions plants requiring even greater winter hardiness. Only extremely frost-resistant plant species are suitable here.

crop_WHZ_Mitteleuropa_2020_MDDG_109_full.jpg

Wulff, E. & Bouillon, J. (2024): Erstellung einer neuen Winterhärtezonenkarte für Europa unter Berücksichtigung mesoklimatischer Effekte.
Mitt. Deutsch. Dendrol. Ges. 109: 81-98. CC BY-SA 4.0.



Adapting Plants to Winter Hardiness Zones

The winter hardiness of plants is a decisive factor in their care and cultivation, which must be considered by professional gardeners, landscape architects, nurseries, and private plant enthusiasts alike. Plants native to warmer winter hardiness zones (such as Zones 9 to 11) can survive in cooler zones (like 7 or 8), but only if they are adequately protected during the winter months. We recommend the following measures to protect plants:

  • Overwintering in frost-free spaces: For frost-sensitive plants, especially those in pots or containers, it is important to bring them to a frost-free location during the winter months. This could be a conservatory, basement, or garage. These locations provide protection from extreme temperatures and prevent frost damage.
  • Protection with garden fleece: Garden fleece is a versatile material that effectively protects plants from frost. It serves as a heat reservoir by retaining warmth in the soil while remaining breathable to avoid mold formation. This method is particularly useful for plants that stay outdoors, such as perennials or small shrubs.
  • Mulching: A layer of mulch made from materials such as straw, leaves, or bark mulch can protect the roots of plants rooted in the ground from deep soil frost. This simple method helps stabilize soil temperature and provides an additional insulating layer against the cold.

By considering these measures, the survival rate of plants in less frost-resistant zones can be significantly increased. This applies both to professional horticulture and private individuals who want to winterize their gardens. Properly adjusting care according to the winter hardiness zone ensures that plants remain healthy and survive harsh winters.


Can Plants Be Grown Outside Their Typical Winter Hardiness Zone?

Yes, it is possible to grow plants outside their typical winter hardiness zone if appropriate protective measures are taken. For example, Mediterranean plants that are typically found in Zones 9 or 10 can survive in Zone 7 if they are well-protected or placed in a greenhouse.

Winter hardiness zones are an indispensable tool for selecting the right plants and ensuring optimal growing conditions. By considering regional winter hardiness zones, one can ensure that plants not only thrive but also survive the winter months unscathed. Given the ongoing climate change and increasingly unpredictable weather patterns, understanding winter hardiness zones and applying appropriate protective measures is more important than ever.



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