Mediterranean Garden Society
Mediterranean Garden Society
Pruning Olive Trees
The photograph at the top of this page shows a walk with ornamental olives in Southern California (Photo Virginia Paca)
Pruning Olive Trees
From The Mediterranean Garden No 34 July 2003
Training and pruning olives
Training and pruning olives by Dr. Peter A. Roussos, Laboratory of Pomology, Agricultural University of Athens. (Dr Roussos presented a workshop on olive pruning at the MGS Symposium: The Dry Garden – Practice and Philosophy, Athens, 2007.)
Pruning Olive Trees
by Brian Chatterton
From The Mediterranean Garden No 34 July 2003
Art or science?
Most books written on olives in English describe pruning as a mystery buried deep in ancient folklore. There are exceptions such as Gucci and Cantini and our own book, but the majority have failed to understand the basic principles. Books in Italian (for example Del Fabro) are more practical and less mystical. A good pruner of olive trees can be compared to an artist where talent and technique are moulded together. The fact that a great artist may be a poor teacher of perspective should not disguise the fact that perspective is a technique that can be learnt. Similarly with olives. The techniques can be learned (the master pruner may not be the best teacher) and a reasonably competent job done.
Why prune?
Most readers will have only a few trees or perhaps a small grove and will pick their olives by hand or use simple hand-held machines to speed up the task. In either case a good density of fruit on each branch is required for productive picking. An unpruned tree will have olives scattered in groups of one or two all over an untidy bush. Such trees are costly and frustrating to pick. Even if you are picking with your own labour and the unpaid help of friends, efficient picking is important. Slow work is most disheartening. A primary objective of pruning is to produce dense clusters that can be stripped off the tree in great showers.
It is inevitable that ladders will be needed for mature trees but good pruning will prevent the trees becoming excessively tall and difficult to pick. In the early days of the New Zealand olive industry when pruning was rudimentary, one grower ended up employing the local fire brigade to pick his tall trees.
There has been considerable scientific research conducted on every aspect of olive growing and oil production. One of the important facts to emerge is that the olive fruit requires strong sunlight at every stage from fruit set to oil production. Olive flowers that are in deep shade will not set in large numbers. Those that do will not produce good levels of oil. Pruning is therefore needed to reduce the density of the foliage and allow sunlight to penetrate into every part of the olive tree. Our pruning teacher from the University of Perugia suggested that every olive should be in direct sunlight for at least some part of the day. This objective is compatible with the need to produce trees that are convenient to pick. By reducing the density of the foliage one reduces the tendency of the tree to race up and out in a desperate search for more light.
Alternate cropping.
Olives are not the only tree crop to produce alternate heavy and light crops. Apples are as bad. The apple tree naturally produces a large crop of small apples one year and a small crop of large ones the next. The supermarket-driven demand for mediocrity in all things forces the apple grower to control the trees and produce a medium crop of medium apples. The mechanism is similar in olives. The tree produces a large amount of vegetative growth one year and has little energy left over to form fruiting buds. The crop is light and the following year there is a surplus of energy to produce an abundant crop. Pruning will help even out the poor and bumper years. My experience has been that our unpruned trees produced bumper crops or nothing. Not a single olive. Now we prune every year and the variation has been reduced considerably but on individual trees it can still be double one year compared to another. The major problem occurs when some climatic event such as a severe frost puts all the trees in the grove into the same phase. Our last bad frost was in 1995. Now most of our trees have broken free of the alternate cropping pattern imposed on them by frost damage. Last year a very poor crop in one part of our grove was compensated for by an excellent crop in another part.
Tree shapes
There are many different shapes for olive trees throughout the Mediterranean region but all the diverse shapes can be simplified down to two. The mono-conical and the poly-conical or to use less mathematical language – the Christmas tree and the vase or wine glass. The Christmas tree is a new idea that has no traditional basis in the Mediterranean. It was invented for mechanical picking using the shakers first developed in California. More recent scientific research conducted in Italy has shown that the Christmas tree shape is no better than the vase for mechanical picking. The theory was that the shorter distance between the main trunk and the fruit bearing branches would transmit a stronger shaking force. Actual experimental work has shown there is no difference in picking efficiency. The Christmas tree is more difficult to manage. Some growers in our comune are converting their Christmas trees to vases.
The vase in various forms is by far the most popular shape throughout the Mediterranean and the most practical for hand or semi-mechanical picking. I would recommend it for growers with a few trees or a small grove.
There is a third shape that is rarely found in the classic olive books (Vallerini is an exception) which is the recovered frosted tree. In central Italy and other cold regions of the Mediterranean all the growth of the olive tree is frosted off above ground at roughly thirty-year intervals. This is when a particularly severe Tramontana blows down from Siberia. Suckers shoot from below ground and three or four are selected as new trunks. They form a ring of new trunks around the stump of the old stem of the previous wineglass-shaped tree. If the tree is frosted again these multiple trunks die and again suckers are selected to form new stems. Each time a severe frost occurs the ring of multiple stems moves further and further out leaving a larger dead stump in the centre.
Young trees
Most small growers will purchase one or two year old trees to establish their grove. These are roughly a metre high, are sold in a small pot and cost about Euro 5 each. These trees will need to be shaped. It is possible to buy larger trees. Trees up to fifty years old are available from our local nursery but they cost over Euro 500. Even a few additional years of growth can be expensive. The price approximately doubles for each additional year and with each re-potting. These older trees are shaped.
The pruning of young trees follows two contradictory principles that must be balanced. The first is to shape the tree into the form that, except for frost damage, will be its structure for life. The other is to allow the young tree to grow and build energy reserves. An old tree can be chopped and hacked with impunity and will rebound with enormous vigour from the energy reserves in the roots and trunk. The young tree will not. A determination to form the perfect shape by excessive pruning will weaken the young tree and stunt its growth for a number of years. Achieving the right balance is part of the realm of art and talent that distinguishes the good pruner from the also-ran.
Cocktail glasses and dry martinis
The shape of the olive tree is loosely referred to as a vase or wine glass but it is necessary to be more precise than this. It is actually one of those silly cocktail glasses that were common in Hollywood movies of the nineteen-thirties, when smart people drank Dry Martinis from little glasses containing rubber green olives on a stick. These glasses were open at an angle of ninety degrees. It is certainly not the shape of a Champagne flute. The whole point of the wine glass shape is to let in the light and allow the sun to shine on the fruit during the middle of the day from the inside through the hollow centre. A flute glass will not allow as much penetration of light into a mature tree as the wide-mouthed glass.
The young tree purchased from the nursery has a central stem and a number of lateral branches. The objective is to select three or four of these laterals to grow up and out to form the structure of the glass. Then you need to find two more (a year or so later) on each of these for an eventual structure of four arms.
There is also some debate about the height of the crown – that is the top of the stem in our cocktail glass simile. As far as I can see, except for the aesthetics of having a nice row of even trees it does not matter much between 80 and 120 cm. If you intend to use a shaker for harvesting the trees when they are mature 80 cm is too short and a minimum of 100 cm is needed. Having selected your three or four arms from upward growing laterals there is no need to cut the others off. You only need to ensure they are growing out and down – not up, as I will explain later.
The arms that you have left will not grow into that perfect cocktail glass shape. Olive trees are never that obedient. More corrective action is needed. They will naturally shoot straight up into a flute glass shape. To push them out requires constant topping. The upward shoot is cut at a point where there is another growing up and out. The relentless upward growth is halted and replaced by another. The upward growth becomes a zigzag of up and out, up and out. As the trees start to crop the weight of the fruit will also bend the shoots outwards.
Finally, you may need to take direct action. I was at first very scornful of bamboo birdcages but I have adopted them as a practical solution to recalcitrant trees that refuse to grow in anything like the right direction. It is a simple task in a small grove to tie on lengths of bamboo canes as spacers to push the arms into the right positions.
Renovating old trees
If you have purchased a grove of mature trees your pruning task is completely different. If you are lucky you will have perfectly shaped trees that have been well cared for and you will only need to continue on the same track. It is more likely that the trees have been neglected and will need some remedial action. I recommend caution when it comes to reshaping the tree. Instead of the three or four arms growing out at a nice angle of forty-five degrees the original arms may have been allowed to split into two and then two again. Instead of three or four they rapidly turn into eight or more. Try to get them back by all means but not all at once or there will be nothing left of your tree. The other sign of neglect is excessive height. Here you will need to be more ruthless. Excessive height creates excessive shade and the lower parts of the tree will not produce fruit. You will be forced to use longer and longer ladders to pick the olives clustered round the top of the tree. The height must be reduced and the top shade removed to allow the bottom part to recover its productivity.
Routine pruning
After radical surgery with a chain saw you will need to understand routine or maintenance pruning. This starts with close observation of the tree at picking time. You can see that the fruit is borne on the hanging and lateral branches and not the verticals. You will also see that some of the hanging branches that bore fruit in earlier years have become exhausted and weak. From your observations at picking time your should be able to distinguish three types of growth on the olive tree. There are the strong vertical shoots that rarely produce a single olive. There are vigorous lateral shoots that are fruitful and there are the hanging shoots. The hanging shoots carry abundant fruit but a more careful examination shows that some are becoming tired and unproductive.
Step by step guide
Firstly do what I call a “clean up” of the tree. Take off all the suckers around base of the trunk and all the shoots that have sprouted from the main arms that are blocking the centre of the glass. Clean them off.
Now you start the hard part where more skill and judgment is required. Your aim is to leave a single up and out shoot on the top of each of your main arms. That is three or four on the ideal trees or more in the neglected tree that you are trying to knock back into shape. All the other verticals should be removed. The problem is to distinguish the verticals. One needs judgment and experience. Some go straight up. That is clear and simple. Others go out at forty-five degrees. These are laterals that will bend down under the weight of olives and produce abundant crops. In between there are all sorts of angles that you will need to make a judgement about whether to remove or leave.
Having removed the verticals which will be mostly around the top of the tree, you need to thin the hanging branches that have become weak. These will be towards the inside of the tree and it is often better to tackle them from inside the skirt of the tree. The growth of this fruiting wood is the mirror image of the upward growth. Whereas the upward growth was moderated into a zigzag path of out before up, the fruiting wood zigzags down. By cutting off the last zig or zag you will allow new lateral shoots above to replace the tired wood. Again judgement plays an important role. The dead wood on the inside is obviously pruned off. The vigorous laterals on the outside will produce abundant fruit. In between you must decide which should come off as weak and which are worth leaving for another season.
Stepping back
Finally step back from the pruned tree and look at it as a whole to judge the overall density. The ideal tree should consist of three or four main arms coming from a trunk about 80 to 120 cm above the ground. Off these main arms are the lateral branches that bear the crop. At the bottom of the tree the laterals are old. They were produced by the young tree. They have grown out from the arms in a series is zigzags with the lowest and weakest hanging branches being cut off every year. They form the base of the cone (hence the name poly-conical) around each arm and combine to form a skirt around the whole tree. If the laterals are long and spindly they are chasing the light, indicating that you have not cleaned out the centre sufficiently for light to penetrate the inside, or the overall density of the foliage is too great. More severe pruning is needed to open up the tree. As you move up each main structural arm the younger laterals are shorter. They are unlikely to shade each other at this early stage but density is still important as they collectively shade the growth below. It is important to keep the upper parts of the tree open to allow the lower parts to remain productive, as they are so much easier to pick.
How can you tell?
Folklore says that you prune the olive tree until it is open enough for a bird to fly through it. Hiding in the olive grove and watching the birds is not a very practical means of judging the overall density of the foliage. Science tells you that you prune until you have a leaf area index of three or four. Leaf area index is the ratio of the area of the leaves on the tree compared to the area of ground covered by the tree. Counting the leaves would take even longer than studying the flight paths of the birds. Neither indicator provides an easy rule of thumb for the inexperienced pruner. My advice is to observe other trees in your district and to be sensitive to the symptoms of under-pruning. It is most unlikely you will have the nerve to over-prune so under-pruning is the usual fault of the beginner.
Let there be light
Light and more light is the overwhelming principle of pruning. Letting the light into the tree improves fruit production. Letting in the light also keeps the tree under control. If the tree is dark and dense branches will race up and out to find more light.
© Brian Chatterton
Training and Pruning Olives
by Dr. Peter A. Roussos
Agricultural University of Athens, Greece
From the Proceedings of the MGS Symposium: Dry Gardening – Philosophy and Practice, Athens 2007
Pruning is rightly considered by many agronomists to be the major cultural practice in an orchard. By pruning, the grower adjusts the tree to the specific climatic and soil conditions of the area and increases the productivity of the orchard. The main aims of pruning are summarized below:
There are three main types of pruning:
1. Pruning during the early stages of the tree’s growth
The aim of this type of pruning is to develop a tree shape during the first years after planting that will facilitate all cultural practices (spraying, soil cultivation, irrigation, harvesting etc) and will enable the tree to best exploit the sunlight and rainfall occurring in the area of cultivation.
The most common shape for the olive tree is the “cup-shaped” tree or “free-cup”. To form this shape the newly planted one-year old trees are cut back to a height of approximately 60-80 cm above soil level. The main aim of such a practice is to force 2-4 side branches to develop around the axis of the tree, at a distance of 30-40cm from each other and at a height of approximately 40 (the first) to 80 cm from the ground. These branches should in future comprise the main branches (primary limbs) of the tree. On these branches new side shoots will be encouraged by cutting the old branches to a length of approximately 50cm, thus removing the so-called shoot tip dominance and making the lower buds sprout.
There are also many other tree shapes that are used around the world, some of which are shown below.
2. Pruning for fruiting
Olive trees produce fruit on the previous year’s branches. This means that in order for us to have fruit every year we must ensure adequate vegetative growth every year. Very vigorous shoots are not productive, as they are mostly full of vegetative buds. The aim of pruning is thus to induce branches that will bear fruit by exposing them to the light and maintaining a vigorous and active fruiting zone.
The olive tree produces fruit mainly at the periphery and top of the canopy. This is because these parts of the tree are fully exposed to sunlight and become fertile. Based on this fact and on the aims analysed above, pruning for fruiting should consist of the removal of any part that shades other younger parts of the tree (photo 5). In this photo we can see that the branch which now has fruit on it will be cut down during the following year in order for the upper branch to bear fruit (due to the favourable lighting conditions), which would eventually lie over the older branch and shade it, thus making it non-fruiting. In this way the branch is not led away from the central axis of the tree, as can be seen in image 6.
When a thick branch is to be cut off we must take great care to avoid tearing the bark of the remaining branch. This is usually done by making a total of three cuts, as it is clearly shown in Figure 1. The first cut is made from the lower side of the branch to the middle of it, some centimetres away from the point where we want to cut the branch. The second cut is made a few centimetres away from the first cut and usually before the completion of this cut; due to the weight of the branch, it will fall, tearing the bark of the remaining branch to the point where the first cut was made. We can then easily cut off the small remaining part, to the point desired.
3. Rejuvenation pruning
The main characteristic of the olive tree is its longevity, resulting from its ability to produce new shoots from nearly every part of the wood, thus making it possible to renovate senescent trees or those that have been damaged by frost or fire. This type of pruning is consists of cutting the tree at the main branches or even at its trunk. The most significant practice, however, is to return during the next months and remove by hand (when they are still young) the shoots that will be of no use to us. We should not wait until the following year to give the tree the shape we desire (pruning for shaping the tree), as we would very likely have wasted almost an entire year before the beginning of fruit production. The new tree enters its fruiting period after 3-5 years, depending on the cultural practices used.
After any pruning or cutting it is advisable to cover the wounds with wound-sealing pastes and to spray the trees with a copper-based fungicide, in order to prevent the tree from developing bacterial or fungal infections. The pruning of olive trees can be done during the period between autumn and winter. It is generally performed after the harvest, but we should wait until the period of heavy rains and frost has passed in order to prevent infections. We should never prune on a rainy day, as this will probably spread bacteria to the cut surfaces or to internal wounds invisible to the naked eye, resulting in bacterial canker of the living tree (for those varieties susceptible to bacterial canker).
Irrigation and weed control
Irrigation and weed control of olives grown in gardens is usually a simple practice and there is nothing special to be noted. Nevertheless, we should know that the critical phases for water stress in the olive tree are principally the periods before and after flowering (during fruit set), when drought can result in a significant reduction in the flowering and thus the fruit set, leading to low production.
© Petros Roussos
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