How Should Hardiness Be Defined?

by Joe B. Parks

Garden Path in Winter
Picture by S & J Perkins
Winter garden path in Salem, NH

I suggest that hardiness should be defined as the lowest temperature at which a particular Rhododendron species or hybrid can be reliably expected to flower. The public's only question is, "Will it flower for me?". And since, except for 'winter burn", "flower hardiness" automatically defines that the plant is also hardy, it is all anyone really needs to know as to the likelihood a given plant will perform at a given temperature.

Some will disagree with this definition as being "too simplistic". But if a definition is made complicated, difficult to apply and understand then only a few technical experts are able to use it. This may satisfy the "experts" ego but deprives those who really need the information. Thus my belief that a hardiness rating should be such that anyone will know what it means without further explanation "Flowers reliably to-15 F." should be an unequivocable statement needing no further explanation.

Having said that, I must say that arriving at such a rating is not a simple task. In collecting hardiness data over the past 15 years, I have observed that great care must to be used in making hardiness determinations. Determinations that are based on observations from a single year or even a single garden over several years are particularly suspect. Word of mouth is even more suspect. Even with data from dispersed locations, I suggest that great care must be taken in making hardiness determinations.

Clearly, even though the available data appears identical, there are variations (micro climes, care, rainfall, etc.) that we are unable to measure. For an example, take a look at the record on 'Catawbiense Album'. The Hardiness Survey shows that in 1994 buds were killed at -30 and in 1995 at -20 in the Wieczoreck garden Yet in 1991 it bloomed normally at -24 in his garden. Still another year, 1984, it flowered at -27 in the Parks garden. Question: What should the hardiness rating be for 'Catawbiense Album'?

The data suggests that establishing a reliable hardiness rating for a given plant may be some what more difficult than many of us have thought. Analysis of the reported data indicates that hardiness can be defined at three levels; 1, the temperature above which a plant (nearly) always performs (defined here as the "universal performance temperature"), 2, the temperature below which it almost never performs - and often dies (defined as the "failure temperature") and, 3, the temperature range between the above two in which performance is spotty (defined as the "uncertain performance range"). Within this "uncertain performance range", a plant often performs well in some gardens but often fails elsewhere as noted above for 'Catawbiense Album'.

The word "nearly" in the first definition is used advisedly because it is not uncommon to find in the Hardiness Report an indication of poor performance within a long run of "Normal" performance. I have no idea what causes these "blips" in the record. They could be errors in reporting, in recording (even after much editing) or they could be an indication that we don't know as much as we think we do. More study is need but currently I am inclined to ignore them.

This discussion applies to flowers, leaves and to overall performance with one exception. What we usually call "winter burn" which damages both leaves and branches (and which I suspect is the source of much "winter kill") usually occurs in New Eng-land at temperatures above freezing. Actually it is not a "burn" at all but in fact is desiccation. It can happen in full shade, semi-shade or in the sun.

Desiccation requires two conditions to be met; 1, the ground or lower stems are frozen and 2, air temperatures are above freezing (usually above +40 F). What happens is this; at some point above freezing the leaves begin to transpire and to call for moisture but the plant cannot respond because either the lower stem or the ground is frozen. Within a short time areas of the leaves and then the smaller stems become so desiccated that they die. The browning occurs a few days later as "winter burn".

In the sun, desiccation can occur at air temperatures below freezing. The sun warms the leaves to the point that they transpire. But since the plant is frozen they cannot replace the transpired moisture. Since part of the leaf is often shaded, often only part of the leaf transpires and thus partial damage results. But make no mistake about it, sunshine is not necessary for "winter burn" to occur. Prof. John Havis of the University of Massachusetts, Amherst has researched and reported on the problem in depth.

Since only a few hybrids and species (in fact I know of no affected species but there must be some) are subject to heavy "winter bum", I suggest we should probably be trying to identify them. The problem must be genetic, and if so, could be solved over the long term. Over the short term, we should be recommending against the use of those plants.

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