Frequently we receive questions regarding the hardiness of the plants we sell. I do not list hardiness zones in the catalogue as I feel they are too rigid. Most of the plants we offer are hardy into very cold winter climates (lows of -30°C). Very few plants survive in places where temperatures dip below -40°C as there is an actual physical change in the water molecule below this level and only a few genera have developed the special adaptations needed to survive. Very few customers live in those regions, so it is not a condition we have to consider.
As a grower these are the factors that matter most:
Geographic origin of the plant sort of like the idea of using native or locally occurring genera; but, a step beyond.
When I go over the various seed lists that come in, I look for seed collected in the "middle latitudes" for two reasons. The climate is likely to be roughly equivalent, with some winter experience, and day length experienced will be likewise similar. Day length can be a problem with some plants from arctic regions or alpine equatorial regions. Collections from southern hemisphere middle latitudes are often frustrating when first encountered, and just as often written off as "not hardy". Perhaps so, but I think the problem is just lack of experience and knowledge of exact growing conditions. There is always a need for the "experimental garden".
Climate type - A very broad category, primarily determining the moisture cycles operating in specific regions to which plants must adapt.
Knowing the native habitat cues you to cultural measures that will accommodate the plant's needs. For example, in the Great Lakes region, where we are located, the precipitation is approx. 75cm - 100cm per year, and in our locale, it is distributed through the year on an equal basis. A drought here rarely lasts more than 4 weeks. Many dry-growing plants do very well here if planted into a coarse, lean gravel/sand mixture as the excess moisture drains and evaporates quickly in the summer heat.
A more difficult problem is excessive night time heat, the converse of prolonged winter freezing. The respiratory rate for plants increases exponentially between 10°C and 15°C. For this reason, grain crops such as corn do well in hot weather because they are annuals that concentrate on fruiting. Alpines are slow growing plants adapted to heat-poor situation. Surprisingly many can accommodate very hot conditions during the day, but need a cooling off period of lower activity at night. The critical temperature is considered to be approx. 15°C. My observation is that above 25°C is stressful and above 35°C damage becomes noticeable. Yet you will find no mention for this in hardiness charts and the usual advice is "water". My own advice is, "Water very carefully." That is, water in the early morning, the coolest part of the day. This allows the water to drip off and evaporate before it can be overheated by intense sunshine, and is followed by the longest period of sun and heat enabling the surface of the ground to dry out sufficiently. A gravel mulch helps immensely. Many plants actually go into asemi-dormant state (Primula spp.), some full dormancy (dry growing iris, Centaruea achtarovii). The survival state of dormancy can arise in response to the harsh conditions of either winter or of summer in different plants. Planting in tufa and using tufa for crevice garden construction does help mitigate heat impact.
Native conditions with respect to acidic or basic (limestone) soils/rocks.
Some plants such as Gentiana sino-ornata absolutely need an acidic condition and this includes the water used to irrigate. Irrigation water can be a significant introductory source of pH-raising salts calcium in particular, and in the absence of significant rain to flush the soil, can lead to a rapid change in soil pH, particularly in containers. Rainwater is still the best.
In summary, hardiness factors outweigh numbered zone charts, which should be regarded with a discriminatory eye. I find the basis of understanding a plant lies in the field notes of the collector. This, combined with a good garden sensitivity, is often sufficient to manage the difficult and the sensitive.
(2007 Catalogue Introduction)