Klimazonenangaben sind eigentlich für A&F.
Dazu sind die Kriterien viel zu schwammig und fast nichtssagend.
Aber viele Leute meinen, daß das das Gelbe vom Ei sei...
Dazu ein guter Absatz einer US-Gärtnerei:
"Plant hardiness
We can repeat this rant until we're blue in the face: hardiness zones do not tell you most of what you need to know to grow many plants. For example, Syracuse, New York and Omaha, Nebraska, are both in zone 5. Are their growing conditions similar? Of course not. The totality of the growing climate, not just the winter low temperature, affects the plant's performance. Snow cover, soil type, presence of pathogens and predators, patterns of temperature change, patterns and amounts of rainfall, drainage, and other things all matter. Is there any simple way to summarize growing conditions? No. A good gardener experiments. A good gardener kills a lot of plants.
To complicate matters, when you attempt to grow plants from ecosystems very different from your own (some of our South Africans, for example), it may be impossible to predict whether they will do well for you, because too little is known about their cultural requirements. Oswego does not much resemble the Drakensberg, but a lot of Drakensberg natives seem to like Oswego. Why? We aren't sure. Maybe we just have a magical mix of conditions. Doubtless you too have a magical mix of conditions, but perhaps it suits a different set of plants. You must be willing to experiment, because we simply cannot tell you whether any given plant will do well for you.
The bottom line here is that we include hardiness zones largely to pander to popular prejudice and give you a small degree of guidance. The fact that your zone number is equal to or lower than the one given for a particular plant must not be construed as an assurance that this plant will grow for you."
http://www.senecahillperennials.com/index.php?page=about-our-nursery