After the rest in trees has become deep or profound a certain amount of chilling temperature must prevail before the rest period is broken so as to permit the buds to open and grow normally on the approach of warm weather. This is often spoken of as the chilling requirement. If the rest period is not broken by a suitable amount of chilling, tree growth is very slow to start in the spring, and then only certain of the longer and stronger twigs may force into growth; water sprouts may develop on the trunks and main limbs; flower buds may not open but fall off; and even though the trees may flower the flowering period is long and few or no fruits or nuts may be set. The most effective chilling temperature is not known but we can be reasonably certain that temperatures of 45°F. to 32°F. are just as effective in breaking the winter rest period as are those well below freezing, if not more so.
This chilling requirement is essentially the same as the rest period. Almonds have a short rest period and require 2 to 3 weeks of chilling, while butternuts, with a long rest period, may require 3 or 4 months. When the tree has been subjected to adequate chilling the rest period is broken and with the oncoming of warm weather growth, blossoming and fruit setting is normal.
A distinction of great importance from a physiological and a practical point of view is made between rest and dormancy in plants. This difference can be simply stated: plants, trees, or seeds that will not grow when external environmental conditions are favorable for growth are in rest, but after the rest period has been broken and they do not grow because of unfavorable conditions they are said to be dormant.
The difference between rest period and dormancy is of great importance in the United States in determining the amount of cold injury that may be sustained by woody plants. Furthermore, it explains why certain plants may be successfully grown in much colder parts of the world and yet fail here. Our winter weather conditions are not uniform, in that it is quite common for us to have quite long periods of alternating warm and cold weather. Too often during mid-or late winter the weather may be quite warm for several days, with above-freezing temperatures even at night, only to be quickly followed by a sudden and extreme drop in temperature. Such conditions are almost certain to result in cold injury to at least certain kinds of woody plants in which the rest period had been broken prior to the occurrence of warm weather, especially so if conditions are favorable for initiation of growth. The plants that were still in the rest period at the time of the warm weather or those with high heat requirement to start growth (as for example, the pecan) would be the only ones that would escape injury. To illustrate with an example: The Chinese chestnut tree has a shorter rest period or less chilling requirement than does the average Persian walnut tree. Now suppose that during the months of November and December a sufficient number of hours of chilling temperatures were experienced to break the rest period or to satisfy the chilling requirement of the Chinese chestnut but not that of the Persian walnut. Then suppose there was a period of two weeks or more of warm weather in January and it was ended by a very sudden drop to below freezing temperatures. Later we would expect to find that some parts or tissues of the Chinese chestnut trees had been injured while the Persian walnut trees had survived without injury. Similar differences would be expected with other crops, such as peaches and apples, that have a difference in rest period or chilling requirement. Under the conditions just described the parts or tissues of the tree that are most likely to be injured are those that first become active with the coming of warm weather, such as the pith in the wood, the lower buds, and later the cambium or the leaf buds. This explains why peach fruit buds and the catkins of the European filbert are often killed in the East during the winter.
Some kinds of woody plants are very much hardier than are other kinds. For example, the butternut is hardier than the eastern black walnut and the almond is hardier than the tung tree. Hardiness is only a relative term and can be determined only when the different kinds of plants are in the same physiological condition as regards growth or activity. Just what it is that makes a difference in the hardiness or ability to withstand low temperatures without injury is not known. However, over the years, experience and research have taught us that there are a number of factors that affect the hardiness of woody plants.
There is a very great difference between the temperature that will cause injury to a tree tissue when it is in active growth and most tender in the spring and that required when it is most resistant in midwinter. With some trees this difference in temperature is as much as 50° to 60°F. or even more. With woody plants, the tissues are least hardy in spring when they are growing rapidly, and as the season progresses hardiness normally increases provided that second or late growth does not occur. There are many changes that take place in the tissues of a tree as hardiness is developed: the moisture content is reduced; cell walls are thickened; the concentration of sugars, starches, and other carbohydrates becomes greater; there is the formation of pentosans, gums, and waxes; and the respiration and other life processes become slower. However, none of these offer a full and satisfactory explanation of why the plant becomes as resistant to cold as it does. All of these changes and probably many others play a part in developing hardiness in woody plants.
Maximum hardiness is developed only by trees that support a large area of normal leaves continuously from the time of foliation in the spring until late fall when they are killed by frost. Attacks by insects or diseases that injure the leave or cause partial or complete defoliation at any time during the spring, summer, or before the occurrence of frost in the fall, not only prevent the development of maximum hardiness of the trees, but such defoliation results in reduced growth of the trees and in poor filling of the nuts. The importance of maintaining a large area of healthy leaves on the trees during the entire growing season can hardly be too strongly stressed. This is because trees that hold their leaves are strong, vigorous trees and are the ones best able to withstand cold, as well as other adversities, without injury. This, however, does not mean that fertilizer applications should be made in late summer or that cultivation should be practiced at that time, which would tend under suitable conditions to stimulate late growth of the trees. This is because some trees like the Persian walnut are slow to go into rest at best and practices that stimulate late growth of the trees cause them to be susceptible to cold injury especially in late fall or early winter. I have seen very severe injury and killing of pecan trees in south Georgia as a result of spring fertilizer applications which, because of drouth, did not become available to the trees until late August and early September and then caused second growth of the trees.
In the case of walnuts and pecans, especially, but also others than are not sprayed for the control of diseases and insects, it is not uncommon for the trees to become defoliated in late summer and while bearing a crop of nuts. Very often this premature defoliation results in the production of a new crop of leaves and some shoot growth. This is one of the worst conditions one can have in an orchard, for the nuts are certain to be very poorly filled and the trees especially susceptible to cold injury.
In such a case as this, the nuts withdraw carbohydrates, proteins and minerals from the leaves and wood of the tree for their development and the production of new leaves and shoots has a like effect. This all results in such a severe removal or using up of the materials involved in the development? of hardiness that such trees are very susceptible to cold injury.
Woody plants to be resistant to cold injury must be well nourished. Unbalanced mineral nutrition of trees is a very important factor in determining the amount of injury they may sustain from cold weather. In the various parts of the United States the soils on which fruit and nut trees are grown generally do not supply in adequate amounts some one or more of the essential elements required in their nutrition. This condition results in unbalanced nutrition, in that too much of certain elements is absorbed by the trees and too little of certain other elements. Under severe conditions this causes the leaves to be abnormal in size or in form, for them to be chlorotic or to scorch or burn, or for them to drop prematurely. Such leaves do not function properly, they are not able to carry on photosynthesis at a normal rate and hence do not make sufficient plant foods of the proper kinds to properly nourish the trees. This results in disorders of various kinds said to be due to mineral deficiencies. Among these deficiencies that have been found to reduce tree growth and yield and to increase susceptibility to cold injury are (1) boron, (2) copper, (3) iron, (4) magnesium, (5) manganese, (6) nitrogen, (7) phosphorus, (8) potassium, (9) zinc, and others. In all cases the corrective treatment to be given consists in supplying the trees with the element or elements in which they are deficient. These must be supplied in an available form and by such methods that they can be absorbed by the trees.