The olive tree (Olea europaea) is a long-lived evergreen plant that is widespread in the Mediterranean Basin. Although cultivated as a tree, it has a naturally bushy growth habit. It is hardy and adaptable to a wide range of climates and soils, tolerating heat (even above 40°C) and surviving even on infertile ground.
Despite being hardy and adaptable, it needs care to avoid disease, fruit drop, alternate bearing (succession of high-yield and low-yield years) and so on, particularly if you grow olive trees not so much as ornamental plants, but to produce oil or table olives. In this article we provide an overview of how to take care of olive trees using some basic interventions: fertilisation, pruning and protection against parasites and diseases.
Fertilisation of olive trees
Despite being frugal plants, olive trees need fertilisation to replace the nutrients they absorb from the soil. Basal dressing with mature manure or compost is the first step to ensuring a good supply of nutrients. It involves preparing the soil with a rotary tiller or other agricultural machine, before planting the olive tree in a hole, mixing fertiliser with the excavated earth and then using that mixture to fill in the hole. This nutritional mix should be supplemented once a year (usually in autumn) by spreading compost, manure or pelleted manure under the entire canopy of the tree. Keep in mind that even pruning waste, shredded onsite using a flail mower, can help to fertilise olive groves. The same goes for clippings mown on grass-covered olive groves.
Among its many benefits, grassing helps to keep the soil of your olive grove adequately nourished by supplying organic matter and making nutrients more available. It’s a soil management system that can be used as an alternative to tillage or alongside it, depending on the context. Whether they are allowed to develop naturally or planted with specific species, grassed surfaces must in any case be mowed regularly using, for example, a flail mower, garden tractor or brushcutter, and the grass clippings left where they are to decompose.
Grassing is a widespread agronomic technique that is also applied in vineyards and orchards. Here we offer you an insight into how to plant an orchard.
Olive tree pruning
Pruning olive trees is an essential task, not only if you grow olives for a living, but also if you have a small olive grove or an individual ornamental specimen in your garden. These are the main aims of pruning, which differ depending on the age of the plant:
To make young olive trees assume the desired canopy form, if you want to cultivate them as a tree: vase (the most common form for olive growing in Italy), globe, monocone, free form, etc. This type of pruning is called training and is done in the first 3-4 years after the olive tree is planted.
To maintain the shape of the plants; to ensure a good quality harvest every year; to ensure that the foliage has good access to sunlight and air circulation, to prevent parasites and diseases; and to facilitate working in the olive grove: in this case it is called production pruning and is performed on mature and productive olive trees.
To extend the productive life of older plants and delay their senescence.
If an olive tree is left to grow unchecked without pruning, epicormic shoots (vigorous vertical shoots in the inner part of the canopy) will start to outgrow the rest of the foliage. The plant will be more prone to disease (due to reduced sun exposure and air circulation further down the canopy) and will only produce fruit every other year.
When should you prune olive trees? Training should be carried out in spring/summer. Production pruning should be done in winter/early spring, when the weather becomes mild and there is no longer any danger of frost, but before flowering, i.e. between January and March/April, depending on the area where you live. In the summer (July/August), you can limit pruning to the removal of epicormic shoots and suckers (on olive trees, as with other plants, suckers are vigorous shoots that grow at the base of the trunk).
Pruning in moderation allows the plants to maintain the right balance, both between development of the foliage and roots, and between vegetative activity and fruit production. On the contrary, drastic pruning of olive trees causes suckers and epicormic shoots to proliferate, resulting in decreased fruit production. Balanced pruning means cutting a maximum of 20-30% of the foliage each year using hand tools, power tools (such as a pruning chainsaw) or pneumatic tools (such as a pruner), whether fixed-length or telescoping, the latter enabling you to work from the ground without a ladder.
If you can't leave the pruning debris on the ground or compost it, here's how to dispose of your green waste.
Olive tree diseases and other problems
Even if your olive tree is disease-free, it may be affected by fungal and bacterial pathogens. The main diseases of olive trees caused by fungi are peacock spot or cycloconium (one of the most serious infections), cercosporiosis, olive leprosy and sooty mould. You can eliminate peacock spot by applying copper-based treatments with a mistblower. The same treatments are effective against leprosy and cercosporiosis. Sooty mould must be treated by targeting scale insects and aphids, which secrete honeydew on which the fungal disease feeds.
Indeed, some insects can be a direct and indirect cause of damage to olive groves. Examples are the black scale, filippia follicularis, the olive fruit fly and the olive moth.
Then there is olive knot, which is a bacteriosis carried mainly by the olive fly. There is no effective treatment against olive knot. You can prevent it through careful pruning, removal of afflicted twigs, disinfection of cuts and pruning tools, protection of pruning wounds and by targeting the olive fruit fly itself.
In general you can prevent disease and insect infestation by adopting a number of precautions, such as: pruning to thin out foliage, protecting pruning cuts, limiting the use of nitrogen-rich fertiliser to avoid excessive foliage growth, and preventing surface water accumulation.