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 UNIVERSITY OF CALIFORN 
 
 PRUNING CITRUS TREES 
 
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 S. B. BOSWELL 
 
 R.M. BURNS 
 
 R. G. PLATT 
 
 K.W.OPITZ 
 
 L. N. LEWIS 
 
 
 
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 CALIFORNIA AGRICULTURAL 
 
 on Service 
 
 CIRCULAR 565 
 
 CAECAG 565 1-16 (1974) 
 
This circular tells the citrus grower — 
 
 • how trees react physiologically to pruning 
 
 • what conditions are necessary for pruning 
 
 • about pruning in the nursery, in the young orchard, and in the 
 mature orchard. 
 
 • about methods of pruning different varieties of citrus 
 
 • how to prune after injury 
 
 • how and when to prune by hand or mechanically 
 
 • how diseases can be spread by pruning and how to care for pruning 
 wounds 
 
 • about pruning equipment 
 
 MAY 1974 
 
 THE AUTHORS: 
 
 C. D. McCarty is Horticultural Technologist, Agricultural Extension Service, 
 Riverside; S. B. Boswell is Specialist, Department of Plant Science, Riverside; 
 R. M. Burns is Farm Advisor, Ventura County; R. G. Piatt is Subtropical 
 Horticulturist, Agricultural Extension Service, Riverside; K. W. Opitz is Sub- 
 tropical Horticulturist, Agricultural Extension Service, Kearney Horticultural 
 Field Station, Parlier; and L. N. Lewis is Professor of Plant Physiology, 
 Riverside. 
 
 [2] 
 
Pruning Citrus Trees 
 
 Poliage cut from healthy, mature 
 citrus trees reduces yield in proportion 
 to the amount of foliage removed. 
 Pruning young, nonbearing trees 
 delays fruiting. Pruning, therefore, 
 should be limited to that required for 
 future tree development and for essen- 
 tial cultural operations — and should 
 be adjusted to the requirements of the 
 individual tree. 
 
 Location of food storage differs be- 
 tween deciduous fruit trees and ever- 
 green citrus. During the summer the 
 majority of excess food manufactured 
 by deciduous trees is stored in the root 
 system. Pruning deciduous trees while 
 they are dormant does not appreciably 
 reduce the supply of stored food, and 
 it remains available for the spring 
 growth flush. 
 
 In citrus trees, on the other hand, 
 carbohydrates are stored in the leaves, 
 twigs, and branches; only a minor 
 amount goes to the root system. The 
 maximum amount of stored food is 
 reached in late February or early 
 March just before spring growth activ- 
 ity. Thus, the foliage of a citrus tree 
 serves as an important food storage 
 area, and pruning, which removes this 
 foliage, causes the tree to produce a 
 vegetative growth flush at the expense 
 of fruit production. 
 
 Hormones or growth regulators which 
 occur in the citrus tree, may affect 
 fruit set considerably according to re- 
 cent experimental work. Just how 
 pruning affects the action of these hor- 
 mones is not completely understood. 
 Citrus trees deficient in both carbo- 
 hydrates and nitrogen grow and fruit 
 poorly. If the tree is low in carbohy- 
 
 1 For detailed information on growing nursery citrus, see Extension Circular 546, "Propa- 
 gation of Citrus," by K. W. Opitz, R. G. Piatt, and E. F. Frolich. 
 
 [3] 
 
 drates, and if nitrogen is available, 
 vegetative growth is produced. When 
 stored carbohydrates and nitrogen are 
 at optimum levels, both fruiting and 
 vegetative growth are more likely to 
 occur. 
 
 Pruning in the nursery 1 
 
 The first pruning often occurs when 
 the young seedlings are removed from 
 the seedbed. In most cases it is neces- 
 sary to trim the roots. Cutting back 
 the top may be needed to bring it into 
 balance with the reduced root system, 
 since the roots must supply the leaves 
 with sufficient water to prevent per- 
 manent wilting. 
 
 Following pruning, the seedlings are 
 transplanted to the nursery row. After 
 growth resumes, suckers which sprout 
 from the lower portion of the trunk 
 to a height where budding is to take 
 place should be removed (preferably 
 by rubbing with a gloved hand). 
 Sucker initiation may be prevented by 
 wrapping the seedling trunk with alu- 
 minum foil. 
 
 Rough lemon, macrophylla, citrange 
 and large-flowered strains of trifoliate 
 orange seedlings grow upright with a 
 single trunk and usually require little 
 pruning. Sweet orange, grapefruit, tan- 
 gelo, mandarin seedlings, and small- 
 flowered trifoliates tend to branch 
 lower and need more pruning. 
 
 A few weeks before budding, the 
 rootstock seedlings may be pruned, to 
 a single bare trunk 12 to 15 inches 
 high, so the stock will be free of growth 
 for bud insertion. Pruning causes a 
 temporary tightening of the bark and 
 should be completed at least three 
 
Fig. 1. Budded seedling. Stock has been 
 pruned above point of budding. Top is bent 
 over to favor growth of scion. 
 
 weeks before budding, so that the cam- 
 bium will again be in an active state 
 of growth and slip freely. Seedlings 
 also can be pruned when budded. 
 
 Topping, lopping or bending is usu- 
 ally necessary to check the vegetative 
 growth of the rootstock, which forces 
 growth of the bud after it unites with 
 the stock. In topping, the trunk is cut 
 off a few inches above the bud. When 
 trees are lopped, the seedling trunk is 
 partly cut through about three inches 
 above, and on the same side as the bud. 
 The top is not removed; instead it is 
 laid over on the ground. This allows 
 the leaves of the rootstock seedling to 
 continue to manufacture and supply 
 food to the roots and developing bud 
 shoot. In bending, the seedling top is 
 bent over by flexing without cutting 
 (fig. 1). The nurse top is not removed 
 until the shoot from the bud 1 as grown 
 to a sufficient height. In some cases the 
 
 rootstock top is allowed to remain un- 
 til the following spring. 
 
 After the bud shoot grows to a 
 height of 2 feet or more, the rootstock 
 stub is cut off evenly and neatly on a 
 slant just above the bud union. The 
 cut should be painted with a protec- 
 tive covering, such as an asphalt-water 
 emulsion paint. This cut should not be 
 made late in the season, since it makes 
 the young tree more susceptible to 
 cold injury. 
 
 Little pruning, except for sucker re- 
 moval, is needed until trees are headed 
 for field planting. Usual heading 
 height is about 30 inches for all vari- 
 eties of citrus. Satsuma mandarin is 
 an exception; it is sometimes headed 
 at a slightly lower height because of 
 its slower growth habit. 
 
 Nursery trees topped to form a head 
 should be pruned again just before 
 digging. Three to five well-spaced 
 branches are left around the top of 
 the tree and cut back to lengths of 5 
 to 8 inches. This pruning brings the 
 top into balance with the root system, 
 which is reduced during the balling 
 operation. If the trees are dug bare- 
 root, a larger root system is left. How- 
 ever, the balance between top and root 
 may be even more critical due to pos- 
 sible drying of roots. 
 
 A second method is to grow the trees 
 as "whips" or "canes," which are 
 topped as a single stem at the time of 
 digging. The whip system is often used 
 with younger trees, which are field- 
 planted one year after budding. Older 
 trees may also be cut back to a single 
 trunk when field-planted. 
 
 Pruning the young orchard 
 
 Most citrus trees need no pruning 
 for two or three years after transplant- 
 ing to the field — except to remove 
 shoots if they sprout from the trunk 
 below the head of the tree. Wraps 
 placed around the trunks to prevent 
 sunburn also provide enough shade 
 
 [4] 
 
Fig. 2. Natural growth sequence. Upright 
 growing limbs will bend from their own weight. 
 Further upward growth will be from new shoots 
 along the upper sides of these limbs and from 
 the center of the tree. The skirt is formed by 
 laterals which droop to the ground. (Lewis and 
 McCarty.) 
 
 to reduce or prevent most trunk 
 sprouts. 
 
 Occasionally, a small, weak tree pro- 
 duces a vigorous sucker, which domi- 
 nates the tree from a point above the 
 bud union on the main trunk. Suckers 
 may also arise from the rootstock. 
 These suckers must be removed before 
 they gain dominance. 
 
 Selection of scaffold branches 
 
 The intended life of an orchard is a 
 major consideration in pruning. Ur- 
 ban development, for instance, might 
 dictate an orchard's life to be no more 
 than 10 to 15 years. Pruning, then, is 
 of little value. On the other hand, if 
 the orchard is a long-term operation, 
 careful and limited pruning to estab- 
 lish good tree form is worth the sacri- 
 fice of a certain amount of fruit. 
 
 Selection of permanent scaffold 
 branches during the first two or three 
 years of the tree's life usually fails. 
 When trees are headed for field plant- 
 ing, only buds close to the point of 
 heading sprout. These are closely 
 spaced and grow upward until they 
 bend under their own weight to form 
 laterals. New buds sprout from the 
 center of the tree and from along the 
 
 upperside of these bent branches, 
 which droop even further to form the 
 tree's skirt. As the new branches grow 
 upward, they also bend, and the proc- 
 ess is repeated. Under these condi- 
 tions, dominance jumps from one 
 branch to another, and what may seem 
 a good choice for a main branch one 
 year may not be the dominant branch 
 the following year. Pruning at this 
 stage often hinders, rather than helps, 
 the normal growth pattern. Figure 2 
 shows a natural growth sequence at 
 this stage. 
 
 As the tree grows older and increases 
 in size, the center fills with closely 
 spaced and crossing branches. When 
 the tree is 3 or 4 years old, depending 
 on size and growth rate, light, selective 
 thinning may be done (see fig. 3) to 
 remove branches which are too closely 
 spaced or are crossed and entangled. 
 This pruning should not be heavy; 
 rather, it should just be sufficient to 
 establish the scaffold framework. 
 
 Small irregularities in the canopies 
 of young trees should be ignored (fig. 
 4). Left alone, citrus trees normally 
 develop a relatively even spherical 
 shape as they mature. 
 
 Fig. 3. Light selective pruning should be 
 done to remove crossing branches and those 
 which are too closely spaced. 
 
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 [5] 
 
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 Fig. 4. Lopsided tree caused by a dominant fast-growing limb. Such cases are the exception. 
 While pruning may be used to force the tree into a more normal growth habit the condition 
 will usually correct itself if left unpruned, and the tree will assume a normal shape. (Lewis and 
 McCarty.) 
 
 Pruning during the next 5 or 6 years 
 should be limited to the removal of 
 occasional branches that interfere with 
 the growth of a sturdy framework of 
 scaffold limbs. 
 
 Undesirable shoots, which sprout 
 following pruning, should be removed 
 when they are a few inches long and 
 are still tender enough to be removed 
 with a gloved hand. Allowed to grow, 
 they deplete the tree of food reserves 
 and necessitate more severe pruning 
 later. 
 
 When pruning, it is best to remove 
 unwanted limbs at the point where 
 they originate, or they should be cut 
 back to a lateral. This reduces the 
 sprouting of new buds in the area 
 around the cut. Heading of a wanted 
 limb may be done when needed to in- 
 duce branching or to strengthen its 
 growth, if it is weak or willowy. 
 
 Pruning cuts should be made as near 
 the vertical plane as possible, since 
 the nearer to horizontal the cut, the 
 greater the sprouting of new buds. 
 
 [6 
 
Mandarins and lemons. Orange and 
 grapefruit trees should be pruned 
 alike during the early years of the or- 
 chard's life. However, some varieties 
 of mandarins may require removal of 
 weak branches and heading back to 
 laterals of limbs to be kept in order 
 to encourage vigorous shoot growth. 
 
 Lemon trees, because of their open, 
 irregular growth habit, require some 
 pruning but less than generally be- 
 lieved. Unpruned lemon trees fruit 
 early and abundantly. However, 
 lemon trees tend to produce long, 
 spindly shoots which are mechanically 
 weak and easily broken. It is often 
 necessary to remove some of these by 
 thinning or to shorten others back to 
 laterals to strengthen them. Lemon 
 trees also send out strong laterals 
 through the center of the tree. With- 
 out pruning, the interior of the tree 
 fills with crossing limbs. 
 
 A good framework of scaffold 
 branches helps prevent limb breakage. 
 To neglect early selective pruning of 
 lemon trees will necessitate heavy cut- 
 ting later on, which causes a delay and 
 reduction in yield. 
 
 Two types of pruning systems have 
 been used for lemons: one, mainly a 
 severe heading back of branches with 
 some thinning where crowding of 
 limbs occurs; two, mainly a thinning 
 of branches with a light heading back 
 to laterals where needed. 
 
 With the heading method, upright 
 growing branches are cut back se- 
 verely. New shoots which sprout near 
 the tops of these cut branches are 
 thinned and cut back in the same man- 
 ner. Some inside wood is removed to 
 prevent the tree from becoming too 
 dense. This pruning produces a small, 
 compact tree — as well as delayed fruit- 
 ing and reduced yield. 
 
 Thinning, on the other hand, re- 
 quires selection of three to five up- 
 right leaders to become the main 
 scaffold branches. These leaders are 
 
 maintained by light pruning. Compet- 
 ing shoots are removed while they are 
 still small. A minimum of foliage is 
 removed with just enough pruning 
 to keep the trees relatively open. In 
 order to discourage the bending of 
 fruiting uprights, the terminals are 
 pruned before fruit production be- 
 comes too heavy. If a main limb bends 
 to the point where it becomes a lateral, 
 it is replaced by a strong, nearby up- 
 right shoot. 
 
 No undercutting or removal of low 
 branches is done until "shading out" 
 begins to occur. 
 
 Young, vigorously growing lemon 
 trees are susceptible to breakage by 
 strong winds. Shortening tall growing 
 uprights plus thinning of unneeded 
 limbs helps to lessen this damage. 
 
 Pruning the mature tree 
 
 Bearing orange and grapefruit trees 
 require little pruning. Most of the ex- 
 perimental work with citrus indicates 
 that yield from healthy trees is re- 
 duced in proportion to the severity of 
 pruning. To facilitate certain manage- 
 ment procedures, there are conditions 
 where light pruning is advantageous. 
 Light thinning in the top of the tree 
 to promote the growth of inside fruit- 
 wood may be helpful. For better pest 
 control and ease of harvesting, limit- 
 ing tree height may have merit, but 
 if excessive its advantages are easily 
 outweighed by the loss of fruit-bearing 
 foliage. 
 
 As the trees age, in uncrowded or- 
 chards, the top branches are usually 
 the first to decline in production and 
 fruit quality. A light thinning of the 
 top promotes the growth of new fruit- 
 wood, but such pruning should not 
 open the tree to the point where sun- 
 burning of exposed branches occurs. 
 If a limb is exposed to sunburn it 
 should be protected by a coating of 
 whitewash. When thinning the top of 
 the tree, branches should not be cut 
 
 [7] 
 

 
 Fig. 5. Weak and dead wood is removed from inside the tree's skirt. This type of pruning, called 
 undercutting, stimulates new growth and maintains the bearing efficiency of the skirt. 
 
 back as stubs. Instead, the limb should 
 be cut back to another branch or lat- 
 eral. Most deadwood may be removed, 
 but it is seldom economically feasible 
 to remove every twig. Weak, non-pro- 
 ductive wood should be removed from 
 the center of the tree, but vigorous 
 shoots should, where possible, be re- 
 tained and bent over to fill thinly foli- 
 ated areas with fruitwood. 
 
 The skirt of the tree bears a large 
 portion of the fruit, and pruning 
 should be slight until the productive 
 wood begins to decline. Non-produc- 
 tive skirt branches should be removed 
 by cutting from beneath, leaving the 
 upper and newer foliage to replace 
 that taken out. Known as undercut- 
 
 ting, this helps insure the bearing ef- 
 ficiency of the skirt (fig. 5). 
 
 As the trees grow larger, it may be 
 necessary to prune to restrict each tree 
 to a given area as limited by spacing. 
 This is particularly true in close 
 spaced orchards. Pruning to hold trees 
 to a given size requires judicious hand 
 pruning and results in some loss in 
 yield. Where trees are extremely close, 
 a better solution is to remove alternate 
 trees. This should be done before 
 crowding reaches the point where most 
 of the skirt fruitwood is lost from 
 shading. 
 
 The growth habit of mature lemon 
 trees is more open and irregular than 
 other varieties of citrus; unpruned, 
 
 [8] 
 
mmmm 
 
 Fig. 6. Pruning the mature lemon tree. Fast growing upright shoots are shortened back to 
 laterals and the inside of the tree is thinned. Light frequent pruning is preferable to heavy prun- 
 ing at longer intervals. 
 
 they usually produce abundantly. 
 However, unpruned trees grow into 
 a tangle of weak and interlaced 
 branches. Such trees are difficult to 
 harvest and efficient pest control is a 
 problem. Crowding also makes other 
 cultural operations more difficult. In 
 addition, nonpruning hastens the de- 
 velopment of sieve-tube necrosis in cer- 
 tain selections of lemon varieties. Dys- 
 function of sieve tubes in the phloem 
 can be reduced, if not eliminated, by 
 proper pruning. 
 
 A systematic pruning program, be- 
 gun when lemon trees are small, 
 should be continued into maturity. 
 Light, frequent pruning is advisable. 
 Periodic thinning of unwanted 
 branches, and shortening of others to 
 laterals, results in the development of 
 a low-spreading tree from which the 
 
 fruit is easy to harvest. (See fig. 6.) 
 
 Mechanical hedging 
 (figs. 7a and 7b) 
 
 The development of mechanical 
 hedging and topping machines has 
 changed pruning methods. Hedging 
 consists of cutting the side of the tree 
 back vertically. If the trees are tightly 
 grown together, hedging will open up 
 the orchard, remove dead and weak 
 wood, and in some cases allow the tree 
 to grow a new skirt. In addition the 
 grove is opened for the passage of 
 equipment needed for grove mainte- 
 nance. A more efficient coverage of 
 insecticidal sprays may result, and 
 harvesting of fruit may be made easier. 
 
 In recent years, closer tree spacing 
 has become the accepted planting 
 
 9] 
 
Fig. 7a. Trees in foreground have been 
 hedged. Compare with unhedged trees in 
 background. A light hedging before trees 
 begin to crowd is more practical than heavy 
 cutting after the trees have grown together. 
 (After Lewis and McCarty.) 
 
 practice in many California orchards. 
 As these trees reach maturity, pruning 
 becomes necessary to prevent the shad- 
 ing out of lower parts of the tree. In 
 some cases the arm holding the cutting 
 blades is tilted slightly toward the top 
 of the tree. Cutting on a slant allows 
 more light to reach the lower foliage. 
 Mechanical hedgers offer a quick and 
 economical way to open close-planted 
 groves. The principal disadvantage is 
 that mechanical hedging is nonselec- 
 tive, removing both bad and good 
 wood. 
 
 Hedging leaves numerous exposed 
 stubs along the side of the tree from 
 which new buds sprout. This produces 
 a wall of new foliage. Depending upon 
 the amount of wood and foliage re- 
 moved, yields are usually reduced the 
 year following hedging. Light hedging 
 reduces yield less than heavy hedging, 
 and in some cases a light hedging 
 which opens the orchard to better 
 illumination and prevents the shading 
 of lower fruitwood results in increased 
 yields over similar non-hedged or- 
 chards. 
 
 Fig. 7b. Tilting the cutting arm toward the 
 center of the row gives a "Christmas tree" 
 effect and allows more light to reach the lower 
 portion of the trees. 
 
 Mechanical topping (fig. 8) 
 
 Convenient picking height and eco- 
 nomics usually determine the need for 
 mechanical topping in lemon or- 
 chards. 
 
 Mechanical toppers are also used to 
 remove the tops of declining orange 
 and grapefruit trees to allow light into 
 the trees to stimulate the growth of 
 new fruitwood. Topping is a heading- 
 back type of pruning, and numerous 
 buds sprout from around the cut stubs. 
 This growth, often referred to as "cab- 
 bage heads" must eventually be 
 thinned by selective hand pruning, 
 since it becomes so dense that it shades 
 out the interior of the tree. Many 
 growers follow a program of alternat- 
 ing mechanical topping with hand 
 pruning. Other growers machine top 
 just before hand pruning so that prim- 
 ers have less brush to handle. More 
 uniform tree height is also maintained 
 by this method. 
 
 Light mechanical pruning of citrus 
 usually does not decrease yields as per- 
 sistently as hand pruning. This may 
 be due to a tendency for some hand 
 
 [10] 
 
Fig. 8. Mechanical topper. Circular saws 
 mounted on rotating arms. Combined action 
 of rotating saws and arms helps throw the cut 
 branches out of the tree into the row. (Photo 
 courtesy Kimball Toppers.) 
 
 pruners to take out too much produc- 
 tive fruitwood. 
 
 Following severe freezes it has been 
 noted that fruit on mechanically 
 topped and hedged trees is more se- 
 verely injured than fruit on unpruned 
 check trees. Fruit on recently pruned 
 trees lacks the protective cover of foli- 
 age and is exposed to greater radiation 
 which lowers fruit temperature below 
 that of unexposed fruit. 
 
 Pruning injured trees 
 
 Trees injured by frost, severe wind- 
 burn, or rodents, will require special 
 pruning. Periodic freezes of moderate 
 to severe intensity occasionally occur 
 in all of the major citrus-growing areas 
 of the state. If the damage is light, and 
 
 only foliage and small twigs are in- 
 jured, pruning is not required. When 
 larger limbs are killed by a severe 
 freeze, pruning is necessary. 
 
 Freeze damage ordinarily becomes vis- 
 ible within a few days on foliage and 
 young growth; it is impossible to de- 
 termine the full extent of injury im- 
 mediately after a freeze on older parts 
 of the tree. Dieback may continue dur- 
 ing the entire season when severe 
 freezes occur. On the other hand, wood 
 which at first appears to be so severely 
 injured that it should be removed will 
 recover. Because of this, pruning 
 should not be done for at least six 
 months to a year after damage has oc- 
 curred. Enough time should be al- 
 lowed for new growth to take place, 
 and for dieback to cease so that the 
 extent of damage is clearly defined. If 
 freeze-injured trees are not pruned 
 until regrowth is well under way, re- 
 covery will be more rapid than if the 
 trees are pruned immediately follow- 
 ing injury. 
 
 When pruning is done, injured 
 limbs should be cut back below all 
 serious bark damage to a point where 
 vigorous new sprouts are growing. It 
 may be advisable to aid the formation 
 of new framework branches by a fur- 
 ther light pruning, if and when it is 
 needed. 
 
 In cases of extremely severe dam- 
 age, where the entire top of the tree is 
 killed and injury extends to the trunk, 
 a new head must be developed. The 
 old trunk and top should be removed 
 as soon as the extent of damage is de- 
 termined. Under these conditions, the 
 new tree which is formed may have 
 a multiple trunk system. Experience 
 shows this is not objectionable. Fol- 
 lowing the freeze, and until the new 
 head is fairly well formed, all sprouts 
 should be allowed to grow in order to 
 bring the top and root system back 
 into balance. Once this occurs, exces- 
 
 [ii] 
 
sive growth should be pruned leaving 
 several shoots to form the new scaffold 
 framework of the tree. 
 
 Where the top of a young tree is 
 killed, it is best to rebuild the top by 
 favoring the growth of one strong 
 shoot. The shoot chosen to form the 
 new top should be forced by pinching 
 back other sprouts as they appear. 
 
 Hot, dry winds will occasionally in- 
 jure trees. Many of the defoliated 
 limbs will recover; however, recovery 
 is usually slower than from frost in- 
 jury. Branches which do not recover 
 should be pruned from the tree care- 
 fully so that a minimum of healthy 
 foliage is lost. 
 
 Breakage from strong winds some- 
 times occurs. Young, vigorously grow- 
 ing lemons are most susceptible. 
 Where a crop is on the tree, and the 
 limb is still attached enough to allow 
 the fruit to mature, pruning can be 
 put off until after harvest. A follow-up 
 pruning will be needed to thin exces- 
 sive regrowth. 
 
 Gopher injury usually confronts every 
 grower. Normally gophered trees are 
 healthy, and will recover unless badly 
 girdled. When root damage is so ex- 
 tensive that remaining roots cannot 
 supply water for the top, the top-root 
 balance should be restored by pruning. 
 If the tree is completely girdled, it can 
 often be saved by immediate inarching 
 followed by severe pruning. 
 
 Time of pruning 
 
 Since time of pruning is not highly 
 critical with citrus, trees are often 
 pruned when other cultural opera- 
 tions are at a low ebb, and it is con- 
 venient for the grower. Experiments 
 have shown that the best results, from 
 the physiological standpoint of the 
 tree, can be expected if pruning takes 
 place early in the spring after danger 
 of frost has passed and before the start 
 
 of a new growth cycle. Rate of foliage 
 regeneration is most rapid on spring- 
 pruned trees and least rapid when 
 trees are pruned late in the fall. Also, 
 fall pruning stimulates a late flush 
 which is tender and more susceptible 
 to frost injury during the following 
 winter. 
 
 Time of pruning may be restricted 
 by the presence of mature fruit on the 
 trees. Little problem is presented with 
 navel oranges and winter grapefruit, 
 when the crop is harvested before 
 spring. With Valencia orange and 
 summer grapefruit trees, both young 
 and mature fruit are on the tree at the 
 same time, and late summer pruning 
 after harvest may be preferred when 
 fruit is at its scarcest. In some cases, it 
 may be desirable to take advantage 
 of the alternate bearing tendency of 
 some species of citrus and prune dur- 
 ing the light crop year. 
 
 In coastal areas, lemons are usually 
 pruned after the last main summer 
 harvest so that fewer nearly mature 
 fruit are lost. 
 
 Rejuvenation pruning 
 
 One of the major problems in Cali- 
 fornia's orange production in recent 
 years has been a reduced tree vigor, 
 especially in older groves, resulting in 
 small fruit and low yields. Loss of 
 vigor has usually been accompanied 
 by a dieback of twigs and small 
 branches from which no primary path- 
 ological organisms can be isolated. 
 
 This decline may be caused by in- 
 creasing age of the tree, low rainfall, 
 faulty irrigation, salt accumulation in 
 the soil, virus and pest problems and 
 air pollution — either separately or in 
 various combinations. 
 
 Rejuvenation pruning forces the 
 tree to produce new fruitwood and 
 ranges from a moderate thinning of 
 the canopy to complete skeletonization 
 of the tree (fig. 9). This latter removes 
 all foliage and wood smaller than an 
 
 [12] 
 

 Fig. 9. Skeletonized grapefruit tree. All 
 foliage and branches smaller than approx- 
 imately 1 inch in diameter have been removed. 
 Cut brush is placed back in tree to help prevenl 
 sunburn. 
 
 inch in diameter, leaving only the 
 main scaffold and adjoining branches. 
 When older and weaker parts of the 
 tree are removed, new buds sprout, 
 and new fruitwood is formed. How- 
 ever, unless the cause of the decline 
 has been corrected, the effects of prun- 
 ing are temporary and the trees soon 
 decline again. 
 
 A well-planned program of irriga- 
 tion, fertilization, and pest control, as 
 well as pruning, is needed to bring a 
 rundown orchard back into produc- 
 tion. 
 
 Heavy pruning of lemon trees af- 
 fected with such diseases as shell bark 
 or sieve-tube necrosis can keep these 
 trees productive for many years. 
 
 Care of pruning wounds 
 
 In the dry climate of California, 
 decay-producing organisms rarely en- 
 ter through pruning wounds. Also, 
 mechanical pruning, which leaves nu- 
 merous cut stubs, has made treatment 
 impractical. Large wounds, however, 
 can be encouraged to heal with a 
 water-based asphalt emulsion. This 
 material must not contain solvents 
 which could penetrate and kill the 
 cambium. Whitewash, bordeaux paste, 
 lead paints, or varnishes and shellacs 
 are not satisfactory for treating cut sur- 
 faces, since they soon dry, crack, and 
 lose their protective qualities. Where 
 gummosis is a problem, fresh cuts may 
 be painted with avenarius carbo- 
 lineum. 
 
 Bark on limbs grown in the shade 
 is susceptible to sunburn, and a few 
 hours' exposure may cause injury. In 
 some cases, especially when trees are 
 topped, the exposed portion of limbs 
 should be treated with whitewash. 
 Treatment of branches other than 
 those exposed to the sun is unneces- 
 sary and may even be detrimental, 
 since whitewash provides an excellent 
 habitat for the development of red 
 spider. It may also delay sprouting of 
 buds. If a few trees are to be treated, 
 application can be done with a brush, 
 but if large numbers are treated, a 
 more practical method is to use a 
 power sprayer. 
 
 Spread of disease by pruning 
 
 Exocortis can be transmitted from 
 an infected to a non-infected tree by 
 knives, pruning tools, and even by 
 hands. It is more readily transmitted 
 to lemon than to orange, grapefruit, 
 or mandarin trees. When the rootstock 
 is trifoliate orange, citrange, or Rang- 
 pur lime, mild to severe stunting of 
 trees can occur. Trees on other root- 
 stocks, even though infected, are tol- 
 erant and show little or no effect. 
 
 [13 
 
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 Fig. 10. Pneumatic and electric pruning equipment. Use of power tools is becoming 
 
 increasingly popular. 
 
 In pruning orchards on exocortis- 
 susceptible rootstocks, care should be 
 taken to avoid cutting exocortis-in- 
 fected trees before moving to non-in- 
 fected trees. Tools should be disin- 
 fected by dipping in \i/ 2 pints of 
 household bleach diluted with water 
 to make 1 gallon. To counteract the 
 corrosive action of the disinfectant, 
 pruning tools should be cleaned after 
 use, in a mixture of 2 teaspoons of 
 emulsifiable oil in one-quarter pint of 
 vinegar diluted with water to a total 
 volume of 1 pint. This mixture should 
 be shaken vigorously just before use. 
 
 Since large mechanical equipment, 
 such as hedgers and toppers, is im- 
 practical to disinfect, known infected 
 trees should be cut last. 
 
 Ferment gum or Rio Grande gum- 
 mosis is another disease which may be 
 transmitted by pruning tools. It occurs 
 in grapefruit and is characterized by 
 an amber to reddish, often frothy, gum 
 which is exuded profusely from cracks 
 in the bark of trunks and limbs. Trees 
 with this disease generally deteriorate 
 and sometimes die. 
 
 To prevent spread of this disease, 
 pruning tools should be disinfected 
 with the bleach solution, as mentioned 
 above, when moving from one tree to 
 the next, particularly from a diseased 
 tree to an apparently healthy tree. 
 
 Also, pruning cuts greater than 1 inch 
 in diameter should be treated with 
 avenarius carbolineum. 
 
 Pruning equipment 
 
 For hand-pruning a good pair of 
 long-handled loppers, a pair of strong 
 hand shears, a pruning saw, and a lad- 
 der are needed. A can of water-asphalt 
 emulsion paint for the sealing of large 
 cuts may also be useful to carry. The 
 pruning saw in general use is a pull 
 saw with a folding blade 12 to 18 
 inches in length with a curved cutting 
 edge. To be satisfactory, the curve of 
 the cutting edge should not be too 
 great, and the form and set of the 
 teeth should readily clear the cut of 
 sawdust. 
 
 Hand-pruning has been made easier 
 by the use of air, electric or hydraulic 
 power equipment (fig. 10). Each has its 
 advantage and disadvantage. Air 
 equipment is lighter than electric or 
 hydraulic, but air saws have less 
 torque. 
 
 Power tools used in conjunction 
 with mobil platforms to position the 
 pruner in the tree, have increased the 
 speed and efficiency of pruning. Sev- 
 eral companies market specialized ma- 
 chines which can be operated by one 
 man. In some cases, growers have built 
 their own pruning platforms, mounted 
 
 [14] 
 
Fig. 1 1 . Mechanical hedger. (Courtesy Kimball Toppers. 
 
 on a tractor or other vehicle, which 
 can be moved through the orchard. 
 
 Hedging machines (fig. 11) consist 
 of a row of power-driven circular saws 
 on a vertical boom which is mounted 
 on a self-propelled or tractor-pulled 
 chassis. Topping machines consist of 
 a horizontal arm of circular saws or, 
 in some cases, a rotating cross arm 
 with independently rotating saws at 
 the end of each arm. The cross arms 
 rotate as the machine moves down the 
 row, and the combined action of the 
 rotating arms, plus the rotating saw 
 
 blades serve to throw the cut branches 
 out of the top of the tree into the row. 
 For lighter wood, hedging and top- 
 ping machines are sometimes con- 
 structed using a heavy sickle bar 
 mower as the cutting mechanism. 
 
 Mechanical hedgers and toppers are 
 usually custom built by commercial 
 operators who contract to hedge and 
 top trees on a per-acre basis. 
 
 Brush left in the orchard is usually 
 windrowed and run through a cutter 
 or chopper. The residue is spread over 
 the ground to serve as a moisture-re- 
 taining mulch. 
 
 [15] 
 
ACKNOWLEDGMENTS 
 
 The authors are indebted to Dr. W. P. Bitters, Department of Plant Sciences, 
 University of California, Riverside, for a general review and suggestions in the 
 manuscript. 
 
 
 10m-4,'74(R6622L)PAD 
 
 "