JARS v45n3 - Defining Hardiness

Defining Hardiness
A new look at hardiness ratings
Herb Spady
Salem, Oregon

Since my wife Betty and I have lived in our present location in the mid Willamette Valley east of Salem, Oregon, there have been at least four episodes of, so called, unusual cold weather. The thermometer has dipped to 0°F or below. This has given us considerable unpleasant experience in the hardiness and lack of hardiness of rhododendrons.
The Pacific Northwest is considered by many as an ideal place to grow rhododendrons. That generalization applies only to specific micro-climates. In our area the cold continental air seeps through the mountain passes and gently settles in the low lying western valley.
There are no significant night winds during the cold periods so radiation cooling is extreme. In areas exposed to the winds, mixing of the air occurs and night temperatures are higher. The most extreme example of this occurred in 1972 when the temperature at George Clarke's nursery adjacent to the Columbia River was 12°F, whereas the temperature at our home was -18°F. That was an astounding difference of 30°F only 40 miles apart. During cold weather the low temperatures in the Portland area are considerably higher than they are in the Salem area. Unfortunately this has given us an opportunity to consider the hardiness and hardiness ratings of many rhododendrons commonly grown in the region.

R. 'Alice' with cold bud damage
'Alice' with bud damage due to cold
Photo by Herbert Spady

There are many factors which contribute to the hardiness of rhododendrons. The primary factor is heredity. Obviously tropical, subtropical and mild temperate plants and their hybrids will not generally be hardy. Plants native to severe climates will be hardy and their hybrids will usually be hardier.
Heredity establishes the basic cold tolerance of the plant, but there are important cultural and environmental factors. The age of the individual plant is important. Older plants are hardier then young plants. A liner planted out in a nursery or in a pot is very vulnerable to cold damage as contrasted to a mature landscape plant. The season at which the insult occurs is important. Extreme cold occurring before a plant is hardened off for winter dormancy or after the plant has been deceived into thinking that it is spring is very damaging.
Not only the season and the degree of cold are important. As the duration of the freeze extends, the ground freezes progressively deeper. The roots are damaged or non-functional, and the plant is unable to pull up adequate water for transpiration. If there are accompanying winds under these circumstances the dehydration can be severe. Snow cover is highly desirable. Often plants will show a visible differential of damage at the snow line with the plant below the snow healthy and blooming in the spring and the plant above dead. Small leaved and compact plants hold the snow much better and do not provide a path for cold air to the interior of the plant. They benefit the most from snow cover. Exposure of the leaves to sunlight increases the dehydration and the leaf damage.

R. 'Vulcan', bud damage Bark splitting resulting in 
plant death
R. 'Vulcan', bud damage, -4F
Photo by Herbert Spady
Bark splitting resulting in plant death
Photo by Herbert Spady

What then can the rhododendron enthusiast do to protect plantings during cold weather? Most important, we can grow and develop plants that are genetically hardy. Second, we can be sure that our plants are healthy as we approach the winter season. Thrifty plants survive better than plants that are under stress from lack of nutrition, disease, insect attacks or other factors. We can avoid cultural practices that reduce dormancy such as late summer fertilization.
There is little that we can do about unseasonable cold weather. On a small scale, covering plants with cloth, plastic or other materials provides the same type of protection as snow and is very effective at preventing damage. Although such protection can be applied on a small scale it is not practical in a large garden. When obtaining plants for putting out in the landscape, it is important to secure plants that are of a reasonable size.
ALL DO NOT BELIEVE THE HARDINESS RATINGS! They may be a general guideline, but they are inconsistent and overly optimistic. Our experience is that this is especially true of new hybrids. One problem is that there is no standard definition of hardiness. When a person registering a rhododendron states that it is hardy to 0°F, one does not know if that person means that it has normal foliage and flowers after 0F or that a 10-foot high mature plant was killed to the ground and then grown out after such a temperature. That is an extreme difference.
We are going to apply a new notation for hardiness in our garden. First, one must define the character of the plant. Since most novices are buying plants of modest size we have chosen a plant size of a caliber of at least one inch at the base of the plant and in a good state of nutrition without any signs of other stress.
Second, we are going to use a multiple notation similar to the quality ratings. Three figures will be used, the first representing the highest temperature at which any damage is detected in the plant, that is, leaf damage or flower bud damage. The second figure will represent the highest temperature at which structural damage occurs in the plant, that is, death of branches. The last figure will represent the highest temperature at which complete plant death occurs. These figures will be used regardless of the season, snow cover, winds or other environmental or cultural factors. Thus they will represent the worst possible scenario. The figures 10°F (-9°C)/0 (-18)/-15 (-26) will be a much better representation of hardiness and be much more useful. There is no doubt that there will be individual plants of a clone that survive or perform below such a definition, but there can be no better knowledge than knowing the worst that one can expect.
It is our recommendation that such a standard definition of hardiness be adopted on a widespread basis and that we abandon the wishful thinking and optimism represented by the current hardiness ratings.
The proposed format is much more informative and less confusing than the various old methods. Abandoning ratings such as the H1, H2, etc., would eliminate much confusion both here and abroad, as the definitions are just the opposite in the two regions and they have no value to the people that are not privy to their meaning.

Herbert A. Spady, M.D., is the newly elected Western Vice President of the ARS. He received the Gold Medal Award at the ARS National Convention in Oakland, CA, in April for his service on chapter, district, national and international levels.