Tree Size Control Hedging and Topping Citrus in High
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10. Passos, O. S., A. P. Cunha Sobrinho, Y. S. Coelho and E. M. Citrus Symp. 1:401-406. Rodrigues. 1977. Behavior of orange trees under three spacings in 13. Sites, J. W. and H. J. Reitz. 1(949. The variation in individual Va- the state of Bahia, Brazil. Proc. Int. Soc. Citriculture 1:169-171. lencia oranges from different location of the tree as a guide to 11. Phillips, R. L. 1969. Performance of closely spaced trees. Proc sampling methods and spot-picking for quality I. Soluble solids in Fla. State Hort. Soc. 81:48-51. the juice. Proc. Amer. Soc. Hort. Sci. 54:1-10. 12. 1969. Dwarfing rootstocks for citrus. Proc. 1st Intl. Proc. Fla. State Hort. Soc. 91:43-46. 1978. TREE SIZE CONTROL HEDGING AND TOPPING CITRUS IN HIGH-DENSITY PLANTINGS1 R. L. Phillips prunes the sides and tops in straight lines, resulting in flat Fruit Crops Department, IFAS, surfaces. New growth is stimulated in relation to the amount University of Florida, Gainesville, FL 32611 removed, being greatest where the most severe cuts are made. Pruned trees, therefore, tend to resume their natural shape. Additional index words, pruning, spacing, size control. Prunning involves both mass removal of vegetative growth by hedging and topping and selective removal by hand-held equipment. Both methods can be effectively Abstract. Proper control of vegetative growth is essential utilized and have their place in controlling tree size and in high density plantings for maintaining maximum fruit shape. Hedging and topping are forms of mass heading bearing surface, obtaining high quality fruit and facilitating back which removes terminal portions of branches, destroy cultural and harvesting operations. Pruning vigorous trees ing apical dominance and stimulating lateral bud breaks. to contain them within an allotted space in a high density This has a tendency to produce a more bushy, compact tree. planting stimulates excessive vegetative growth at the ex Selective pruning may be helpful by removing excessively pense of fruiting wood. Pruning should be started early in vigorous shoots at their base to prevent their regrowth. The such plantings with maintenance pruning at intervals which larger the shoot, the more vigorous the regrowth would be. would avoid severe cutting. Slow growing trees respond Upright growth is generally more vigorous and less fruitful more favorably to pruning and can be maintained at a than is horizontal growth. Upright shoots may be removed given size and shape without sacrificing yield. Optimum and horizontal growth favored by selective pruning. tree dimensions of pruned trees are based upon interrela The importance of sunlight in the production of high tionships among width, foliage depth, height, hedging angle yields of good quality citrus fruit cannot be overstressed. The and middle width which expose the maximum amount of amount of light intercepted by the leaves determines pro foliage to adequate sunlight. duction potential. Light provides the energy for photo synthesis which produces the basic foods upon which the High-density plantings of small trees, designed with their trees live and bear fruit. This process is carried out mostly foliage positioned for maximum fruit yield and for in the leaves which must be exposed to sufficient sunlight efficiency in cultural and harvesting operations, may be the for it to take place. Few fruit are produced where sunlight answer for highly profitable citrus groves of the future. How is inadequate and branches die when light is insufficient to ever, the advantage of high initial fruit yields will be short maintain them. A reduction in light also leads to a delay lived if proper control of vegetative growth is not main in fruit maturity and poor color development. Therefore, tained. High-density plantings have been quite successful in tree size, their shape and the distance between their sides the apple industry where dwarfing rootstocks and inter- should be adjusted to expose the maximum amount of stocks, together with various tree training programs, have foliage to direct sunlight. been used to control and direct vegetative growth (2, 3, 7). The quantity of light required for fruit set and Most high-density citrus plantings in Florida have not been maintenance of bearing foliage is not specifically known for successful for very long because the trees were too vigorous citrus. Light intensity within the tree canopy decreases for their allotted spaces. The trees soon competed with drastically with the distance from the outer periphery be each other for light, water and nutrients as they increased cause of shading by outer leaves (7). The depth of fruit in size. Crowded conditions resulted in loss of lower foliage, bearing foliage depends upon the penetration of adequate decreased fruit yield and quality and interference with light into the tree. Light which is transmitted through the cultural and harvesting operations. Pruning, to alleviate leaves also changes in quality, making it still less effective in crowding, often resulted in stimulation of excessive vegeta photosynthesis. tive growth at the expense of desirable fruiting wood. The High fruit yields have a tendency to reduce vegetative success of high-density plantings will depend upon main growth. It is very important for trees in high-density plant taining the trees at an appropriate size and shape without ings to begin producing good fruit yields before crowding sacrificing yield potential. occurs and pruning is required. Otherwise, vegetative growth control will be more difficult and poor fruit yields will Some Pruning Principles probably result. The balance between tree growth and Most citrus trees tend to develop a hemispheroidal shape fruitfulness appears to depend to some extent upon a re when left to grow natraaWy. Hedging or topping by machine lationship between carbohydrates and nitrogenous com pounds (8). Moderate growth and high yields occur when iFlorida Agricultural Experiment Stations Journal Series No. 1525. both are adequate, while a tree low in carbohydrates and Proc. Fla. State Hort. Soc. 91: 1978. 43 high in nitrogen tends to produce vigorous vegetative growth received more light because of wider middles. Studies with at the expense of fruit production. Therefore, a reduction in 'Valencia' orange did not produce results that were nitrogen application may be useful for controlling tree vigor particularly promising but, if the trees had been less vigorous before or after any substantial removal of foliage. However, and less pruning had been required, the outcome might have unthrifty and eventually unproductive trees could result if been more favorable. A problem with a vertical hedge of this practice is abused. vigorous trees is that the upper portion grows more vigorously and soon shades the lower portion. Therefore, it Tree Size and Shape Control would be desirable to hedge at an angle which would delay this effect. Pruning practices are largely directed by tree spacing 'Hamlin' orange trees, spaced 16' x 12.5' (4.9 m x and vigor. Slow growing trees generally perform best in 3.8 m), were hedged at angles of 5°, 10°, 15° and 20° closely spaced plantings and they respond much more from vertical (12). Longer shoot growth was stimulated with favorably to pruning when it is required. Unfortunately, the 2 greater angles. There were no differences in average most citrus cultivars possess greater inherent vigor than is yield after 6 years. This planting averaged 591 boxes per desired for high-density plantings. Pruning vigorous trees in acre (59.6 kg/ha) in the last half of the study. No topping a closely spaced planting can lead to a perpetual problem of was necessary with the 2 greater angles since the hedged excessive regrowth at the expense of fruit production. The sides came together at an acceptable height. Also, a greater more vigorous the trees and the closer the spacing, the sooner proportion of the fruit wras located where it could be har pruning should be started and the more often it should be vested from the ground. done so that only light cutting is necessary and crop re Many groves have been planted with close spacings duction is minimized. between trees within the row and standard spacings be Pruning will eventually be required for the control of tween rows. This spacing arrangement allows standard trees tree size and shape in high-density plantings. Optimum tree to grow for many years before hedging is required to main dimensions are based upon interrelationships among width, tain middle widths adequate to accommodate grove equip height, hedging angle, depth of foliage and width of spaces ment and to ensure adequate light conditions on the hedged between trees. Optimum tree width increases as the depth of sides. The sides between the trees are gradually lost and bearing foliage increases. Most citrus fruit is borne in the hedgerows result as the trees within the rows grow together. outside 3 feet (0.9 m) of foliage where light is adequate for Whether or not it is to the grower's advantage to hedge fruit bud formation while the inside area is occupied mainly between these trees depends on the need, tree spacing and by supporting structure and is mostly wasted space as far as tree vigor. Solid hedgerows have some advantages but they fruit production is concerned (14). Theoretically, the great present an obstacle to worker movement between rows and est amount of fruit bearing volume per unit area could be there are no spaces between trees for fruit containers. attained with narrow hedges about 6 feet (1.8 m) thick Attempts to maintain vigorous trees as individuals by cross- with their height and distance between rows adjusted to hedging are generally impractical when in-row spacing is allow sufficient light to fall upon their sides. Optimum very close. A compromise, with hedging at less frequent tree height depends upon width of the middles, the hedging intervals across the rows, may be more practical.