Popular Science Monthly/Volume 18/April 1881/The Relative Hardiness of Plants
THE isothermal line, curving up and down the map, is no inapt illustration of the course another line would take on the chart which sought to explain the relative hardiness of plants, only the curves of the latter would be more complex than those of the former. Who, indeed, could direct aright such a line for even individual species? and for varieties it would be wellnigh impossible. Scarcely could reliable data be furnished for the broader division of genera. And yet the investigation that does not take into account varieties misses a large number of plants possessed of the most noteworthy and valuable individual traits. The question may be easily asked, Wherein lies the difference between a variety and a species? but the answer evidently is not so easy, when we consider that every individual plant varies in a degree from all other plants; and, to render it still more difficult, we find botanists very properly ignoring the existence of varieties that may have individual characteristics invaluable to the planter.
Latitude, moreover, we find is only one factor, and a very vague one, in the problem of determining the relative hardiness of plants. Climate is the real governing. element—climate, that varies with the special conditions of every valley and hill-top. Yet the knowledge and insight that determine the relative hardiness of plants seem easy of acquisition in the eyes of the inexperienced. Still, no one, really an expert in such matters, will venture to express a decided opinion on such data as are usually obtainable. A canny Scot of our acquaintance, whose knowledge of plants in a practical way is encyclopedic—if such a term in such a connection be admissible—is a notable example of this wise cautiousness. Never was he known to positively commit himself, at least on the subject of the peculiarities of plants. "They might be hardy," he would say, "and again they might not. Circumstances alter cases." There was always a profound consideration shown for the incalculable effects of any particular environment.
That we may better realize the strangely complex character of the relative hardiness of plants, let us consider briefly the special behavior of various kinds under apparently similar conditions, and then note with care their behavior under evidently different conditions. Take Japanese plants, for instance. They illustrate how, sometimes, on two sides of the globe, in the same latitude, almost identical species and varieties appear. Whether it is the similarity of the course of the oceanic currents along their shores and the trend of the mountain ranges inland that modify the two climates into a peculiar likeness, or still more recondite causes, the fact remains that whole genera of Japanese plants resemble in the strangest way indigenous American kinds. Retinosporas, the most popular evergreens of Japan, seem, in some cases, though confessedly distinct botanically, nearly identical in appearance with Thujas or American arbor-vitæs, and they also behave alike. In grafting, for instance, members of one genus require members of the same genus as stocks. Yet the Retinosporas, which graft well, of course, on their own stock, graft also with entire success on the American arbor-vitæ, which the botanist tells us is an entirely distinct genus. They are, likewise, equally hardy, but the variation in form and color, in both Japan and America, is much greater among Retinosporas than among American arbor-vitæs. This capacity for endurance of like conditions which appears among Japanese and Eastern American evergreens runs also through deciduous trees. There are maples on the Amoor River and in Japan which have little to distinguish them from at least one variety of maples in America, and there is a similar kinship in reference to hardiness.
These instances of resemblances in hardiness and in other features, between plants botanically widely different, might be greatly multiplied, but those mentioned will suffice to show the widespread likeness, in these respects, of many different plants throughout the globe. In face of these extraordinary instances of similarity in hardiness of plants the native haunts of which are widely removed, we have to recognize the existence of many kinds in the same region, closely related botanically, and yet entirely different in their several degrees of hardiness. One of the salient instances of diversity in hardiness of nearly related plants is the behavior of Japanese persimmons in this country. It was confidently expected that they would prove hardy in America, because native persimmons were hardy, and because the general hardiness of Japanese plants in America had been often demonstrated.
But fifteen years' trial of a few detached plants, and five years' trial of thousands together of these Japanese persimmons, prove their hardiness more uncertain than that of the American persimmon. What shall we say of the other Japanese plants that fail to prove hardy in America north of Tennessee, or even Florida—of the Osmanthuses, best described as resembling the hollies in appearance, of the privets, live-oaks, Ancubas, etc.? They are common enough plants in our greenhouses, but only in very sheltered positions, and during mild winters, will any of them live uninjured in this climate. Such facts must be very perplexing to any theorist who attempts to explain why and where this or that plant is hardy.
Or let us change somewhat our problem, and consider why plants belonging to countries much nearer home than Japan, but in similar latitude, fail to prove as hardy here as many Japanese plants. Note the fact, moreover, that these trees I shall next refer to come from even colder regions than Japan, and yet Japanese plants of the same genera are usually more hardy. The ways of plants, verily, become still more puzzling when we find such evergreens as Thuiopsis borealis and Thuja gigantea, natives of northern Oregon, fail even under the best treatment sometimes, during winters of New York and Philadelphia. Some explanation may of course be attempted by adducing the peculiar climate of the Pacific coast in its rainy seasons, but then consider that many of these plants are found eight and ten thousand feet up in the mountains, and also that, when we pass a few hundred miles farther east in the same parallel of latitude, we find the same varieties and even species such as the Douglas fir becoming hardier. Few, comparatively, of the California native deciduous trees are hardy in the East, and even for many Oregon trees of the same class, such as Acermacrophyllum, there is much suffering in store during hard winters on the Atlantic coast. Passing over to Northern Europe, the behavior of trees is still more perplexing. To be sure, as a rule, the Gulf Stream insures milder climate in the same degree of latitude, but away up in northern Scotland, and even in Norway, we find many evergreens more hardy than in the more temperate latitude of New England. Rhododendrons, hollies, and all evergreen shrubs, if not all evergreen trees, do better there, which is doubtless to be attributed in part to a moister climate and more equable temperature, but it can hardly be that altogether. On the other hand, what can we say to the evergreen Thuiopsis Standishii doing better here than in England, and Thuiopsls dolobrata better in England than in America? Japanese maples, that seem to grow more thriftily and vigorously here than in Japan, give evidence also of being less adapted to England than to the United States. Besides, why is it that evergreens thrive better, and are more hardy, in a cultivated state in Europe, and deciduous trees in this country? We know answers can be given by experienced observers to all these questions, that are more or less comprehensive, but we believe also that, when such answers come to be closely scanned, it will be found that they do not entirely meet the case. How is it, otherwise, that the same peculiarities in a minor degree are evident in the behavior of trees growing within a few miles of each other? One might understand why the same plants act differently farther inland, but here in the neighborhood of the coast it naturally strikes us as curious that on the Hudson River some plants are hardier than on Long Island.
There are more inexplicable facts than these. Mr. Hunnewell can grow plants on his lawn that will hardly live through some winters, even under the most favorable conditions, on any other spot about Boston.
Nor is this the strangest feature to be noticed in the behavior of plants under apparently like influences of soil and climate. Plants a few feet from each other, of the same species, will suffer in very different degrees during many winters. Rhododendrons are a notable instance of this. It is not simply that Rhododendron ponticum and its hybrids are not as hardy as Rhododendron Catawbiense, nor that the more of the Catawbiense strain there is in a Ponticum hybrid, the hardier it is, but it is that sometimes a Ponticum hybrid, usually entirely unreliable, will pass the winter unscathed, when nearly the hardiest pure Catawbiense of the plantation will be killed. But our expert says, "One did not ripen its wood as well as the other." Yes, but is it not also strange that sometimes the one which finally died was the one that had ripened its wood most thoroughly?
A few striking examples like these should be sufficient to illustrate the great difficulty that must always attend the determination of the relative hardiness of plants. Many more instances of the same character might be readily selected, but it is not necessary. We have simply endeavored to give sufficient data to warrant the general statement that the varying and complex conditions of the environment of any given plant are difficult to understand or explain on the basis of experience of another environment which, to a superficial observer, may seem to be identical with the first. Our intention is not to insist on any explanation of the facts adduced in regard to the relative hardiness of plants, but only to show distinctly the difficulties that must attend such explanations, and to point out that experience is now being purchased too dearly, and that it is, moreover, not of a sufficiently varied character. Hundreds and thousands of plants are killed every year under very similar circumstances, and it seems evident that human intelligence should be sufficient to compass some method of reducing this loss to a minimum.
The Cambridge Botanic Garden and the Arnold Arboretum, adjoining the Bussy Institute, being well organized and both managed under the auspices of Harvard College, would be perhaps the best repository of reports on the relative hardiness of plants.
The proper sources of these reports would be botanic gardens such as those of Washington and Cambridge, and parks such as those of New York and Philadelphia, the superintendents and gardeners of which might be directed to make careful investigations and fill up printed forms month by month on the behavior of plants in different localities. Above all, private individuals—and they need not be trained observers—all over the country should be encouraged to investigate in the same systematic way and report to the central repository. Consider how valuable such records of actual hardiness would be, coming from interested observers everywhere, if the resultant tables were published in a compact form! The perplexing question of the behavior of rhododendrons, for instance, would probably be explained, whereas twenty-five years of unsystematic observation has been very barren of results.
It may not be out of place in conclusion to say a word concerning the so-called acclimatization of plants. The name seems to imply the use of some peculiar treatment whereby a half-hardy plant is made hardy. There are many people who really fancy that tender plants may be rendered hardy by first protecting them carefully and then exposing them more and more by degrees until they are taught to endure a manifestly greater amount of cold than they did at first. Natural selection carried on for hundreds and thousands of years may accomplish a change of nature of this sort, but, under ordinary limitations of time, the attempt to acclimatize, in this sense, is practically futile.