What Shall We Plant
By J. Harold Clarke
This is a frequent question as we in the Northwest continue to assess the damage caused by the freeze of mid-November. Rhododendron growers are not alone in this for many different kinds of trees and shrubs, ordinarily quite hardy, were killed or injured. But many rhododendrons did get hit and, partly because there are so many varieties to choose from, some of which were uninjured, and because there are hardiness ratings available, rhododendron fanciers may be inclined to give special consideration to hardiness when buying stock for replanting.
The rose grower who saw all his plants killed or badly injured, and with no "ironclads" in the field, will probably replant his favorites on the basis of growth characteristics, resistance to pests, and beauty of flower with little thought about relative hardiness. The rose grower, for various reasons, is more accustomed to replanting than is the rhododendron fancier, who may have come to think of his plants as good for a lifetime-and many of them are.
An A.R.S. member recently mentioned losing an 8 ft. specimen of 'Sunrise' which he had had some 10 or 12 years. "Was it worth the effort?" There was no hesitation in his answer that it was. If I were in his place, and particularly liked the variety, I would replant it as I do not feel the injury caused by such an unseasonable freeze is necessarily an indication of lack of hardiness.
A Freak Season
As Dean Collins, veteran garden writer for the Oregon Journal said in a recent column regarding the effects of the freeze on woody plants in general, "Don't allow your experience with shrub and plant damage by the weather freakishness of the past three or four months lead you to make final decisions as to the hardiness or non hardiness of plant material in your garden." Pointing out that it was a freak season and that such early and severe freezing had never happened before in the memory of anyone, and that the late summer and fall had kept the plants from going into dormancy he continued, "Plants that had proved fully hardy by all normal tests were wrecked by this combination of conditions. Such material, if it was killed in the freeze, should not be marked off your list as not hardy enough for this climate.' It can be replaced and the odds are in favor of its going on for years through any winter we may reasonably expect."
And so, agreeing with the above quotations, my answer would be, in general, to replant with those varieties you would have used before the freeze. Where two varieties have equal appeal otherwise, it would of course, seem the part of wisdom to plant the one showing the most resistance to the freeze, if we can be sure that its survival was due to inherent qualities and not to having been in a more favorable location.
Should Ratings Be Revised
Several A.R.S. members have asked me if we should not use the information developed by the November freeze to revise our hardiness ratings at this time. I have given it careful thought and would like to present my reasons for giving an answer in the negative, insofar as immediate revision is concerned.
In the first place cold hardiness is a very complicated problem. It covers resistance to early freezes such as we have recently experienced, to low midwinter temperature, to spring frosts or late spring freezes, and by leaf, bud, twig, trunk and roots. Varieties which are very cold resistant in all parts but flower buds have often been observed. Furthermore hardiness is affected by the nutritional status of the plant, by the soil fertility, water supply, exposure to light, seasonal light conditions, summer temperatures, and occurrence of light frosts in the fall, in addition to actual minimum temperatures. These environmental factors affect that general condition of the plant which we call its 'stage of maturity.' It is well known that poorly matured plants are more susceptible to cold injury.
There is also an inherent pattern, typical of the variety, which may cause it to mature relatively earlier or later in the fall than other varieties, and to start growth earlier or later in the spring. We are conscious of early spring development because we can see the buds swell, but early or late fall maturity are essentially chemical conditions within the plant, of which we are ordinarily not conscious, until some catastrophe, such as we have experienced, points it out.
It is also a fact that rapid drops in temperature are more likely to cause injury than are slowly falling temperatures. A plant will be hardier in winter following relatively cold weather than it is after a warm spell, and this may cause the hardiness of an individual plant to fluctuate several times during one winter.
Ratings Indicate Specific Kind of Hardiness
Obviously no single rating could express the ability of a plant to resist cold under all these conditions. The A.R.S. hardiness ratings indicate, based on the experience of a large number of growers, the minimum winter temperature which well matured plants of a particular variety may be expected to withstand without appreciable injury to any of its parts, buds, leaves or woody tissues. This "dormant" hardiness is, in the long run, the phase of most importance to rhododendron growers throughout the country. Although the growing season was late last fall, the November freeze probably does indicate fairly well which varieties are early maturing, and as that is worth knowing it is hoped that considerable data will be accumulated and published, covering a large number of varieties. This information will not necessarily indicate the hardiness of the same varieties to midwinter cold.
The A.R.S. hardiness ratings are not perfect as they are not based on scientific research, but on observations, and they will become more accurate as more people make observations during more years. The ratings, therefore, should be revised every few years, to increase their accuracy, as well as to cover more varieties. The data accumulated as a result of this freeze should be studied carefully as they may indicate that certain varieties should be given special attention when hardiness rating revisions are being considered. But if they will still withstand a certain minimum temperature, when well matured, then that should govern the rating, even though they might have a tendency to grow late in the fall or to start so early in the spring that spring frosts frequently injure the opening flowers. Such information could well be given in special lists or descriptions of the particular varieties.
I have checked a large number of varieties on my own place and grouped them in 5 classes, based on amount of injury. In general the hardiness ratings and the amount of injury correspond fairly well, but there were many exceptions. In a few cases this may be because the rating is inaccurate, but in many others it may mean that a relatively tender variety matured early, or a relatively hardy one was a bit slow in maturing. Continued study over many years will improve our knowledge of this complex problem, but it will be a problem as long as there is a possibility of freak weather.