As you may have noticed, we specify the hardiness zone on every plant listing in our catalogue.
For example, a plant which is described as ‘hardy to zone 7’ means that it can withstand minimum temperatures of -12°C to -17 °C. A plant ‘hardy to zone 8a’ will be able to withstand low winter temperatures of between 12.2°C and -9.4°C.
However, plant hardiness ratings must be treated with caution as it is impossible to be precise about them. Furthermore, the exceptional weather events we have experienced in recent years have shown that we need to be prudent.
The hardiness zones were first developed by the United States Department of Agriculture (USDA) and were then adopted by many other countries.
They were worked out by calculating the average minimum temperatures over the last 20 years. The zones can be considered as indicators which give the “probable lowest temperature’ in your region in the winter.
In Great Britain and Ireland, the maritime influence is particularly strong and therefore produces mild winters in these countries despite their northern location. Their hardiness zones are therefore high (7 to 10).
Only Scotland (Grampians, Highlands) and the Pennines in the North of England have a USDA hardiness zone of 7.
In France, the transition from an oceanic climate in the west to a continental climate in the east is very noticeable. The maritime frontage, from the Channel to the Basque coast has a high hardiness rating (USDA Zone 9), with temperatures of between 6.6°c and -1.2°C.
The large western part of France which stretches to the Paris/Ile de France region has a USDA zone 8 rating, whereas the east of France and the Massif Central have a zone 7 rating. The high altitude regions (Alps, Pyrenees, Jura, Vosges) have a zone 6 rating.
In Central Europe, Belgium, Holland, Germany, Austria, the climate is a good example of transition from a temperate oceanic climate to a continental climate.
This transition is clearly visible when you look at the USDA zones. The zones tend to go down in number moving eastward rather than northward.
In addition, the sea, rivers, hills and plateaux have a significant impact on the extent of hardiness zones. Local topography is also a significant climatic factor all year round and therefore important for what can be grown in a given region.
Hardiness zones for major European cities
Amsterdam (zone 8), Antwerp (zone 8), Belfast (zone 9), Berlin (zone 7a),
Cardiff (zone 9), Copenhagen ( zone 8), Cork ( zone 9b),
Dublin (zone 9), Düsseldorf (zone 8), Edinburgh (zone 8), Glasgow (zone 9a),
Hamburg (zone 7), Lisbon (zone 10a) , London (zone 9a),
Madrid (zone 8-9), Marseilles (zone 9),
Milan (zone 7-8), Munich (zone 6), Paris (zone 8a),
Portsmouth (zone 9b), Rome (zone 9b), Santander (zone 10a), Stockholm (zone 7),
Strasbourg (zone 7), Vienna (zone 6-7), Zürich (zone 7)
Pros and cons of the hardiness zones
Plants – and we cannot stress this enough – are living beings in their own right. They suffer the vagaries of their environment: rain, wind, frost and sunshine, etc.
Hardiness zones are effective in many situations because low winter temperatures very often determine whether a plant can grow outside or not.
Unfortunately, the USDA hardiness zones have their disadvantages because they do not take into account other key factors for plant survival or growth, such as heat and drought in summer, rainfall and humidity in winter, snow, soil and growing conditions.
One factor in plant hardiness is moisture and consequently the dry matter content of the plant. Dry and healthy growing conditions in the winter will help plants resist frost.
Methods: protect potted plants from excessive moisture in winter, provide drainage elements (coarse sand, pozzolan); dry bracken or straw mulch, overwintering covers, alpine greenhouse protection, overwinter frost protection.
bibliographical source: http://en.wikipedia.org/wiki/Hardiness_zone