Citrus trees are a perennial crop. As long as they remain healthy, they can flower and fruit for years on end, sometimes for as long as twenty or thirty years. However, if the trees are not maintained at a proper size, the height and canopy shape of mature trees in a citrus orchard will not be uniform and the branches will be too crowded.
In a crowded orchard, diseases and pests can spread quickly. Fruit quality tends to be poor, and trees may not bear fruit every year. A proper training and pruning program is essential for the maintenance of a healthy, productive citrus orchard.
Objectives of Training and Pruning
There are three main objectives. The first is to increase the total effective leaf area, and promote photosynthesis by exposing the leaves to light and air. If branches are well spaced and properly oriented, they have plenty of space and light. This improves the efficiency of their water use, and also their conversion of available plant nutrients. In turn, this increases the yield and quality of the fruit.
Secondly, proper training and pruning of the tree keeps it in the right size, so that the grower can easily manage his orchard and tend his trees. It also increases the vigor of the tree, enhances its tolerance of various stresses, and helps maintain the most efficient balance between vegetative growth and fruiting.
Thirdly, by removing diseased or infested branches and exposing leaves to light and air, a good training and pruning program helps control pests and diseases in the citrus orchard.
Shoot Growth and Bearing Habits of Citrus Trees
General Characteristics of Growth
All cultivated citrus species are evergreen trees, with one exception (trifoliate orange: Poncirus trifoliata Raf.). Neglected trees tend to have overgrown tops and produce crowded shoots. In a few years, the canopy becomes very dense and grows into a dome. Branches in the interior may die from lack of sunlight. The tree will bear fruit only on the surface of the canopy, and will become progressively less productive.
In tropical and subtropical areas, new shoot growth occurs in spring, summer, autumn, and sometimes even in winter. The spring and summer shoots are the most important ones. They should develop properly, and not be too vigorous.
In a mature citrus tree, the bearing wood develops mainly from the spring and summer shoots. The spring shoots which sprout from the bearing woods are the most productive ones. Single flowers or inflorescences may develop from the shoot apex or axillary buds.
Tree Form and Training System
Citrus trees can grow very tall. They should be trained into a suitable shape, with an open center. There are several advantages if growers follow such a system. The trees are easy to manage, including spraying and harvesting the fruit. There is rapid growth of the canopy in young trees, which also bear fruit early. The trees are easy to prune, and the bearing canopy covers a large surface area.
Growers should select the correct time for their pruning. Since citrus trees are evergreens, they do not have a period of true dormancy. However, the metabolism of the tree is less active in the period after fruit harvest.
In countries with a cool winter, the metabolism is less active before the spring flush, because temperatures are lower and less water is available to the tree. In countries with a monsoon climate, trees may have a lower metabolism in the dry season.
It is this period of reduced metabolic activity which is the time to prune. Light pruning can also be conducted at other seasons to remove unwanted and overcrowded shoots.
Fig. 5 shows the different parts of the citrus tree. When the trees are being trained and pruned, growers must consider how these different parts should be distributed in space.
Pruning Principles and Procedures
Fruit should not be allowed to ripen until the tree is mature. If the tree sets fruit during its second and third year, the fruit should be removed while they are small and green.
Fruit thinning in young trees will promote the growth of the canopy. Fruit thinning in mature trees will improve the fruit quality. Trees which bear heavy crops tend to produce small fruit. Thinning will also correct the tendency found in citrus trees to bear a heavy crop of fruit only in alternate years, with few or no fruit during the year in between.
Don'T Remove Too Many Shoots
In general, growers should be careful never to remove more than 15% of the total shoots. They should study the structure of each tree before they prune it. Pruning should start at the top third scaffold branch, followed by the second and finally the first. For each scaffold, pruning should start from the secondary branch, and then the side shoots.
Undesirable Branches and Shoots
Diseased branches, or branches heavily infested with pests, should be removed. So should branches and shoots which are in the wrong position, or growing in the wrong direction. The undesirable branches and shoots of the citrus tree are shown in Fig. 6.
How to Make the Pruning Cut
Heading back and thinning out are the two basic types of pruning cut. Heading back will promote the growth of lower buds and branching (Fig. 7). Thinning removes the entire branch or shoot, to reduce the total number of the side shoots (Fig. 8). The branches and shoots which should remain after pruning are shown in Fig. 9. How the tree appears after pruning is shown in Fig. 10.
Rejuvenation of Old Trees
In an old citrus orchard, the trees may lose vigor. Their yields decline and their canopies become too large. This is the time they need to be rejuvenated (Fig. 11). The procedure is shown in Fig. 12, Fig. 13, Fig. 14 and Fig.15. It should be carried out over three or four years before it is completed. Fig. 16 shows the shape of the tree before and after rejuvenation.
There are many different cultivated citrus species, all of which have different growth and fruiting habits. Optimal light interception for the entire orchard is the first thing to be considered when choosing a training and pruning system. The second is the individual tree form and tree vigor.
Tree age is another important factor that should be taken into account, because the tree's response to pruning varies according to its age. Therefore, citrus growers have to recognize the characteristics of the different cultivars they are growing, in order to select the best training and pruning system for their orchards (see Fig. 17, Fig. 18 and Fig. 19).
Index of Images
Figure 1 First Year of Planting. after Planting, When the Leader Grows to a Height of 70 - 80 CM, It Back Cut to a Height of 30 - 40 CM Above the Ground, and Remove the Side Shoots
Figure 2 Second Year. Keep Branches No. 1, 2 and 3, Which Will Grow into Scaffold Branches. Pinch the Ends of Branches No. 4, 5 and 6, Which Should Be Cut off in the Next Two Years.
Figure 3 Third Year. Remove Branches No. 5 and 6.
Figure 4 Fourth Year. Remove Branch No. 4. Looking down on the Trees, the Angle between Scalffolds Should Be around 120o. the Vertical Distance between Scaffolds Should Be 20 - 30 CM.
Figure 5 Arrangement of Tree Parts of a Typical Open-Center Training System
Figure 6 Undesirable Branches and Shoots
Figure 7 Heading Back: Shorten Long Shoots, Side Branches, and Secondary Branches
Figure 8 Thinning Out
Figure 9 Branches and Shoots Remaining after Pruning
Figure 10 Pruning of Side Branches
Figure 11 Old Tree before Pruning.
Figure 12 The Tree Form after Pruning
Figure 13 The Short Heavy Lines Mark the Positions of the Cuts Made at the Second Winter-Spring Pruning
Figure 14 Tree Shape after the Second Year's Pruning. the Short, Heavy Lines Mark the Positions of the Cuts to Be Made the Following Year
Figure 15 Tree Shape after the Third Year's Pruning. the Short, Heavy Lines Mark the Position of the Cuts to Be Made the Following Year
Figure 16 Tree Shape before and after Rejuvenation Pruning
Figure 17 A Well-Shaped Citrus Tree
Figure 18 Showing Growers the Right Way to Prune
Figure 19 (Left) a Badly Pruned Tree, with Poor Fruit Set on the Lower Branches. the Upper Branches Have Almost No Fruit at All
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