Following are some basic concepts of pruning olive trees. For more in-depth information, we recommend the books by Riccardo Gucci and Claudio Cantini, by G. Steven Sibbett and Louise Ferguson, and the article and book by Paul Vossen, all listed in our Sources below.
What are the main reasons to prune mature olive trees?In mature trees, pruning is mainly required to renew the fruiting surface of the tree and achieve high yields, maintain vegetative growth of fruiting shoots, maintain the skeleton structure, contain tree size, favor light penetration and air circulation inside the canopy, permit control of pests and diseases, prevent aging of the canopy, and eliminate dead wood. Under certain circumstances, pruning can be required to alleviate the effect of abiotic stress, to re-form the canopy after damage by frosts and pests, to rejuvenate old or abandoned trees, and to adapt an obsolete training system to mechanical harvesting. In modern olive growing, the training system should permit easy movement of machinery in the orchard; little attention needs to be paid to specific tree shapes.
Pruning shears are used to cut shoots less than 1 inch (25mm) in diameter. Double-bladed shears are more suitable than single-bladed shears for cutting flexible shoots. Professional pruners prefer pruning shears with shock absorbers to reduce fatigue.
The hand saw is the most practical tool to cut shoots and branches up to 3 inches (76mm) in diameter in the internal part of the canopy where the vegetation is dense. Saws can have either rigid or folding blades. The best results are obtained with a rigid blade of at least 15 inches (0.38m) in length, especially for heavy work.
The use of a chain saw can reduce the time and cost of pruning. The chain saw must be light (to avoid fatigue) and robust. For large cuts (either to major branches or to the trunk), the chain saw must have at least 14 inches (0.35m) of free blade to be operated efficiently. Note that using a chain saw is dangerous. This equipment should only be used by pruners in good physical condition, standing on the ground or a stable platform, wearing a helmet, goggles, gloves and heavy-duty clothes for protection. Rest should be frequent and all precautions taken for the worker’s safety.
Pneumatic tools, both shears and saws, can be installed on poles to prune plants up to 138 inches (3.5m) in height without using ladders. A tractor and a compressor are generally required for every two pruning units. Two to four workers for each compressor unit are required for efficient operation of pneumatic tools, in order to justify the initial investment.
All tools should be kept sharp (to make clean cuts without tearing the bark) and clean. Blade sharpening often requires professional skills but many types of shears are sold with replaceable blades. Periodic cleaning of blades to remove wood particles can be done simultaneously with disinfection, using 70% ethanol. If blades are dipped in pesticide or copper solution to prevent spreading of diseases during pruning, they should be carefully rinsed with water and then dried with a cloth to avoid corrosion.
Pruning techniques vary depending on specific cultural conditions and social factors. The type of pruning must be adjusted in relation to plant age, training system, crop load, product use, environmental conditions, soil fertility, and farm structure. Evidently, there is not a single technique that can be recommended for all conditions. The most limiting factor, such as the availability of labor or the cost of pruning, becomes the main criterion for choosing between alternatives.
A few general rules, however, hold true in most conditions. According to Riccardo Gucci and Claudio Cantini, the main ones are:
- When in doubt, less pruning is better.
- Not all trees in a grove need to be pruned every year.
- Pruning should be adjusted to the age of the tree.
- It is best to proceed from top to bottom.
- Large cuts should be made before small ones.
- One main goal should be to correct differences in vigor between branches.
- Pruning should be rapid and simple.
- Cost is more important than appearance, except, of course for landscape trees.
- All cuts that can be delayed until the following year should be.
The Thinning Cut
The thinning cut consists in suppressing the whole shoot, or in reducing the length of the main axis by cutting close to a lateral shoot or branch, which then assumes the terminal role. Thinning cuts reduce the length of the branches and the overall volume of the canopy, and maintain fruit and foliage closer to the center of the plant. Correct thinning requires that the cut be directed to oppose the natural growth habit of the plant by leaving the more erect shoots in plants with pendulous habit and vice-versa.
The Elimination Cut
Entire shoots or branches are removed when fruiting is excessive, when there are too many competing shoots, and when sunlight cannot penetrate the canopy. The elimination cut reduces the competition between shoots without modifying the main axis. The remaining shoots and branches are selected with different orientations to avoid mutual shading and overlapping, and spatially separated along the main axis.
The cuts should be made close to the insertion point of the lateral branch, external to the branch collar and bark ridge. Wound repair may be delayed if the cut is made too close to the main axis or if a long stub is left. The delay in healing makes the exposed tissue a preferential point of entry and diffusion of pests and diseases. Slanting the cuts avoids accumulation of rainwater, which may infiltrate down inside the bark and cause rot.
TIME OF PRUNING
Pruning should be performed between the end of winter and flowering. Cutting stimulates metabolism and growth, which makes the plant tissue more susceptible to plant injury. In mild climates, with no spring frosts, pruning can be started in winter. Pruning before bud break is risky in cold climates, however, because of the high probability of frost that may damage the remaining tissue and delay wound repair. An advantage of pruning after bud break is that even the inexperienced grower is able to assess the number of flowers and the potential crop removed by pruning, whereas flower buds cannot be distinguished macroscopically from vegetative buds at or before bud break.
Waiting to prune until emergence of inflorescences is feasible in small orchards, but may be difficult to manage in large ones, where a longer period for pruning is necessary. Pruning should not be delayed until after full bloom, since it will remove tissues towards which nutrients and carbon reserves have already been remobilized, resulting in a net loss for the plant. Late pruning does not damage the plant but can reduce seasonal vegetative growth substantially.
Summer pruning is done during the growing season when the plant is actively growing. It is not common in cultivars used for oil and it is usually limited to the elimination of suckers and water sprouts.
The timing of pruning also influences the plant response. Removing shoots at bud break results in much more vigorous growth of the remaining shoots than if the same operation is performed at the beginning of the summer.
INTENSITY AND FREQUENCY OF PRUNING
Before going into details about the intensity and frequency of pruning, it is useful to note that the current tendency is to prune olive trees as little as possible. Concepts of minimum pruning should be applied in all possible cases to reduce costs substantially and simplify pruning management. These concepts can be summarized as follows:
- Prune only the trees that need it.
- Reduce the frequency of pruning.
- Adopt free-canopy systems.
- Use irrigation and fertilization to stimulate growth and sustain fruiting.
Note, however, that minimum pruning does not mean neglectful pruning.
The intensity of pruning should be adjusted by taking into account all the factors affecting plant vigor, including age, cultivar, crop load, soil fertility, water availability, and length of growing season. As a general rule, the greater the intensity of cutting, the stronger the vegetative response of the plant will be. Hence, pruning should be more severe on old trees and trees of low vigor than on young plants, or on trees growing in irrigated conditions and in fertile soils.
The intensity of pruning should also take the crop load into consideration. It is especially important because of the alternate bearing habit of the olive tree. In heavy cropping years, the growth of the tree is reduced, so pruning should be limited to the elimination of water sprouts and weak shoots. Alternatively, trees should be pruned more severely after years of low yields.
When trees are not pruned every year, the intensity should be increased.
Under most circumstances, olive trees are pruned each year. Annual pruning is strictly recommended when a rigid frame and a specific shape must be achieved. It is indispensable in table cultivars or when shoot growth is limited by external constraints, such as low soil fertility, long summer drought, short growing season, or old age of plants. In these cases, annual pruning renews the fruiting shoots and stimulates vegetative growth.
Less frequent pruning reduces pruning costs and the need for skilled labor as compared to other types of pruning. In this respect, olive trees for oil production are exceptional in tolerating not being pruned every year without yield losses.
The most critical factor in deciding how frequently to prune olive trees is the rate of the current year’s shoot growth. If active growth is maintained, pruning can be postponed until the following year. Pruning every two years or longer can be more easily implemented in irrigated orchards, in fertile soils, and with trees planted at wide spacing. A biennial frequency can be adopted in the majority of cultural conditions, but intervals longer than three or four years are not always suitable. Otherwise, yields decline markedly, and at the end of the four-year cycle, pruning will have to be drastic, with consequences on the vegetative-reproductive balance of the tree. Cultivars with an upright habit and those sensitive to foliage disease are less suitable for infrequent pruning because the excessively thick canopy and upright growth will make harvesting and pest control more difficult and time consuming.
Following is a description of some common training systems. The question of what is the best training system is often asked but does not have a single answer. The choice really depends on the characteristics and the objectives of the orchard, as well as cost and labor availability.
This is probably the most common training system in many olive-growing areas. The vase has a greater surface-to-volume ratio per tree than systems with a full canopy (e.g. globe, bush). There are several types of vase, including the cone, the inverted cone, the cylinder, a cylinder on top of an inverted cone, or multiple cones. The main characteristics are that there is a single trunk with three to five primary branches. The secondary branches are arranged in a symmetrical, regular way. There is a “window” in the central part of the canopy.
There are several advantages. The light can penetrate the interior of the canopy and is quite evenly distributed. This shape is suitable for different growth habits, for table olives, and for mechanical harvesting if the branches are kept relatively short and a rigid structure is formed. The disadvantages are that skilled labor is required for pruning and that pruning can be time consuming.
The globe is a system with a single trunk and a full canopy. The main difference with the vase is that the center of the canopy is occupied by either secondary branches or by the terminal part of the main axis. This shape is widely used in areas where plants grow vigorously and sunlight is high. The high-density foliage protects the bark for direct sun light. This shape is suitable for mechanical harvesting by trunk shakers and for different growth habits. The foliage can become excessively dense if pruning is neglected, however, and skilled labor is required for pruning.
The vasebush is a vase without a proper trunk, and with primary branches originating from the soil line or inserted on a short trunk. The secondary branches are arranged similarly to the vase configuration. The main advantage of this training system is that the trees are kept shorter than if trained into the vase or globe configuration, so that most operations can be done without a ladder. There is generally an early onset of production. This shape is appropriate for different growth habits. It is easily formed from suckers after cutting the plant at ground level. It is inadequate, however, for mechanical harvesting so must be reserved for hand picking or hand operated equipment. It is an excellent system for table olives.
The central leader, also called monocone or monocone vase, consists of a single trunk, free of any lateral shoot up to 1.0-1.2m (39 to 47") in height. Primary branches are numerous and arranged in elicoidal fashion along the central axis for maximum occupation of space and minimum overlapping. The primary branches are of decreasing length from the base to the top of the tree. The resulting shape is conical. It is suitable for mechanical harvesting by trunk shakers and for table olives. The visual appearance of the orchard is homogenous. The pruning costs are high, however, and pruning requires skilled labor. These trees tend to get high. There is no advantage in the onset of production, the yield, or the quality of the oil. This shape is not suitable for pendulous cultivars.
Single Trunk Free Canopy
Free canopy systems include all those requiring little or no pruning. Single trunk free canopy systems combine the features of a single trunk with the low cost of and flexibility in pruning. It is similar to the globe but primary branches are not necessarily regularly distributed along the main axis and minimum pruning concepts are applied to the whole canopy. The lateral branches are selected among those growing naturally on the main axis. The pruning costs are low. This shape is suitable for mechanical harvesting and for different growth habit. The appearance of the trees is not very homogenous in the orchard and experience is required to determine the intensity and frequency of pruning.
The bush is a free canopy system obtained with minimum pruning from the time it is planted. The canopy is allowed to grow as free as possible so that the final shape resembles that of naturally growing plants. The trunk is short or absent with lateral scaffolds that make it unsuitable for harvesting by shakers. Hardly any pruning is required during training, and only minimum pruning is done on mature trees. No skilled labor is required. The production onset is generally early and this shape is suitable for different growth habits. It is also easily converted to the vasebush or free vase form. The orchard appearance is not homogenous.
Riccardo Gucci, Claudio Cantini: Pruning and Training Systems for Modern Olive Growing.
G. Steven Sibbett and Louise Ferguson: Olive Production Manual, University of California.
Paul M. Vossen: Organic Olive Production Manual, University of California.
Paul M. Vossen: Ten Basics of When and How to Prune Fruit Trees