The size of the crop of fruit or nuts borne by a tree and the length of time between harvest and a killing freeze are important factors in determining the cold resistance of fruit or nut trees. In test winters many cases have been observed in which trees that matured heavy crops during the previous summer were severely injured. Cases have been observed in which the degree of cold injury sustained has been largely in proportion to the size of crop matured the previous growing season. Trees that mature the crop of fruits or nuts late in the season may be less hardy than those that mature the crop early. It seems not only that some material or materials are made in the leaves during late summer or early fall which move out of them into the wood and cause it become resistant to low temperatures, but that when a tree is maturing a crop so much of this material goes into the fruits or nuts that if the season is not a favorable one the wood may not attain its maximum hardiness. We have learned that a high percentage of certain of the minerals, carbohydrates, and oil that go to make up the kernels of the oily nuts are transported into them during a period comprising a month to six weeks before they are mature. In the production of a heavy crop the amount of minerals and elaborated food materials such as proteins, carbohydrates, and fats removed from a tree is very large. If the trees do not carry a large healthy leaf area at the time of harvest or if there is a killing frost at that time, the leaves have no opportunity to elaborate more carbohydrates and other materials to replace those removed in the crop, and as a result the trees do not develop maximum hardiness.
To cite an outstanding example of this effect of the crop on hardiness, I want to describe some observations I made several years ago. The late J. B. Wight of Cairo, Ga., had a few hundred Satsuma orange trees that bore a very heavy crop of fruit. The fruit had all been harvested from certain of these trees for two weeks or more before the occurrence of a freeze the last of November. From other trees the fruit crop had only been partially harvested and none had been harvested from most of them. The day and night temperatures had been warm but there was a rather sudden drop into the low 20's during one night with the result that all of the trees from which no fruit had been harvested were killed to the ground. The trees from which a part of the fruit had been removed were defoliated and all but the large limbs were killed. The trees from which all the fruit had been removed two weeks or more before the freeze were defoliated, but little or no injury to the woods occurred. The severe injury was probably because the materials making for hardiness in the wood had been transported to the maturing fruits and the temperature dropped quickly before the trees had time to develop cold resistance.
It is a well-known fact that many kinds of non-woody as well as many woody plants develop hardiness or cold resistance on exposure to very gradually falling temperatures. This change, in the case of non-woody plants such as cabbage or wheat, is spoken of as "hardening off." It is not known how important this is in developing cold resistance in flower and leaf buds of woody plants. It is quite possible that buds that have become extremely tender as a result of rapid growth might, if held for some time at temperatures too low for further growth, become quite resistant to low temperatures just as do wheat or cabbage.
Generally speaking, the greatest amount of cold injury to the buds or above-ground portions of a tree occurs on a single night. The length of the cold period is of only indirect importance as influencing the rate of temperature fall or the acquiring of cold resistance by the trees. Trees that are subjected to low temperatures over a considerable period of time are not nearly so likely to be injured as are those that are subjected to a low temperature suddenly. That is really why there is so much severe cold injury to woody plants in the South. In the deep South freezing weather may be uncommon but when freezes do occur usually they follow a period of comparatively warm weather and the temperature falls quickly. It is this sudden change in temperature that causes the severe injury. Two different places may have had the same mean monthly temperature yet at one place severe injury may have occurred and no injury at the other place with plants normally having equal hardiness. A careful analysis of the situation, however, would probably show that at the place where the injury occurred a period of warm weather had existed which was followed by a rapid drop in temperature to a killing low on a single night, whereas the trees at the place where no injury occurred were not subjected to such changes in temperature. On the other hand, injury to the roots usually occurs only after prolonged periods of cold weather. This is largely because the soil cools slowly and it requires a long period of cold weather to reduce the soil temperature sufficiently and to such depths as to cause injury to the roots.
Under northern conditions where low temperatures for a rather long period are sometimes experienced, injury to the portion of the trees above ground may occur as a result of drying out of the wood. It is well known that a cake of ice will gradually evaporate and disappear when in the open and exposed continuously to below-freezing temperatures. We all know that the family wetwash when hung on a line and frozen will soon dry, especially if the wind blows. The principles operating in these cases may cause severe injury to trees. In the wintertime the root systems of trees take up water from the soil that is not frozen and this water moves in the tree to replace that lost by evaporation. Under conditions where the soil is frozen to such an extent that the water absorbed by the roots is continually less than that lost by the top of the trees by evaporation, drying out of the top occurs. If this is continued over a period of time a dryness of the wood and other tissues occurs that causes death of the dried-out portions. This type of injury does not show the typical symptoms of cold injury but rather those of drying out. The conditions that are most likely to cause such injury are a soil frozen to the effective rooting depths, a dry atmosphere, and a moderately high wind velocity. Injury of a similar nature to that just described very often affects trees transplanted in late fall or early winter, especially those that did not have their tops cut back to balance the loss of roots sustained in transplanting. During even very mild winters the tops of such trees dry out to such an extent that the small branches and even the leader may die. In extreme cases the entire top may die back to the root. In planting bare-root trees regardless of the time of the year they should be rather severely cut back immediately after transplanting to prevent such drying out and dying back of the wood. Cut-back trees generally will make more growth the first season following transplanting than will similar trees not cut back.
One of the most common types of injury to young nut trees as well as others is that known as "sun scald" or "winter injury". This occurs generally on the south or southwest sides of the trunk and for some distance between the ground and the head of the tree. Usually the injury is not evident until a year or so after it occurred and then it may be observed as a narrow strip of discolored and sunken bark which may crack where it meets the live tissue. This dead or injured area is usually invaded by borers of one or more kinds. This so-called sun scald injury is thought to be caused by the alternate freezing and thawing of the tissues on the south and southwest sides of the tree. On a bright, sunshiny day, even though cold, the sun's rays striking the bark of the tree quickly raise the temperature of the bark and wood. When the sun is obscured by clouds or at nightfall the temperature of the tissues drops rapidly and they may freeze again. It is thought that the rapid and rather great change in temperature of the bark and wood is the primary cause of sun scald. Whatever the cause, we know that it can be prevented by shading the tree trunk. This can be done by heading the trees low so that the branches shade the trunk, or by shading the south side of the trunk with a board 6 or 8 inches wide, or by wrapping the trunk with burlap or similar material. Much of the injury to Chinese chestnut, pecan, and hickory trees, especially, is caused by inexperienced growers who cut off the low branches in an effort to raise the head of young trees. The Chinese chestnut generally forms a very low-headed or bush-type tree. Most of the cold or winter injury I have seen on Chinese chestnut trees has been on the trunks and has resulted from removing the lower limbs so that they were not shaded.
Hardiness in woody plants is only a relative term and is determined by the condition of the plant at the time the low temperature occurs. Woody plants are most tender when they are most actively growing and most resistant to cold injury when they are in deep or profound rest. Strong, vigorous, well-nourished trees are much more resistant to cold injury than weak, poorly-nourished trees. Hence, the successful grower makes an effort through disease and insect control and proper fertilization and cultivation to keep his trees strong. These practices should be so carried out that the trees will make a strong, vigorous growth in the spring and early summer and then go into rest without a second or third flush of growth. The trees should carry their leaves until frost as there are some things made in them that cause the trees to develop resistance to cold injury. Winter or cold injury can destroy in a single night the hopes and expectations of several years' work but, in the main, if one grows well only those trees that are suited to the environment such losses are only rarely experienced.