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The Physiology and Psychology of Pruning

The Physiology and Psychology of Pruning

The and Psychology of Pruning

E. L. Lord, University of Florida, Gainesville

I doubt if there is a subject that could methods of acquiring knowledge are sat be selected for discussion before this So isfactory, it cannot be said that pruning ciety in which there is a wider range of is one of them. opinion than the one which I have been It is only within the last few years that asked to present to you; and before going horticulturists havebeen able to reach any further I wish to assure you that I do not definite conclusions, and these have been intend to offer you a text on this sub the result of the work of physio ject, but rather tot>ring to your attention logists. Studies on growth responses, certain phases of this important matter , carbohydrate- ratio, which are often overlooked. absorption and translocation of food ma In the first place this paper will not terials, differentiation and de take up the question of many pruning velopment, relations and the like, practices the wisdom of which is self-evi have given us more real insight into the dent, such as the removal of dead or in pruning problem in the last fifteen years jured branches,the removal of limbs than had been learned through the prac close tothe trunk, the burning or destruc tices of the preceding centuries. tion of pninings which may cause Why do we prune our ? I suspect in the planting, or the protection of large that any grower can give an answer to wounds from infection by the use of this question, but I am not so sure that dressings. I assume that my topic "The he can give a rational answer. In a great Physiology and Psychology of Pruning/' majority of cases his answer would rest will explain why I confine myself to the oil the dogmatic assertion that he learned discussion of more fundamental facts. to prune the plant properly at' a certain It is difficult to reach final conclusions time and a certain place, and that that on the subject of phming, because much is the way to do it because that is the way of our pruning is tied up with traditional that he learned it. It is probably true th&t practices brought over from Europe. if he learned his practice ffdm a good Most growers prune as a matterof course grower, that is one getting satisfactory re rather thalri because of any rationaliza sults, he is probably not harming his trees tion concerning the process. Most of our very much, although it is doubtful if he pruning practice.is the result of trial and is benefiting them as much by his method error acting over a long period of time, of pruning as he believes. The man who and while many horticultural practices is careful and conscientious in his grove which arethe result of sudi unscientific work can usually ascribe thecondition

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Let us examine some of the answers is wise. If his pruning of young trees that we get from growers. One grower consists further in the removal of the fine will tell you that he prunes his trees in brushlike near the base of the scaf order to avoid disease. If he removes all fold limbs, or if his pruning of older oi tile dead and badly injured or diseased trees consists in taking out a portion of wood he is certainly wise, and if he has the surface shell with the avowed purpose any melanose infection he is certainly of changing the shape of the , he is busy. He would be still wiser if he re certainly unwise, as by this means he re moved most of the dead wood that he moves the earlier productive wood of the could get conveniently, sprayed with tree or changes the type of wood pro Bordeaux-oil if his infection were very duced ; both of these practices tend to les heavy, and by proper fertilization and soil sen or delay production and are certainly treatment kept his trees in such a vigorous of questionable value. and healthy condition that infection was Why are these last processes of doubt reduced to a minimum. He is certainly ful value ? This is an easier question to unwise if he depends on pruning alone to ask than answer. Let us consider the keep severe infections of melanose in tree as a factory which exists for the pur check. With all that can be said about pose of producing fruit. It has certain the grower who limits his pruning to the raw materials, water, dioxide, soil removal of dead wood, he at least is not nutriments. It is like many factories in likely to damage his grove severely, as the that it has sections devoted to the assem removal of dead wood has no physiologi bling of raw materials, to direct manufac cal effect on the tree; valuable as it may ture, and to the storage of the finished be in helping to control and in products. The system is primarily sects by removing sources of infection. an assembling system for the plant. It If you ask another grower why he collects raw materials from the soil and prunes he will tell you that he does it so the air; water with its contained nutri that the tree will have the proper shape ents from the soil and carbon dioxide so that it will bear heavy and regular from the air. The direct manufacture crops of high quality fruit. If at plant takes place only in the parts of the ing time the prospective grower heads plant, particularly in the . The - back the young tree to a proper height he factory produces carbohydrates, (, thereby helps in forming a low head ). The rate at which the plant which enables the tree to produce its crop produces carbohydrates depends on the nearer the ground on strong limbs; if he leaf area, , sunlight, its sup tops old trees at a height of twenty-five ply of water and other , and oth or thirty feet, so that the fruit can be er factors. If there is a fruit near this * reached easily in spraying and harvesting, leaf it will manufacture them more rap he is making the production of fruit more . idly because the finished sugars are stored economical; to this extent he is pruning in the fruit and cannot accumulate near for shape; certainly this type of pruning the leaf and slow up its activity. The tree FLORIDA STATE HORTICULTURAL SOCIETY 111

with fruit developing upon it manufac and yet the tree be unproductive because tures much more rapidly than the one of unsatisfactory pruning methods. without it, although it may not grow as Given a satisfactory root system to sup rapidly. The manufactured food may be port the top, let us see why it is possible used to enlarge the assembling plant (root for some trees to be unproductive. To system) or. the factory (leaf area), or do so it is necessary for us to examine the stored. If the assembling plant can col tree from a different point of view than lect more raw material from the soil than is usual arid I ask you to bear with me the leaf factory can use, then the manu during the short time that is available for factured food from the leaf is combined the discussion of this intricate subject. with the raw material (water, nitrogen, In the more recent discussions of horti etc.,) from the root system, and new culturists it is common to find terms used growth is produced, enlarging the leaf which were unknown to those of an old area. Suppose you reduce leaf area by er school. One of these terms is fruit pruning away active leaves, the result is bud differentiation and another is car shoot growth and new leaves, and your bohydrate-nitrogen ratio. A third term, assembling plant and factory again are in fruit setting, has long been a familiar balance. Suppose your assembling plant term to fruit growers, although the fac breaks down or is reduced in area by root tors which govern it have not been thor pruning; then your factory (leaf area) oughly understood. It will be necessary has to run on part time (wilt or drop to explain these terms in order to under leaves) until the root system has been re stand what goes on in the tree when it generated. is pruned. Fruit bud differentiation is the It has been said that a people has as process which takes place when fruit good a government as they deserve; it are formed. All buds on a tree are poten may also be said that a tree has as large tially fruit buds, that is, any bud will if and as good a top as its root system can conditions are favorable develop support. Consequently the root system buds. If you will examine the spring should be as large and as healthy as pos growth of a mature orange tree a month sible, not injured in any way by poor or six weeks from now, cutting lengthwise drainage, too high a concentration of fer through the buds in the axils of the leaves tilizers, lack of food or root pruning. As with a sharp razor or microtome and suming that the root system is as large mounting the thin section on a slide, the and efficient as may be produced, then the small flower buds which would become top (leaf area and supporting branches) the next year's bloom may be easily dis will be capable of being equally as large tinguished. These flower buds will nor and productive. It must be kept in mind mally remain in the leaf axil until the that the first condition to be met lies in next spring, when, if conditions are the root system. However, the root sys favorable, the shoot below them will tem may be large and capable of support lengthen and the appear. If con ing a large top with a good crop of fruit, ditions in the trees-are unfavorable at the 112 FLORIDA STATE HORTICULTURAL SOCIETY time when this fruit bud differentiation idea of the importance of this ratio is normally takes place, then the next spring based, but it is necessary to take this im no bloom will appear, for the simple rea portant relationship into account in any son that none has formed. It is curious modern discussion of pruning. There are that growers will often talk about the four different carbohydrate-nitrogen re amount of bloom that a tree has as some lations that may be found in ; they thing that is caused by spring treatment may be expressed by these four ratios alone, when no amount of right treatment (i)2> (*)*, (3)S and (4)g. In these can bring out bloom which has not been formulae the small letters represent more formed during the previous year. The or less of a deficiency of the material only way in which a grower may affect (Carbohydrate or Nitrogen) and the cap the amount of fruit buds differentiated is itals represent a supply adequate to the by practices which affect the tree before needs of the plant. Remember that the or during the period when differentiation carbohydrate (C) is the result of leaf ac may take place. tivity, and is im lessened supply when the The problem of fruit setting may be di leaf area is reduced or the work of the vided into two problems, that of length leaf is checked, and that the nitrogen sup ening the containing the bloom ply (N) is the result of root activity, and sufficiently so that as many flowers as is correspondingly lessened when the root possible may be developed to the point system is reduced or its work is checked. where as many young are formed Let us examine the condition of trees as possible, and that of holding these under the first condition (^). In this young fruits on the tree until they reach tree, the root system is not active or is maturity. This is largely a growth prob limited in area. The leaves are yellow lem and seems to depend mainly on the and sparse, and the tree is in what the amount pi water and nitrogen available grower calls a starved condition. If you at this time. If these are abundant and cut it back heavily the remaining parts the root system is functioning at its best, get a higher portion of nitrogen, although the only factors which will prevent an the total amount that the tree gets is abundant crop are injurious, environmen small. The new shoots that form will tal conditions, such as frost damage or an be greener in and the tree will look imperfect functioning of the floral parts. healthier; but you have not really bene What do we mean by the carbohydrate- fited it, all that you have done is to re nitrogen ratio and what has it to do with duce the top to fit the amount of nitro the pruning problem? The ratio between gen that the root system has been able to the atnount of carbohydrate in a part of collect. It would have been wiser and the plant or the plant as a whole and the more sensible to have found the trouble amount of soluble nitrogen is called the with the root system and fed it until the ciirbohydrate-nitrogen ratio* It would be tree was restored to health and vigor. unwise and tiresome to discuss hefe the This is a general statement and has this and research on which the exception, if the tree is in the starved con- FLORIDA STATE HORTICULTURAL SOCIETY 113 dition temporarily because of transplant statement is based on plant physiology ing, then it is important to reduce the top alone, and does not take into account cer to correspond to the size of the root sys tain conditions that may be necessary tem, or it would have been better to have from economic reasons. But, it may be reduced the top so that this starved con stated as a general rule that the removal dition did not occur. Then top and root of active leaf area from young trees, di growth could develop together naturally. rectly or indirectly, by reducing support This first condition is only brought into ing tissues, simply delays profitable fruit the discussion in order to show that it is production in two ways; one by produc primarily a proposition of feeding the ing conditions within the tree which are tree, rather than a pruning problem, and unfavorable to fruit production; the oth the pruning is only a temporary adjunct er by lessening the amount of wood avail to use in helping the tree to get estab able for bearing fruit. The primary effect lished. of a reduction of leaf area by pruning is to increase the amount of nitrogen available The second condition (&) is that of to each growing point, and decrease the the young tree after it is established, amount of carbohydrate in the plant. when it is growing vigorously. Here the This causes an increase of growth, espec supply of carbohydrates is low, and the ially in the neighborhood of the removed supply of nitrogen is high. This is due part, but also thoroughly and effectively to the fact that the root system is well prevents the formation of fruit buds. Re established and collecting nitrogen faster sult, a tree in this condition will bear no than it can be used in making shoot bloom on the new growth. growth, while the leaf area has not been developed so that carbohydate can be The third physiological condition (£ ), made rapidly enough to supply the needs where the carbohydrate is abundant and of the plant. The trees are deep green and the nitrogen supply low is most favorable are in vigorous growth whenever suffi to "fruit bud differentiation. It can only cient moisture and heat are available. This occur when the root system is less active condition is not favorable to fruiting as than the leaf area. In the mature fruit leaf area is not sufficient to make a sur- tree it is a characteristic condition in the . plus of carbohydrate, the moderate sup summer, when the intensity of heat and ply of carbohydrate that is made being is high. During this period the used immediately for shoot growth. In synthesis of carbohydrates by the leaves order for this tree to get to the fruiting takes place very rapidly with the result stage, it is necessary for it to have a leaf that more is produced than the tree can area which balances the root system and use in growth, and this material accumu makes more carbohydrate than is needed lates in the region at the base of. the for shoot growth. Any reduction in leaf leaves. When this accumulation reaches area by pruning a tree in this stage is very a certain stage, all growing points in the unwise, as it simply postpones the period axil of the leaf begin to differentiate of profitable fruit production. This flower parts. Soon after this period 114 FLORIDA STATE HORTICULTURAL SOCIETY

starts it is easy to distinguish them from has accumulated a high nitrogen reserve the ordinary leafy shoots. This is a criti which, due to low , it has had cal time in the of the bloom. If the no opportunity to use up in growth, and is leaf area has been reduced so that carbo in a condition favorable to fruit setting, hydrate production lags behind nitrogen but not to fruit bud formation. Heavy absorption, no bloom can form. Heavy .applications of nitrogenous fertilizers applications of nitrogen in summer tend to with a regular and sufficient supply of prevent carbohydrate accumulation and water, favor fruit setting because they consequently fruit bud differentiation, favor growth. As the bloom develops pruning of the top also has the same ef this nitrogen reserve is used up in making fect. Heavy applications of potash tend new , carbohydrate production in to stimulate carbohydrate accumulation creases until a balance is reached. But as and thereby favor fruit bud. production, the spring advances the nitrogen supply root pruning has a similar result. Bend of the shoots becomes gradually smaller, ing limbs, ringing, spiral girdling, are growth stops and carbohydrate manufac processes which tend to cause carbohy ture and accumulation becomes so great drate accumulation in the top and in con that the tree gets into the third class, sequence also favor fruit bud differentia where fruit bud formation is grealty fa- tion. All top pruning of normal trees focer. When the rainy season begins, the (trees in balance between root and top) same process may be repeated, but the ex tends to prevent fruit bud formation. tremes are not so great as in the spring However, any pruning process that re cycle. Heavy applications of nitrogen, moves a portion or all of the fruit with heavy pruning, a constant moisture supply out also reducing the leaf surface to a are all factors which may prolong or pre corresponding extent, tends to produce ac vent the normal cycle of development that cumulation of carbohydrates, and in that I have tried to picture. Root pruning, respect helps to produce a heavy bloom heavy applications of potash, frequent dry the next year. This is due to the fact that periods, injury to the root system, partial all fruit tends to remove the carbohy girdling due to foot rot, an inadequate drates as soon as they are produced; when stock, are all factors which hasten the stored in the fruit they have little physio cycle or suppress its early stages. logical effect on fruit bud production. Now that I have tired you thoroughly Heading back of peaches and grapes is .by this long and dry dissertation on what really to be classed as a thinning process is going on inside your trees, I wish to rather than a typical pruning process. talk about pruning. All top pruning of The fourth class (£) where carbohy the active portions of normal trees is a drate and nitrogen are both abundant and dwarfing and injurious process. It may in equal balance is more theoretical than be economically sound because dwarf real. It is really a transitional condition. trees are sometimes desirable, but it is Let me illustrate; in. the spring the tree physiologically unsound from the stand has its leaf activity far below normal, it point of the tree. Because heavy pruning FLORIDA STATE HORTICULTURAL SOCIETY 115 results in more activity does not mean pearance to the lord of the manor. We that the tree has benefited. Any lizard Americans for a hundred years have done which through some accident loses his this or that because some German or Eng tail may grow a new tail very rapidly, and lish gardener from Munich or Kew told to all appearances be in a very desirable us that that was the way it should be done condition because he is growing. Un because that was the way he learned. Even fortunately he has to take much of his in California the growers tried to copy surplus energy, which he might be using the pruning of the Continental lemon for the enjoyment of life, and use it for growers, but are becoming aware of their this abnormal growth. In the same way mistake. In Florida the citrus grower, a tree which has had much of its wood being an observer in a strange country removed, has to waste much energy in with no traditions behind him, knowing producing new wood in order to balance only one thing, that he did not know how its root system, but it has not been bene to treat citrus trees, watched the effect fited. All of its apparent benefits are of pruning of live wood away from cit merely optical illusions. rus trees and did not think much of it. He Why has the pruning of fruit plants was intelligent ahead of his generation. been emphasized so much? Here is The grower who fertilizes, and cares for where my title comes in. In Europe the his trees and soil, has the proper variety gardener is hired by the year, he has to on the proper stock, gives the trees suf have an excuse for working part of his ficient room,, sprays against disease and time, so when he can find no other job , and after he has done all of the that is showy, he goes around with a pair things necessary to keep the tree in good of pruning shears, shaping up these two health, limits his pruning to the removal trees so that they look like a pair of fight of dead, severely injured, or diseased ing roosters, snipping out a bold shoot that mars the perfect smoothness of his wood, is much wiser than the one who clipped hedge, and arranging his dwarf takes his pruning problem as a sort of pear trees so that they will lie against a "Made in Germany" affair. Pruning is wall or appear like a hug fan. His work no substitute for intelligent grove prac has no relation to the health or produc tice, and in the normal grove it should be tiveness of the plants but only to their ap^ reduced to the above mentioned minimum.

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