Nut trees of most species commonly thrive at both latitudes and altitudes much greater than the limits of regular or even frequent crop production. This fact is seldom fully appreciated by prospective planters, particularly in the North, who, not unnaturally, assume that the presence of a group of vigorous appearing trees, or even of a single tree, particularly in a fruitful year, is sufficient evidence of local hardiness to justify commercial planting. However, practically all of our native species of nut-bearing trees are indigenous well beyond the range of regular crop production. This is made possible by occasional seasons favorable to seed production which enable such species to reproduce themselves. A crop once in a quarter century would be sufficient for this purpose.
Taking the pecan as an illustration of how a species may be affected by latitude, it has been found that, as the limits of hardiness are approached, the ill effects on the species in approximate order are:
(1) reduction in size of nut, especially with oblong varieties
in length,
(2) increased proportion of faulty kernels,
(3) increased irregularity of crop,
(4) practical crop failure, and lastly the
(5) partial, then complete, destruction of the tree.
On the other hand, the fact that a tree is subject to occasional winter injury, or that it bears irregularly, or not at all in a particular site, is not necessarily to be taken that the same tree in a different site or under slightly changed environment would not perform satisfactorily, even in the same locality. A change in exposure or of cultural treatment, or of rootstock, or of variety, or a modified association of varieties, might and frequently does bring about entirely different results. Sometimes a southern exposure causes trees to respond to mild weather, in winter or early spring, and to be caught by subsequent, violent drops in temperature. Some of the best known and best performing Persian walnut trees in the East are on a northwestern exposure, yet the species is commonly not hardy in the temperate portions of this country.
To a certain extent the ability of orchard trees to withstand frost injury is subject to control. The danger is greatest with trees which have grown late or those which have become devitalized for some reason or with those which are in poorly drained soils. The kind of root stock which has been used, is known to have had an influence in some cases. Doubtless this will be better understood as different stocks are used by the leaders in pecan breeding. Varieties also are known to differ greatly in their degree of hardiness. However, failure upon the part of otherwise normal trees to bear paying crops with regularity is not necessarily due to low temperatures. Other factors, such as self-sterility, may be wholly responsible for at least the lightness of crops.
So far as the orchardist is concerned, a tree is not hardy unless it is capable of bearing crops the average of which are profitable. On the other hand, occasional winter injury does not prove that a species cannot be grown successfully in the same locality. Neither the peach nor the apple industries of the North nor those of the citrus in the South and California nor, in fact, any of the other horticultural commodities of this country are wholly unaffected by frost damage. Our forest trees may be more subject to winter killing than we suspect. A certain amount of winter-injury is to be expected in any part of the country no matter what the species of plant may be.
The frequency with which winter or spring injury is definitely known to occur gives color to a rising theory that freezing temperatures may play a vastly greater part in the development of the nut industry over the entire country than is commonly supposed. Much of the evidence of damage from this cause is of such nature as to be easily overlooked or attributed to other causes. Trees and plants of many kinds have become so accustomed to injury by freezing that they are able to recover without the injury always being apparent. A few illustrations of this which have come to the writer's attention might be cited.
In December 1919, a sudden drop in temperature of from 32°F to 24°F occurred at McMinnville, Oregon, with fatal result to cultivated trees and shrubs of many kinds. The damage was greatest in flat bottoms, especially those where neither land nor air drainage was good. Under such conditions, numerous apple orchards were killed outright. Prunes and Persian walnuts were so badly injured to the snow-line that subsequently great numbers of trees were cut down. Both staminate and pistillate buds of filberts above the snow were practically all destroyed. Later on, the entire tops of many of the older-bearing filbert trees succumbed. An instance of particular interest, in so far as this discussion is concerned, was afforded by the behavior of a shagbark hickory tree in McMinnville, some 20 or 30 years old, which had been grown from a Missouri seed. In February, when examination was made of the condition of this tree, it was found that all visible buds had been killed, yet the bark on the branches between the buds was in apparently perfect condition. The question as to what the tree would do, therefore, became one of great interest. The following September, when revisited, this tree was found to have such a wealth of luxuriant foliage that the observer felt that the accuracy of his February records was challenged. However, closer inspection showed that growth had entirely taken place from adventitious buds, and that the dead buds and spurs were still in evidence. There were no nuts on the tree but otherwise the casual observer would not have suspected that the tree had been affected in any way. In all likelihood, the owner of the tree would deny that it had been injured.
Another case of somewhat similar kind occurred early during the present year in a pecan orchard in South Georgia. The trees had been set in 1917, and in 1919, a portion selected by the Bureau of Plant Industry for conducting a series of fertilizer and cover-crop experiments. The summer of 1923 was extremely dry. This was followed by warm rains in the late fall and early winter. On January 6, during a period of high wind, the mercury dropped to within a few degrees of zero, official reports recording temperatures of from 6 to 8 degrees above zero at various nearby stations.
On March 31, Dr. J. J. Skinner, of the Office of Soil Fertility Investigations, in attending to the spring fertilizer applications, discovered that a high proportion of the trees had been badly winter injured, as indicated by the usual characteristic evidence. These included a considerable exudence of sour and frothy sap from the trunks of the trees, particularly those having smooth bark. This invariably occurred on the west side. Shot-hole borers, which not infrequently follow such injury, were already at work.