JARS 49n4 - ARS Hardiness Reporting Criteria

ARS Hardiness Reporting Criteria
Richard A. Houghton, Jr.
Orleans, Massachusetts

Do you have trouble understanding the new hardiness definition adopted by the ARS (Vol. 48, No. 2)? I have prepared a simplified example that may be helpful in clarifying the concept of temperature range. The challenge is in the last sentence of the definition:
Cold hardiness is defined as the temperature range through which damage to flower bud, leaf or plant structure can be expected to occur, in a plant at least five years of age and in good health. Flower bud damage is defined as that which detracts from a normal floral display. The range is given by two figures, expressed in both Fahrenheit and Celsius. The lower figure is the lowest temperature after which the plant has been observed to perform normally; the upper figure is the highest temperature after which cold damage has been observed.
My first reading of that last sentence produced a "How's that again?" I did a double take. The higher temperature is associated with damage while the lower temperature is not. That seemed intuitively wrong. I read it again and thought about it, and one thing leading to another, I developed the accompanying curves to aid my understanding.

temperature-time histories for two 
winters which defined the hardiness range for hybrid plant
Figure. Simplified temperature-time histories for the two winters which
defined the hardiness range (0F to -10F) for Hybrid X.

The two temperatures are minimum temperatures for particular seasons. These temperatures are from temperature-time histories gathered over many seasons for a plant we will call "Hybrid X". This hybrid has been determined to have a hardiness rating of 0°F to -10°F. The two extremes can be plotted for the two years that provide the significant data. The two extremes in the example are identified as years two and five.
Year two had an unusually mild early winter followed by a sharp drop and prolonged cold spell with strong winds and no snow cover. Because of the mild fall and early winter, the plant was unprepared for the sudden and prolonged cold when it came. Damage followed. Year five grew gradually colder until late winter when a brief cold period occurred. The plant was much better prepared for the low temperature when it finally came. There was no damage.

Richard Houghton, a member of the Cape Cod Chapter, first became interested in rhododendrons in 1966 on Long Island, where he assembled his first collection as an active member of the New York Chapter. He now lives on Cape Cod.