Beginners Glossary: Cold Hardiness
The term "cold hardiness," often referred to as just plain "hardiness," is so complicated a concept and so difficult to measure that the ARS has adopted its own definition - which in itself is difficult to grasp on first reading. In general, cold hardiness refers to the coldest temperature a plant can withstand. The difficulty in accurately assigning a plant a cold hardiness rating arises from the multiplicity of factors involved, i.e., temperature, wind exposure, hardening off period, snow cover, etc. On top of this, the various parts of the plant may be damaged, such as flower buds, while other parts are not, such as foliage. Nevertheless, attempts have been made to assign rhododendrons hardiness ratings.
The British have developed four ratings based on their climate, and these ratings will be found in the Royal Horticultural Handbook 1980 in each species entry. They are:
H1 normally requiring greenhouse protection.
H2 requiring protection even in the most sheltered gardens.
H3 hardy in the west and in favored areas in the south and east.
H4 hardy throughout the British Isles.
The United States Department of Agriculture has devised zones based on average annual minimum temperatures. Frequently a plant will be labeled by the coldest zone in which it can live, but this designation is so general that exceptions abound.
A more common designation found in plant catalogs and the ARS Plant Name Register is simply a temperature, such as -5°F, indicating it is hardy to that temperature - but maybe even hardier or, according to some gardeners' personal experience, not that hardy!
Frustrated by the shortcomings of all these ratings, the ARS is taking a new perspective: pinpointing the range of temperatures between which one can expect some plant damage. There are two temperatures involved here. The low figure is the lowest temperature at which a plant has been known to perform normally. The high figure is the highest temperature at which cold damage has been observed. Since much data must be collected before our rhododendrons will have the ARS cold hardiness designation, we have to rely on the systems in use, as inadequate as they may be.