Some Elementary Facts About Hardiness
By J. Harold Clarke
One of a series of talks for beginners given before the Portland Chapter
The disastrous freeze of last November is still a very common topic of conversation wherever northwestern Rhododendron fans get together. The final score as indicated by the survival of individual plants and the per cent survival of varieties is still being computed and lessons are still to be learned. The beginner, with two or three plants, however, may be somewhat puzzled by the whole matter especially since the reports of damage may not be very consistent.
The following simple facts about hardiness are given in the hope they will be helpful to such beginners. Although it sometimes has other connotations the word 'hardiness' means ability to survive cold. The word commonly used to mean the opposite of hardiness is 'tenderness.'
Hardiness is not a simple problem. This is well illustrated by the fact that some of our Rhododendron species which were collected from high mountains, sometimes over 10,000 feet elevation, have on occasion proven to be surprisingly tender when grown in this country. Included among many factors which may be responsible for this phenomenon are the following. By a process of natural selection only the very hardiest plants of a species would survive at such altitudes but seed collect average d much less hardy than those from such plants would probably except. In many, and probably most, cases plants in the high mountains are covered with a thick blanket of snow during the severe parts of the winter. Furthermore, conditions of weather and available plant food cause the plants to 'mature' relatively early in the fall.
In considering hardiness we should keep in mind that there are several forms of cold injury to plants, or `winter injury' as it is often called. One form doesn't even occur in the winter but during frosty nights in spring or fall. If the temperature goes a little below freezing, open flowers, actively growing shoots, and recently expanded leaves are likely to be killed or injured more or less severely. Sometimes the injury is so slight it is invisible immediately but will appear later as deformed leaves which were damaged while just pushing out of the bud.
Other types of injury may involve complete death of the plant although this may b-a evident at once or only after considerable time has passed. The 'lingering' death is usually associated with damage to the crown of the plant which is that part of the stem just above ground level. Serious damage here is usually accompanied by bark splitting and loosening which is evident only on close examination. Just why it is so, is hard to explain but all kinds of woody plants seem to be especially tender at this point where the tissues are apparently the last to mature. What happens to grafted plants during severe freezes is interesting. Sometimes the flowering part of the plant is relatively tender and the top will be killed back to the understock which may send up shoots and make a new bush entirely different from the original. Then the flowering variety may be a very hardy one grafted on a relatively tender stock, in which case the roots may be killed but the top uninjured directly by the freeze, although of course it will die because of lack of nourishment from the roots. Under some conditions the leaves or the flower buds may be injured with little or no injury to trunk and branches.
A complex part of an involved subject is the exact way in which freezing kills plant tissues. Much research has been done but no definite answer is forthcoming. It seems to be associated with the formation of ice crystals in the cells and death may result from the mechanical rupturing of the cells or from chemical changes associated with the withdrawal of water when the crystals form.
What is more pertinent is, what factors affect hardiness and what can be done about them. In the first place varieties differ greatly, some being killed by a light freeze, others being able to withstand -25°F. or lower. Information about most of the common varieties can be secured from the Hardiness Ratings published by the A.R.S. in "Rhododendrons 1956." Obviously the minimum temperature reached is directly associated with the amount of injury to be expected. But the rate of fall is also important, a rapid drop usually resulting in greater injury than a slow decline. The temperature for a few days before the freeze is a controlling factor. It is well known that a severe freeze in spring or fall while the plants are still growing is likely to be disastrous but it is less well known that warm spells during the winter tend to make the plant more tender and that cold weather tends to condition the plant to withstand lower temperatures. The rate of thawing also seems to be important in some cases although this relationship has not been very clearly established.
There is no doubt but that the condition of the plant is a very important factor and one which may, to some degree, be controlled. The stage of maturity of the plant, as it emerges from dormancy in the spring, grows actively and then undergoes the changes associated with approaching dormancy in the fall, is definitely correlated with its resistance to cold, the more mature it is the more resistant. Associated with this maturity and affecting it materially is the relative succulence of the plant which depends on temperature, moisture and especially the relative abundance of available nitrogen. Hence the oft-given advice to avoid over watering and over fertilizing in late summer. Exposure to sun and wind are often mentioned in connection with hardiness. Such exposure, or lack of it, will affect the growth of the plant, its rate of maturing and its relative succulence. Also strong winds striking a plant directly will cause it to freeze more rapidly and often result in more injury than may occur on a plant of the same variety in a more sheltered place. In some localities specimen Rhododendrons receive considerable protection from a barrier of burlap placed around the plant as a windbreak.
Since much of the injury to rhododendrons in the Northwest has taken the form of girdling or damage just above the ground level it would seem practical to do some mounding around the trunks of particularly valuable specimens, with soil or sawdust or other mulching material, to cover the most susceptible part of the trunk. Such mounding is an accepted practice with fruit trees in some areas where crown damage has been prevalent in the past. Such mounds are knocked down in the late spring to facilitate normal maturing of the trunk tissues.