Report from the Constitution and By-Laws Committee—Dr. MacDaniels.
Dr. Crane: "I move we accept the report of the Committee and suggested changes be voted on item by item."
Mr. Silvis: "Second."
The question of whether the entire Constitution and By-Laws should be read at this meeting or mimeographed and mailed to each member was considered.
Prof. Slate: "I move the Constitution be taken up now."
The motion was carried. Dr. MacDaniels read the Constitution and By-Laws and they will be voted on at the 1948 meeting.
J. Russell Smith: "I move that '10 days' notice for change in the Constitution be changed to '30 days'."
Seconded by Mr. Silvis.
Motion carried.
On fiscal year—Dr. Rohrbacher: "I suggest the fiscal year be changed to January 1 through to the end of December."
Mr. Snyder: "I can see no improvement in changing the fiscal year. If we are to hold our meetings the first part of September each year it would be better to have our fiscal year ended August 31."
Dr. MacDaniels: "I move that our fiscal year be from September 1st to August 31st and I move that the annual dues include a report for only the year you join."
Motion carried.
Factors Influencing the Hardiness of Woody Plants
H. L. CRANE, Principal Horticulturist[1]
There is hardly any soil or climatic condition found in the world where it is not possible for at least one or more kinds of plants to be grown. This is possible because the plants that can be grown under the most adverse conditions have special structures and adaptations with regard to periods of growth and rest or dormancy. One of the most important adaptations of nearly all trees and shrubs that shed their leaves in autumn and survive freezing weather without injury for a part of the year, is that of rest. This rest in plants is somewhat similar to sleep in animals in that it is a period in which the life process activities take place slowly. In other words, the plant physiologist defines rest in living plants as that period in which their buds will not open and grow even though the temperature, moisture, and other external environmental conditions are highly suitable for growth.
Different kinds of deciduous plants have or require rest periods of different lengths, just as some people require more sleep than do others. Two or three weeks may be enough for soft-shelled almonds but three or four months may be required for butternuts, to cite extremes. The Eastern black walnut requires more rest than most Persian walnut clones, and they more than the Southern California black walnut. Even within a species there is considerable difference in the rest period of individual seedling trees and certain clones. For example, it has been found that the varieties of Persian walnut grown in northern California and in Oregon, such as Franquette and Mayette, have the longest rest period; and those grown in Southern California, such as Placentia, Ehrhardt, Chase, and others, have the shortest rest period. It is quite possible that the clones and seedlings of the Persian walnut brought to this country a few years ago by the Rev. Paul Crath from the Carpathian Mountains of Poland may require the longest rest period of all.
The question may be asked what causes or brings on this rest period in plants and what breaks it? The scientific answers to these questions are not known at this time, but we do know some of the factors which cause the initiation of rest and how it is broken.
Tree growth is initiated in the spring with coming of warm weather and other suitable conditions. At first the rate of growth is slow; but the rate increases and goes through a maximum and then slows up again and finally ceases. On the cessation of growth in length, a terminal bud is formed and the tree begins to go into rest. This period of growth is determined by the age of the tree, the suitability of moisture and nutrient supply. Young trees grow longer during the spring and summer than do old ones. Deficiencies of soil moisture or nutrients or both cause the cessation of growth and the beginning of rest. In some trees, such as tung, cessation of growth and the initiation of rest is caused by the change from long to short day-lengths.
After rest has begun, the longer it continues the more profound or deeper it becomes until a maximum is reached, i.e., it becomes increasingly difficult, up to a certain time, to make the trees start growth again even though optimum conditions are provided. Some trees such as Persian walnuts and pecans, for example, are slow to go into deep or profound rest in late summer or fall. For this reason, there may be several cycles or periods of growth during the summer and early fall, depending on weather conditions and whether the leaves on the trees have remained in a healthy condition. Under conditions of dry weather growth stops on the Persian walnut and pecan and when this is followed by a rainy period and warm weather growth begins again. In fact in early summer a walnut or pecan tree may form terminal buds on all the shoots and remain without growth long enough for an apple or pear tree to go into complete or profound rest; then later, new shoot growth may be made from all or nearly all of the walnut or pecan shoots. Not only is this an important factor in promoting susceptibility to cold injury but in the case of bearing trees more often than not this late growth prevents the proper development of the kernels in the nuts and they are poorly filled or shriveled at harvest. Should the leaves of these trees in midsummer or later be so seriously damaged by disease or insects as to result in partial or complete defoliation, new growth is generally sure to follow even in late fall if growing conditions are suitable. This habit permits such trees to grow so late that there is much greater danger of severe injury from late fall or early winter than is the case with most other deciduous fruit trees. Furthermore, it explains why we see so much cold injury in the shoots and limbs of trees; they had grown late and had no chance to develop hardiness before killing temperatures occurred.
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.
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.