What Is Hardiness All About?
Fred E. Knapp, Editor, N. Y. Chapter Newsletter
We are all puzzled by hardiness ratings and by the ability of others to grow plants we cannot, but possessed of definite opinions about the hardiness of our own favorite plants. The hardiness concept is basically easy to understand yet difficult to apply. In the narrow meaning of cold-hardiness, as it is generally applied, the whole area leads to confusion and contradiction.
The best single word for what hardiness means is adaptability, another word that brings to mind a whole collection of ideas. That again means confusion until the collection is summed up into its whole. In England, I am told, plants are often referred to as "good doers". A hardy plant then is a plant adaptable to your purposes, a good doer for you in your situation. With a broader point of view, you must admit that it is not very hardy if it is not a good doer for all your friends who have tried it. How many rhododendron buffs have claimed hardiness for their favorite clone only to be attacked by near neighbors with the vehement statement, "Well, it is not very hardy for me!"? This means the plant is not very adaptable to the range of conditions for the area.
The narrow interpretation of hardiness referred to above is simple cold hardiness, stated in terms of the minimum winter temperature a clone is known to withstand. This is the basis of published ratings. Most of us are familiar with the USDA hardiness zone map, also based on temperature extremes, and these tools are often used to guess the future success of a newly purchased plant. Such an approach can be deceiving. My own first rhododendrons were planted in Bethpage, in the heart of the central Long Island cold belt. I had excellent results with plants I should not have been able to grow at all. There were several reasons for this, but one was the inaccuracy which had grown into the zoning conclusion of the USDA map since it was published. In the late mid-sixties, an article on hardiness appeared in Horticulture magazine, by Andre Viette, a prominent local horticulturist. He specifically referred to the Bethpage/Plainview area as a portion of the cold belt no longer cold! The climate had been significantly altered by the proliferation of development houses, each a very nice source of heat for its surrounding grounds and each overlapping the zone affected by its neighbor. This was one reason for my early successes, but this is by no means the whole story. Other favorable factors included excellent air drainage, good winter wind protection, and most of all, good luck. The latter covers all those things we do not understand about hardiness.
In our May 1970 Newsletter, an article by Charles Trommer, who has gardened extensively in New Jersey and Massachusetts, made some pertinent observations about relative hardiness of particular clones in different gardens in these two areas. His experiences made it quite clear that moving to a "warmer" area could as easily reduce the apparent hardiness of a clone as enhance it. Temperature extremes or averages simply do not explain what we see happening under the name of hardiness.
What are the other hardiness factors if temperature is not sufficient to explain the great variations we see? Some plants try to give us clues to this question. A number of plants of our genus, and indeed of others also, reach hardiness only with some degree of maturity. They must become large, woody of trunk to some minimum dimension, thoroughly established in their site with adequate root system for their winter needs, and suddenly they show a quantum improvement in hardiness. Other plants and this is virtually a rule, live and die at the same temperature extremes in different areas. This demonstrates that hardiness is not an invariant primary characteristic of a plant which displays always the same defiant face to its environment. It is the product of a process, an annual process which affects the chemical and physiological nature of the plant to help it accept cold temperatures and drying winds. The process is complex, requires time and timing for its completion, and above all requires a maximum of synchronization with the environment to produce good effects. This synchronization is the culprit responsible for our confusion about hardiness. It is behind the failure of ironclads in mild winters, the success of one gardener with another's nemesis, and of course the long misunderstanding of West Coast/East Coast hardiness requirements shared by nurserymen and amateurs alike.
Synchronization as used here refers to all the factors which contribute to regulation of the life process of the plants we grow. These include temperature extremes and durations, rainfall - amount and timing, soil temperature and texture, mineral and organic matter available to the roots and when these arrive in the root zone, pruning whether natural or artificial, light and shade mixture and timing through the day, wind exposure and typical humidity, and anything else the reader may care to add. All of these factors tell the plant when to grow, how much to grow, when to go dormant, when to prepare for coming events such as winter. The summer history of a plant thus has as much to do with its fate in winter as has winter itself. All of these factors have a sort of Jekyll and Hyde nature. For them however, it is not an evil potion which triggers their schizoid reactions, but simply timing. We must have nutrients and moisture at the right time; we must not have them at other times or we destroy the ideal rhythm of the plant's micro-climate. When these are well synchronized, the plant seems hardy even though it may be two zones away from its temperature rated hardiness. If they are not well synchronized you will know it, and you will perforce believe that your plant's rated hardiness has been set too high.
There are many typical examples of synchronization problems. One of the common ones is spring transplanting with a handful of nitrogen bearing fertilizer, often accompanied by a well meant pruning to urge the plant to branch strongly in its new location. In so doing, the bush rapidly outgrows its roots which have been sharply limited by transplanting. Thus in midsummer dry heat or in winter cold, a clone usually hardy in the area may die. Early fall or late summer pruning or feeding may also prove unfortunate, and the grower is lucky to lose only the untimely new growth forced out by these practices. The weather itself, by a moist late fall, or an unusually early spring thaw, may alter a plant's rhythm enough to destroy its synchronization with the environment and thus reduce its hardiness. The list goes on, and each of us can no doubt add his examples.
The Long Island area, with wild swings of temperature, fierce summer heat, uncertain rainfall distribution and occasional severe drought, and little snow cover, requires a maximum of adaptability to produce a good doer. In many cases this has nothing to do with simple temperature hardiness. In a climate like ours, the best plants are perhaps very like a dull phlegmatic person. Both have a life rhythm that responds slowly to external stimuli and can often go through temporary but severe stress without any significant reaction. In any case, the gardener's job is to use only the best cultural practices, to avoid excess in doing even the right things, and to be aware of and experiment with the different micro-climates, soil conditions, and environmental rhythms which even a small property can provide within its own limits. To a very considerable degree, the gardener can enhance the hardiness of his plants. It takes patience, time, and experimentation, but whenever you meet a friend who is growing successfully a "tender" clone, say to him, "If you can grow it, I can grow it! " - and go do it."