Historic, archived document Do not assume content reflects current scientific knowledge, policies, or practices. ae £ ce Washington, D. C. October, 1927 VARIETAL CHARACTERISTICS OF PLUMS IN THE PACIFIC STATES IN RELATION TO PRUNING By C. F. KINMAN, Associate Pomologist, Office of Horticulture Bureau of Plant Industry CONTENTS Ditutiacce pi erunms Operatreanss 3. 2250S ke eink a a aS eee te Faplanatanne Ok Perms: Used. V9 co eine oss hot Sie i US EG Medea Se ae 4 Growing Habits of Plum Trees inRelationto Pruning . . . .....-.------ 4 Fruiting Habits in Relation to Growth .. ...... Te Gertie) | SHRM ePAE Aas! POL eae Ree 9 eomereni of Prov Spurs’). 522) Se So ee 9 Effect of Low Summer Temperatures on Wood Growth ......-.--.--.-- 13 Habits of Growth and Production of Some Plum Varieties Grown in the Pacific States . 14 Comparison of Behavior of Plums under Eastern and Western Conditions . .... . UNITED STATES GOVERNMENT PRINTING OFFICE WASHINGTON 1927 UNITED STATES DEPARTMENT OF AGRICULTURE Ue 2 “+ es : yy atvetetes VARIETAL CHARACTERISTICS OF PLUMS IN THE PACIFIC STATES IN RELATION TO PRUNING By C. F. KINMAN, Associate Pomologist, Office of Horticulture, Bureau of Plant Industry CONTENTS Page Page Importance of pruning operations___ 1 | Habits of growth and production of Explanations of terms used______~_ 4 some plum varieties grown in the Growing habits of plum trees in rela- PaACiIhC, StaLeS ee see a= ee 14 ipenvioy puuning= 31 2 Tiitts thie ie 4 | Varieties of the Prunus trifiora Fruiting habits in relation to growth_ 9_; group and its derivatives or Habits of growth of fruit spurs_____ 9 J Nad OW eI) (sped Stee ace see Reg = gL athe oA 14 Eifect of low summer temperatures Varieties of the Prunus domes- on, wood growth —---—_= 2... = 13 Lica. Croup. = == see 2a Comparison of bebavior of plums IMPORTANCE OF PRUNING OPERATIONS Probably nowhere in the United States is the pruning of deciduous fruit trees more important or more varied than in the plum-growing sections * of the Pacific States, where this important fruit industry is distributed over a large area. In this region the summer rainfall is very light, soils and temperatures are variable as between different _ fruit-growing sections, and only large fruit is profitable for ship- ment to distant markets. Pruning has become a very important factor in obtaining the regular production of large fruit and in _maintaining the vigor of the trees. It has become one of the prin- cipal operations of orchard management, and the variations in the operation which may prove beneficial in different sections and with the different varieties are of great interest and importance. A greater number of varieties in general cultivation are included in the plum and prune-producing industry than may be found with _almost any other commercially grown fruit in this region. As each _varlety possesses its peculiar and usually striking habits of growth _and production and responds in its peculiar way to local environ- ment and treatment, it is essential that the pruner have an intimate +For information on plum culture, see KINMAN, C. F. PLUM AND PRUNE GROWING IN THB PACIFIC states. U.S. Dept. Agr. Farmers’ Bull, 1372, 59 pp., illus. 1924. 27428°—27——1 1 2 BULLETIN 1477, U. S. DEPARTMENT OF AGRICULTURE knowledge of the behavior of his different varieties if the best results are to be obtained. Plums, as well as other fruits in this section, are quickly affected by lack of water for irrigation, variation in depth and fertility of soil, and extremes in summer temperatures; and various combina- tions of these conditions are found in the plum-growing localities. The great diversity found between the performance of different commercially grown varieties may also be explained in part by the fact that among them are represented several species including Prunus domestica, P. salicina (triflora), P. simoni, and probably native American types either by true forms or hybrids. When planting a western plum orchard for the purpose of pro- ducing fruit for fresh shipment, varieties are ‘chosen which will ripen their fruit in such order as to make an uninterrupted and extended harvest. As the ripening period of a variety continues for a few days only, depending upon the locality and the variety, several varieties, usually from 1 to 24 or more, are needed to extend the harvest over a satisfactory period. The orchard of only a few acres often has as many varieties as the large one, if plums are the main crop or are grown exclusively. Where the fruit is grown for drying to prunes, only a small number of varieties is chosen, often only one or two; but here, as with the fruit for fresh shipment, the large- sized fruit is usually the most profitable and the variations in habits of growth of individual varieties are striking, and a real knowledge of them is needed by the grower and pruner just as with fruit for _ fresh shipment. © Tf pollination is provided for, there need be little fear in the Pacific States as to the setting of a large crop of plums. Unless frost injures the blossoms or small fruit soon after it sets, the crop will be excessive and an expensive fruit-thinning operation necessary. This is regardless of any of the common pruning practices. Though the pruning is often relied upon to remove some of the bearing wood in order to lighten the crop and to distribute the load over the tree, a variation in the pruning method to encourage fruitfulness is sel- dom, if ever, necessary when bearing plum trees are pruned. The general pruning practice which has been evolved to meet the various conditions consists largely in the cutting back of the previous season’s growth and some thinning out of branches. The quantity of wood removed is determined very largely by local con- ditions and the use which is to be made of the fruit. When water for irrigation is scarce or soils poor, severe and regular heading back of the previous year’s growth is needed to induce the fruit to grow to sufficient size to be profitable for fresh shipment (pl. 1, A and B). Where soils are fertile and well irrigated, regular head- ing back and thinning out of fruiting branches is done for the purpose of preventing too tall a growth of the tree, to admit light, to lighten the crop on the tree, and to insure large fruit (pl. 1, Cand D). By following these general rules in pruning, the habits of growth and production peculiar to the variety, although often very pronounced and important, are hable to be overlooked or con- sidered in only a general way. In the preparation of the following discussion of the principal commercial varieties of plums grown in the Pacific States, it has PLUMS IN RELATION TO PRUNING 3 been the intention to record the varietal characteristics of growth and production which should influence the pruning practice, and to in- dicate some of the effects of pruning and of natural conditions on the behavior of the tree. It 1s not the intention to give a detailed pruning method for each different variety, but rather to point out the ‘important differences in the habits of varieties and to guide and en- courage the grower in the study of his trees. It has not been the aim to include the general subject of ‘plum culture, but rather to note the behavior of the tree; not to suggest changes in treatment or to point out faults in environment, but to observe the general results of these things. The purpose has been that of a variety study rather than a pruning experiment—an inquiry into the peculiar habits of growth and production of trees of different varieties, rather than to apply a peculiar pruning practice or cultural treatment and observe the reaction. The object has been to observe the response of the dif- ferent varieties to the type, variations, and severity of pruning prac- ticed by different orchardists and under different soil and climatic environment, for the purpose of learning the fundamental character- istics of the variety and what the variety wishes to do and how it wishes to grow, rather than to show the benefits of certain pruning treatments or to test its ability to withstand them. Trees of some va- rieties make an upright and compact growth and those of others open and spreading. Some bear their fruit on spurs and others on twigs which grew the previous season. Some retain their spurs longer than others, and some are more inclined than others to replace spurs which have been broken off. These and many other tendencies which may be found by studying the commonly grown varieties of plums are all readily infiuenced, although to a varying degree, by the local environ- ment and cultural treatment. ft is necessary to have a thorough knowledge of the tendencies of a variety in order to decide upon the peculiar phase of a pruning method best suited for it. By carefully observing trees of different varieties and their growth in different sections and noting their response to the pruning treatment given by different orchardists, many varietal traits which should have a bearing on the type of pruning given them may be readily seen. The general understand- ing of the effect of different growing conditions and treatments on a variety gives a true idea of its habits and tendencies and therefore provides a reliable foundation on which a pruning practice may be based. A thorough knowledge of the tendencies of a variety is in- dispensable to the adoption of a wise pruning method. It is also of importance in choosing varieties for an orchard and in plum breeding. An arbitrary and restricted pruning practice, if applied to more than one of the small groups of varieties which are similar in their performance, can but oppose the development of some varieties. To maintain thrifty, vigorous trees which produce good and reg- ular crops of large-sized fruit—all highly desirable if not essential to profitable plum-growing—it is important that the grower acquire an accurate Inowledge of the characteristic habits of his trees so that he may cooperate with them rather than oppose some or many of them by giving all the same pruning treatment. Regular, although moderate, opposition to the tree in pruning may be said to throw 4, BULLETIN 1477, U. S. DEPARTMENT OF AGRICULTURE the tree out of its regular balance of growth and production. An occasional excessive pruning may strongly oppose the natural tend- encies of growth and prove to be a costly mistake. EXPLANATIONS OF TERMS USED Training, or the shaping treatment given the young tree, is con- sidered somewhat distinct from pruning and is not discussed in this bulletin. Unless otherwise stated it is understood that the trees under consideration are those of normal vigor, growing in suitable soil which is of moderate depth and fertility, given some irrigation, making a moderate annual growth, and bearing regular and mod- erate crops. | Heading back, as used here, refers to the cutting back of 1-year- old shoots. With some fruit growers heading back is used to mean cutting weli back into the old wood for the purpose of developing a new top of fruiting branches. This is done particularly with peaches in some parts of the country, but is rarely practiced in the West with plums. The terms used to indicate different parts of the tree are presumed to be self-explanatory. Framework branches denote those which grow from the trunk of the tree, and secondary branches those which grow from the frame- work branches. Next are the fruiting branches, and on these are twigs, spurlike twigs, and spurs (pl. 2, A). A spurlike twig is a growth which, because of unfavorable conditions, has failed to make more than a very short development and resembles a spur more than a twig. Such twigs are often unbranched or the branches are short and spurlike. When favorable growing conditions recur a normal twig grows from the terminal bud. Twiglke spurs are normal spurs which branch but little, if any, but become long by the annual growth from the terminal bud (see pl. 9, D). Spur twigs are short twigs that grow from vigorous branches or spurs which pro- duce lateral spurs and a terminal spur but are not inclined to produce new twigs (see pl. 32, C). If twigs do grow from them they produce spurs the same as the ones on which they grow. Twigs grow from the main wood of a shoot, branch, or twig, but are slender and grow to a few inches or more in length. Some twigs branch more or less freely, and the terminal buds of both the main twig and its strongest branches send out a few inches of new growth each year. The weaker lateral branches of twigs often make but a short spurlike growth and die after fruiting. When growing conditions are uniavorable, many or all twigs are short, and many make but a spurlike terminal growth for one or more years or until conditions for growth improve. GROWING HABITS OF PLUM TREES IN RELATION TO PRUNING In training the young tree and in maintaining its shape and vigor when in bearing, the pruner should carefully consider the varietal characteristics of growth and production, such as the tendency to grow erect or spreading, open or compact, to make long or short an- nual wood growth, to maintain vegetative vigor, and the inclination to produce twigs or spurs. PLUMS IN RELATION TO PRUNING 5 Trees which make an erect growth often produce numerous long, stout, new shoots and a compact top and require thinning out of branches at pruning time to admit hght to the interior of the tree. If the seasonal growth of shoots is short on varieties inclined to make a vigorous growth, yearly cutting back at pruning time is usu- ally required to induce the production of large fruit. This is more especially true if the season’s growing conditions are somewhat un- favorable or where the plantings are in a location unsuited to the variety. If the trees grow well and are naturally spreading or open, a pruning practice is needed which will insure strong branches to carry the load of fruit as well as to insure fruit of large size. Be- tween the decidedly upright and the typically spreading trees men- tioned there are intermediate types representing both the triflora (Prunus salicina) or Japanese group and the domestica or European group. Of the varieties which are naturally upright in growth, the Santa Rosa and Wickson are examples of the Japanese group and Giant, Pond, and Sergeant of the domestica. Left unpruned or pruned but lightly, bearing trees of these varieties will assume a spreading posi- tion, because of the relatively slender growth of the new wood and the weight of fruit. Upright-growing trees of the domestica group are much more easily trained to the desired open shapes than are those of the triflora, as their branches are inclined to grow at a wider angle. With some of the domestica varieties many of the smaller fruiting branches and twigs are slender and hang from the larger branches after they have been brought to this position by a load of fruit. The Giant, Diamond, German Prune, and Lincoln illustrate this type as contrasted with trees with stouter fruiting wood, such as the Clyman, Sergeant, Imperial Epineuse, and President. Burbank of the Japanese group, Climax and Appie of the American-Japanese hybrid group, and California Blue, Reine Claude, and Peach of the domestica group are examples of varieties which are naturally open and spreading rather than drooping. These varieties represent ex- tremes in habits relative to the form of tree. Most varieties are between them in natural form. The round-topped, open tree of moderate growth is the most de- sirable from the standpoint of convenience in cultivation, pruning, and spraying, for productiveness, and for strength required to carry a heavy load of fruit. Most varieties deviate somewhat from the ideal in their natural growth, but a satisfactory form may be ap- proached by training the young tree. With the more extreme types, however, the ideal form may be only approximately maintained and this only by a careful and persistent selection of branches when pruning. The Jefferson, a variety inclined to make but little wood growth, and the Santa Rosa, one which grows vigorously, are examples of two of these types. The removal of enough wood at any one pruning to cause a radical change in the form of the tree will stimu- late a rampant growth and make much subsequent reshaping of the tree necessary. This rank growth of new wood of undesirable form resulting from severe pruning is more apparent with trees which make an erect, compact growth, as does the Santa Rosa, than with those which are naturally spreading, like the Burbank. 6 BULLETIN 1477, U. S. DEPARTMENT OF AGRICULTURE Trees of many varieties which produce a heavy new growth and form compact tops require thinning of branches to admit sufficient heht for the thrifty growth of fruiting wood in the interior of the tree. With other varieties which are very open and make but a short new growth, it is preferable to cut back the larger new shoots rather than to remove them. This provides for the growth of new wood needed to maintain the desired vigor for the production of large fruit and to protect the larger branches against sunburn. Jefferson, California Blue, and Golden Drop are among those which grow more slowly. Clyman and Sergeant are representative of those that are also open but are more vigorous. ‘The trees of these varieties soon become very spreading and open from lack of new branches, if pruning to encour- age branching and to maintain vegetative vigor is neglected. A few varieties are inclined to produce their new shoots by an outgrowth of the tips of branches rather than from spurs or the production of new branches. This tends to keep the tree open, and heading back to induce the growth of the desired number of branches is often neces- sary. Varieties in which this characteristic is prominent include the Sergeant, Clyman, California Blue, and Sugar. | The health or thrift of the tree and the conditions under which it is growing, whether favorable or otherwise, are indicated by the new growth made during the current season, as is also the tendency toward fruitfulness. New shoots that make a strong growth may be headed back less severely than weak, slender ones and a larger crop of fruit of good size produced, provided good cultural conditions are maintained. If the tree is severely headed back each year, it is burdened with the production of a new crop of wood as well as with that of fruit. The danger of too little pruning lies in reducing the vegetative vigor of the tree to a point below that needed for the production of large fruit. If the season’s wood growth is short it indicates drought, other unfavorable growing conditions, or insuffi- cient pruning; and small fruit may be expected if the trees are allowed to bear even moderately heavy crops. Aiso, a much smaller number of lateral twigs will be produced by the varieties which nor- mally produce spurs. Under the same conditions the trees which bear their fruit on twigs will come to production more promptly than those making a heavy growth. If the current year’s shoots which develop near the point of head- ing back are comparatively short, stout, and rigid, it indicates that the tree will be more open, will produce fewer fruiting branches, and will bear its fruit on spurs or short spurlike twigs. If the new shoots are long and slender, as they are on the Beauty and Santa Rosa, a thrifty wood growth is needed to enable the tree to produce large-sized fruit, and the tree is inclined to become compact and re- spond quickly to either favorable or adverse growing conditions. These rapidly growing shoots often branch during their current season’s growth. Several of these branches, which often grow to considerable length, sometimes appear on a single shoot and cause the tree top to become very compact and shade the lower portion. This branching indicates that the shoots on which they grow will not produce heavy crops as promptly as shorter, stouter, unbranched ones. By removing the large shoots and the small, the slender, and the more frail ones, leaving the stoutest of those of medium length _— ee ae PLUMS IN RELATION TO PRUNING 7 and cutting these back sufficiently to insure the desired new wood growth, spurs and twigs should develop promptly. With a few varieties the large vigorous shoots growing from stubs of branches of thrifty trees which were severely headed back send out short and slender but strong twigs along the lower part of the shoots during their first summer’s growth. These twigs appear early in the summer and grow at right angles to the main shoot. They produce fruit at leafless nodes along much of their length when one year old, but spurs are produced at a few nodes near the end. Such twigs make but little length growth after their first season, but their spurs are prolific and long lived and retain vigor well even in moder- ate shade. ‘Their appearance on new shoots indicates that the tree is thrifty and is inclined to bear its fruit on spurs instead of twigs. As these twigs appear well down along the shoots and remain prolific for a period of years, they should be spared and given light, and care should be taken that they are not broken during cultural operations. This type of growth is common with only a few varieties and these are in the domestica group. The President furnishes the best exam- — ple (see pl. 82, B). In the Japanese group the Duarte is similar to the President in regard to the branching of the current season’s shoots, but with the Duarte the twigs appear at somewhat less than a right angle te the parent branch, are slender and become drooping after bearing fruit, but are vigorous and continue fruitful for a number of years (see pl. 8, A). Trees below normal in vigor or those growing under unfavorable conditions and those not headed back in pruning seldom produce this type of twig. Short right-angled spurlike twigs sometimes appear also near the tip of shoots during their current season’s growth if the growth of the tree is checked by drought, the attack of insects, or otherwise. Shoots which make a long, vigcrous, whiplke growth start, for the most part, near a cut made in pruning, along the main limbs, and at the highest point of bending branches. The number of these shoots and the extent of growth made vary decidedly with the va- riety and with treatments which influence wood growth, the prin- cipal one of which is pruning. On 1-year-old wood which has been headed back, the growth of new shoots is, to a large degree, pro- portionate to the severity of the pruning. With some varieties the vegetative vigor appears to be very largely centered in the stubs of headed-back branches and almost all new shoots appear there. With other varieties given the same treatment the appearance of new shoots is distributed more or less throughout the tree. Trees of varieties which produce stout lateral spurs along the branch, but make only a few twigs and fruiting branches and remain open, are inclined to produce their new shoots near the ends of stubs of branches which have been headed back or near where branches have been removed. With those more inclined to produce twigs, new growth often appears along the older branches as well as on stubs of headed-back branches. Gaviota, Beauty, Tragedy, and Santa Rosa belong to the latter group, and Sergeant, Formosa, President, and Peach to the former. If the trees have been pruned lightly or not at all and the soil moisture is somewhat below their needs there will be but a small extent of new wood growth, and the upper branches of open-grow- 8 BULLETIN 1477, U. S. DEPARTMENT OF AGRICULTURE ing trees which produce spurs will often lose their vigor earlier than those more inclined to produce twigs. Also, the new shoots which appear are often well down on the older wood. On the upper branches of such trees the fruit as well as wood and leaf growth is usually small. Under similar conditions, trees which produce twigs instead of spurs are inclined to retain their vigor better, although the twigs are not much longer than spurs. If grow- ing conditions are favorable, some twig growth will take place at the terminals of some of the spurs of the spur-forming varieties, even where little or no pruning has been done. Trees of varieties which are decidedly inclined to produce twigs rather than spurs, of which Giant, Agen, and Italian Prune are typical, continue to form new twigs from nodes of 1-year-old wood and to extend the terminal growth of twigs throughout the tree even where not pruned, if other conditions favor wood growth. By cutting back in pruning, the length growth of the twigs is greatly increased, and but few of them remain as short spurlke twigs, such as are found on unpruned trees. On trees which tend to produce twigs, the twigs appear principally at nodes of 1-year- old wood, but with trees of the spur-forming groups, any twigs which appear are usually the terminal outgrowth of spurs and seldom grow from 1-year-old branches unless these branches have been cut back. Few twigs are produced on most trees of the latter varieties, unless they are cut back in pruning or unless the crop is light. With a number of Japanese varieties, many long, slender twigs appear as an outgrowth of spurs, if the trees are severely cut back, the soil fertile, and moisture plentiful. With spur-forming varieties the setting of spurs and their vigor and long life should be encouraged. Much toward this end may be accomplished in pruning by the thinning or heading back of brang¢hes, or both, to suit the needs of the variety and the individual tree. With such varieties excessive heading back may result in the loss of spurs in the interior of the tree through injury from shade caused by the dense new tops or by the outgrowth of spurs and twigs that follow such pruning. Too light pruning of such varieties may result in but little growth of the framework branches and in small fruit. Light pruning of varieties inclined to produce twigs instead of spurs may cause but a short growth of twigs and the formation of but few new ones. - The spurs of some Japanese sorts, of which the Apple variety is an example, are slender and brittle, and many of those on which twigs are being produced are broken during the fruit harvest and by strong winds. Spurs of the Japanese variety Formosa and of most domestica varieties, which increase in length considerably from year to year, become much strengthened as twigs grow from them, and they do not break as easily as those which grow slowly and remain slender. If the vegetative vigor of the spur is low, as is often the case with compact trees of some Japanese varieties such as Beauty and Santa Rosa, twigs often spring from the branch at the base of the spur instead of from the spur itself. If these twigs are not well thinned out, the tree soon becomes compact and the fruit spurs in that part of the tree are lost (see pl. 12, A). Bul. 1477, U. S. Dept. of Agriculture PLATE 1-—— PLUM TREES BEFORE AND AFTER PRUNING A.—A California Blue plum tree growing in an unirrigated section, before pruning. This tree is severely headed back and thinned out each winter, to maintain vigor and insure large fruit. (Compare with B) B.—The same tree shown in A, after the annual pruning under the system followed to insure vegetative vigor and large fruit C.—A 6-year-old Duarte plum tree growing in fertile irrigated soil, before pruning. This tree has been severely headed back each winter D.—The same tree shown in C. The severe type of annual pruning here illustrated is often practiced to prevent the tree from growing too tall and to admit light. However, arank growth of branches follows and insures large fruit. Dense shade in the interior of the tree results by early summer Bul. 1477, U. S. Dept. of Agriculture RESPONSE OF DIFFERENT PLUM TREES TO PRUNING AND TO SOIL AND OTHER CONDITIONS.—I A.—Fruiting branch of the Imperial Epineuse plum: a, Twigs; b, spurlike twigs; c, spurs B.—A 16-year-old Kelsey plum tree growing in the cool coastal region of California. This tree has been severely headed back each year and is growing in fertile irrigated soil. Note the very short annual growth and barren branches. (Niles, Calif., July, 1924) C.—A 16-year-old Gaviota plum tree growing in the cool coastal region of California. This tree has been severely headed back each year and is growing in fertile irrigated soil. Note very short annual growth and barren branches. Niles, Calif., June 29, 1923) (Compare with pl. 10, A. e Bul. 1477, U. S. Dept. of Agriculture PLATE 3 4 | RESPONSE OF DIFFERENT PLUM TREES TO PRUNING AND TO SOIL AND OTHER CONDITIONS.—II A.—A 16-year-old Maynard plum tree growing in the cool coastal region of California. This tree has been severely headed back each year and is growing in fertile irrigated soil. Note the vigorous growth and compact tree. (Compare with pl. 2, B and C. Niles, Calif., June 29, 1923) B.—An 18-year-old Grand Duke plum tree growing in the cool coastal region of California. It has been severely headed back each winter. The new growth is short, although the soil is fertile and irrigated. (Compare with pl. 20, B. Niles, Calif., July 9, 1924) C.—A 16-year-old Washington plum tree which has been severely headed back each year. It is growing in fertile irrigated soil in the cool coastal region of California. Note the vigor- ous, compact growth. (Compare with pls. 20 B, and 34 B. Niles, Calif., June 29, 1923) Bul. 1477, U. S. Dept. of Agriculture PLATE 4 = eae : GROWTH AND FRUITING HABITS OF THE BEAUTY PLUM UNDER DIFFERENT SOIL AND OTHER CONDITIONS A.—An old tree which has been headed back and thinned out each year, on deep, fertile, well- irrigated land, showing fruiting twigs throughout the tree. Most of the new shoots will be removed and the remainder headed back in pruning. (Courtland, Calif., , January, 1924) B.—Fruiting twigs and spurs. These form on branches of vigorous, well-opened trees if con- ditions are favorable to wood growth C.—Tree showing growth of twigs and spurs. This tree has been headed back and thinned out in winter and the new shoots thinned out each year about the 20th of May. This tree is not under irrigation. Vigorous twigs and spurs are formed throughout the tree. (Vacaville, Calif., July 7, 1923) 7 > Bul. 1477, U. S. Dept. of Agriculture PLATE 5 GROWTH, FRUITING HABITS, AND FRUIT SPURS OF THE BURBANK PLUM a} A.—Tree which has been headed back each year, growing in fertile irrigated soil. Note the numerous fruiting branches, twigs, and spurs on the newer wood B.—Fruit spurs ranging from 1 to 7 years of age from an irrigated tree which has been moder- ately headed back and thinned out each year Bul. 1477, U. S. Dept. of Agriculture PLATE 6 GROWTH AND FRUITING HABITS OF THE BURBANK PLUM _ IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDI- TIONS.—I A.—Old Burbank plum grafts, topworked on apricot, after a severe annual pruning. Almost the entire crop of fruit is borne on twigs 1 and 2 years old. The tree is growing in deep unirrigated soil where severe pruning is often necessary to insure large fruit. (Vacaville, Calif, February, 1923) B,—The same tree shown in A, the following summer. The new growth is very vigorous, but too compact for the growth of spurs in the interior of the tree. (Vacaville, Calif., June, 1923) Bul. 1477, U. S. Dept. of Agriculture PLATE 7 GROWTH AND FRUITING HABITS OF THE BURBANK PLUM IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDI- TIONS.—II A.—A 10-year-old tree which has been well thinned out each year in pruning but only lightly headed back. Note the vigorous spurs throughout the tree. The soil is fertile and under irrigation, and only moderate pruning is required. The tree has not as yet received its annual pruning. (New- castle, Calif.) B.—An 8-year-old fruiting branch of the Burbank plum tree shown above, showing the fruiting habit of old spurs Bul. 1477, U. S. Dept. of Agriculture GROWTH AND FRUITING HABITS OF THE DUARTE PLUM IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDITIONS A.—Very vigorous shoots of the current season, growing on a thrifty tree which was severely headed back the previous winter. Note the growth of lateral twigs. (Compare with C and also with pl. 1, C. Newcastle, Calif., May 5, 1928) B.—Fruiting twigs and thin spurs on framework branches 5 and 6 years old. The twigs were produced the same summer as the branches on which they grew. The tree has been Galil) back each winter and is in fertile irrigated soil. (Compare with A. Newcastle, Calif. C.—A few 4-year-old fruiting twigs which were produced during the first season’s growth of the large branch. (Compare with A and B) D.—A vigorous branch of a tree which has been pruned by heading back to lateral branches. Numerous spurs but only a few twigs have been formed. (Compare with A) Bul. 1477, U. S. Dept. of Agriculture \ \ / { x GROWTH AND FRUITING HABITS OF THE FORMOSA PLUM IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDITIONS A.—A tree which has been moderately headed back and the branches thinned out each winter. Note that the fruit spurs have been retained throughout the tree. It is not under irrigation and is growing in fertile soil B.—Some 8-year-old branches on which the new shoots have been severely headed back } each vear. Note the long growth and but little branching of spurs. The tree is i) growing in fertile soil that has not been irrigated. (Vacaville, Calif.) C.—Two 3-year-old branches which have not been headed back. The tree is growing in fertile irrigated soil. (Neweastle, Calif.) D.—A 7-year-oid branch of an open, vigorously growing tree. Note the long persistent spurs. The tree has been moderately headed back and the shoots thinned out each year. It is growing in fertile irrigated soil E.—An 8-year-old tree which has been severely headed back each winter. The tree is growing in fertile irrigated soil. Note the heavy growth of new shoots. (Newcastle, Calif.) Bul. 1477, U. S. Dept. of Agriculture PLATE 10 Eos GROWTH, SPURS, AND FRUITING HABITS OF THE GAVIOTA PLUM IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDITIONS A.—A thrifty tree which has been moderately headed back and thinned out each winter. There is a thrifty growth of foliage, spurs, and twigs, but only a few large new shoots. The tree under this treatment is inclined to remain open. The soil is fertile and irigated. (Compare with pl. 2, C. Newcastle, Calif., July 3, 1919) B.—Branch of a 9-year-old tree which has not been headed back but has been lightly thinned each year. Note the persistence of old spurs and that few of them have grown to twigs. (Compare with D. The trees in the two figures are growing in the same type of soil in the same locality, and both are under irrigation) C.—Spurs from vigorous irrigated trees which have been moderately thinned out and headed back each year D.—Twigs growing from spurs along framework branches of a tree which has been headed back and thinned to admit light each year. The vigorous growth shown is due to the fertile soil, irrigation, and pruning E.— Branch of a tree which has been moderately headed back and the branches thinned out each year, showing production on old spurs Bul. 1477, U. S. Dept. of Agriculture PLATE 11 mranapnadesiinicrnravenenenteqmervercsecererencenmmeryrencrencnrceeusatatasinapMtiictnirésitesnesscamneiinrrnercenercennmrerereverprnetced GROWTH, SPURS, AND FRUITING HABITS OF THE SANTA ROSA PLUM IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDITIONS.—I A.—A tree which has been moderately cut back and thinned out each winter. It is growing in deep irrigated soil. (Photographed before annual pruning. Courtland, Calif., January, 1924) B.—Some 1-year-old branches, showing habit of producing spurs at almost all nodes on L } larger shoots and of bearing fruit at nodes where spurs are forming. The more slender shoots produce fewer spurs along the basal segment and here produce fruit at leafless nodes C.—Spurs (above) from a tree which has been well thinned out and moderately cut back each year and (below) from a tree which has been heavily headed back each year but only lightly thinned out Bul. 1477, U. S. Dept. of Agriculture PLATE 12 GROWTH, SPURS, AND FRUITING HABITS OF THE SANTA ROSA PLUM IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDITIONS.—II A.—A tree which has been severely headed back each year. Shade from the dense new tops has caused the loss of spurs on the lower branches of the tree. (Compare with B. New- castle, Calif.) B.—A tree which has been well thinned out each year and only lightly headed back. Note that spurs have been retained well down on the older wood. ‘The tree is in fertile irrigated soil. (Newcastle, Calif.) C.—An 8-year-old secondary branch of a tree which has been kept open by annual thinning out. Note the thrifty spurs and twigs Bul. 1477, U. S. Dept. of Agriculture PLATE 13 DEVELOPMENT OF SPURS, TWIGS, AND FRUITING WOOD OF THE SATSUMA PLUM A.—Branch from a vigorous tree, Showing spurs and twigs on wood 1 and 2 years old. These live and remain productive for years if given light and the tree is kept thrifty. (Com- pare with pl. 13, B. Courtland, Calif.) B.—A 25-year-old secondary branch of a thrifty tree, showing the persistence of twigs and spurs. (Courtland, Calif.) C.—A 16-year-old tree which has been severely headed back each winter. Note the abun- dance of fruiting wood in all parts of the tree Bul. 1477, U. S. Dept. of Agriculture PLATE 14 ee GROWTH AND FRUITING HABITS OF THE WICKSON PLUM IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDITIONS A.—A 12-year-old tree which has been moderately headed back each year and kept open so that the spurs have not been killed by shade or induced to grow to twigs such as those of most trees severely pruned and irrigated. The soil here is fertile but not under irrigation. (Vacaville, Calif., June, 1923) B.—Fruiting twigs showing fruiting spurs and cutting back which has been practiced in pruning to maintain vigor C.—An old tree growing in fertile unirrigated land. The new growth has been severely headed back each winter. Note the short growth of shoots and barren framework and secondary branches Bul. 1477, U. S. Dept. of Agriculture PLATE 15 %y x LR pene et RESPONSE OF THE WICKSON PLUM TREE TO MODERATE PRUNING A.—Branches from an irrigated tree which has not been pruned for 15 years. Note the lack of new shoots and twigs, but the very numerous old spurs. (Newcastle, Calif.) B,—An old tree in fertile irrigated soil which has been moderately headed back and thinned out each year. Photographed after pruning. Note the numerous spurs and vigorous twigs. (Newcastle, Calif.) Bul. 1477, U. S. Dept. of Agriculture PLATE 16 GROWTH, FRUIT SPURS, AND FRUITING HABITS OF THE AGEN PRUNE IN RELATION TO PRUNING AND TO SOIL AND OTHER CONDITIONS A.—A thrifty old tree which has not been pruned recently and is not irrigated. This tree produces heavy crops of medium-sized fruit. Thinning out of fruiting branches is needed to increase the size of the fruit. (Vacaville, Calif.) B.—A 10-year-old tree. The short spurlike growth instead of longer twigs indicates lack of thrift, although the tree is in fertile irrigated soil C.—Framework branch of the tree shown in D, indicating the fruiting habits of a tree on which all shoots and young twigs have been severely headed back each winter D.—A 16-year-old tree which has been severely headed back each year and is growing in fertile irrigated soil, It is very vigorous and produces heavy crops of fruit, (Niles, Calif.) PLUMS IN RELATION TO PRUNING 9 FRUITING HABITS IN RELATION TO GROWTH For the purpose of studying fruiting habits of plum trees in relation to the portion of the tree on which the fruit is borne, the varieties grown commercially are considered under two groups, the Japanese and the domestica. In the Japanese group, fruit is borne both at nodes on 1-year-old wood and on spurs throughout the tree. It is found on spurs with leaves or at nodes of 1-year-old wood where spurs are forming, except that shaded or crowded spurs sometimes set fruit but fail to produce leaves and die after the fruit ripens. Some leafless nodes of slender, crowded, 1-year-old twigs also bear fruit. As with varieties of the domestica group, little, if any, fruit is borne on large 1-year shoots which develop on branches which were headed back or on water sprouts lower in the tree. On the 1-year-o!d wood, the crop is often heaviest on the shorter, more slender wood; and in many cases the fruit on such wood is not of profitable size and market quality unless both fruiting branches and fruit are well thinned. Many varieties of the domestica group bear much of their fruit at leafless nodes of 1-year-old wood. Most of these varieties bear only an occasional fruit, if any, at a node where a spur or twig is form- ing. The California Blue is one of the exceptions to this. Leafless nodes which bear fruit remain barren after the fruit ripens, and varieties of the twig-forming type often require new twigs each year to provide fruiting wood for the following season. A few domestica varieties produce spurs instead of twigs, and with these the crop is produced largely on spurs. If for lack of moisture or other causes the wood growth on such trees was short the previous year, fruit production will often be heavy on the 1-year-old wood, as with twig-forming varieties, and be produced almost entirely. at leafless nodes.