What is alternate bearing and how does it affect my olive tree?

Before we get straight to the point, let’s look at exactly how an olive tree bears fruit. Olive trees bear fruit on the lateral parts of the shoots that were created last year. The germination is created thanks to the storage substances (such as organic matter, carbohydrates, nitrogen, phosphorus and potassium) carried by the tree. This process begins in spring and ends in late autumn with the harvest of the fruit.

What is alternate bearing?

The term ‘alternate bearing’ is used in agronomy to describe the phenomenon in which a year of high production is followed by a year of low production. This phenomenon can be ‘total’, where the tree is fully fruiting in the first year and completely defoliated in the second, or ‘partial’, where the fruit set rates are 60-70 % in one year and 30-40 % in the next.

But what is the cause of this phenomenon?

This phenomenon is multifactorial and its causes have not yet been fully established. One of the main causes of alternate bearing is considered to be the imbalance created by the competition between germination and fruiting, in other words, in years when we have a lot of fruit, we have few shoots and vice versa. For example, let’s say that in 2023 there are few shoots, in 2024 there will be few fruits, as the former have to grow on the latter. In practice, this is because, as we have said, organic components, carbohydrates and nutrients such as phosphorus, potassium and especially nitrogen and boron, which play an important role in fruit setting, are not present in quantities sufficient to generate new production the following year. On the contrary, elements such as calcium and magnesium are, due to competition with the above, in abundance.

Still, weather conditions, as we are talking about trees, is a factor influencing the phenomenon. In particular, in years where there is not the necessary winter cold (temperatures below 16oC) that olive trees need, as we have explained in a previous article, the resulting fruiting will be low. Furthermore, when rainfall is reduced and accompanied by hot winds, flower drop, fruit drop and general dehydration of the tree will occur. This explains why dry olive groves are more susceptible to the phenomenon of alternate bearing.

The variety of olive tree also plays a role. Koroneiki, or the queen of the Greek varieties as it is considered, is the most resistant variety to alternate bearing. Its resistance to drought, its small fruit size and its rapid growth help it to produce a steady yield of fruit over the years (from 50 to 150 kg per tree).

How can my olive tree avoid alternate bearing?

From the following, it is easy to see that alternate bearing is a major challenge for farmers and olive tree owners in general. In order to reduce the uncertainty and to have a stable production every year, it is necessary to have a proper crop management program, which includes:

  • Pruning and thinning of fruits plays a decisive role in the alternate bearing of fruit. The crown of the tree must be evenly distributed and adequately illuminated in order for the olive tree to be robust. As a general rule, in years when fruit set is expected to be reduced, pruning should be more severe so that the olive tree is well lit and productive the following year. On the other hand, in years when production is expected to be high, pruning should be light so that the shoots that will bear fruit the following year are not removed from the tree.
  • In turn, thinning of the fruit has had positive results, since it promotes germination and results in a heavier fruit (better value on the table olive market). It should be noted that fruit thinning can be carried out either mechanically (by hand) or chemically (with the hormone naphthyloxacetic acid NAA).
  • Fertilization with phosphorus, potassium and especially nitrogen and boron can strengthen the upcoming germination, reducing the phenomenon. Specifically, research has shown that transfoliar spraying in the spring in years of low bloom helps to balance the tree. However, nitrogen fertilization should not be excessive in years when fruit set is high, because it will increase even more, to the detriment of the vegetation.
  • Irrigation should also be at a sufficient level to avoid the risk of fruit set and dehydration. Limiting any weeds will certainly help with this. Of course, anything goes, because if irrigation is overdone, the nitrogen available to the olive tree will be leached out, leading to flower drop and excessive germination.
  • Harvesting is recommended not to be done too early, as it works in a competitive way in the next fruiting season. Still, it is important that fruit picking be done by means that do not injure the tree and break the annual vegetation, as various raking machines do.
  • Implement an integrated pest and disease management program, particularly in early spring when populations are at their peak.
  • Lastly, infestations of insects, such as the olive fly, olive moth and respectively cycloconium and other olive tree fungi, reduce the fruit and, of course, its quality.