What is biennial bearing in fruit trees?

What is alternate bearing in fruit trees?

This phenomenon is defined as irregularity of a tree’s production from one year to the next. In other words, fruit trees produce an abundance of fruit one year and almost no fruit the next.

This phenomenon is a major problem for professional fruit producers, who see the price of their harvests fluctuate greatly from year to year. For the sceptical amateur gardener, it often raises many questions.

This major orchard problem mainly affects high-stem orchards (fruit trees whose graft point is at 180-200 cm in height) and especially species known as pome or stone : Apple trees, Pear trees, Cherry trees and Apricot trees.

In warmer Mediterranean-type climates, olive trees and certain citrus trees are also susceptible to alternate bearing.

Apple trees and pear trees are prone to alternate bearing, as are apricot trees

What are the causes?

There are both external (climatic) factors and intrinsic factors (tree genetics, species, variety, rootstock and age) that explain alternate bearing in fruit trees.

Climate

As any good gardener knows, one year’s climate is never the same as another’s. Productivity of a fruit tree depends on climatic conditions during flowering. If, unfortunately, a succession of rain and frosts were to occur during the tree’s flowering, virtually the whole harvest would be lost that year. This can start a cycle of alternate bearing because your tree will instinctively (to reproduce) deploy a profusion of flowers the following season to compensate for its losses. This will induce an overabundant fruit set that will inevitably lead to reduced fruiting the year after, and so on… Alternate bearing can therefore occur because of an adverse climatic event.

Droughts that have affected our temperate climates for years can also trigger an alternate-bearing cycle. Indeed, fruit trees are then deprived of water and certain mineral elements, causing fruit abortion. Thus begins the vicious cycle of alternate bearing.

Factors intrinsic to trees (endogenous)

When a tree is fruiting, the seeds forming inside the fruits produce hormones (called gibberellins) that inhibit formation of fruit buds (spurs) for the following year. This competition between fruit formation and vegetative differentiation of buds is the principal cause of production alternation in orchards.

When fruits are forming, they also draw on large amounts of sap and mineral elements to develop, which leads to reduced formation of fruit buds for the following spring.

Frost, as much as drought, particularly affects fruit production (here a pear tree)

How can I limit the phenomenon?

Several preventive measures exist for this phenomenon, but it remains very difficult to counter completely, even for professionals.

Here are the best ways to limit alternate bearing:

• Choose varieties least susceptible to this phenomenon by asking advice from your nursery.
• Thin surplus fruits in years of “overproduction” : After flowers have been pollinated, assess the fruit load of your tree. If your tree seems to carry too many fruits, shake some branches and remove fruits that are too crowded together. This will help obtain fruits of good size and encourage your tree to form fruit buds (fruiting spurs) for the following year. This operation is generally carried out in June.
• Ensure tree fertility and water supply : Make sure soil is well manured (amended) with organic matter beneath the tree and that it suffers as little as possible from summer droughts.
• Remember to prune your trees (at the appropriate time) so the crown of your tree is open and well lit. This will stimulate production of future fruit buds. Overall, we train the tree into a conical rather than rounded shape to improve sun exposure.
• Treat cryptogamous diseases affecting the foliage that could impair production.
• Limit growth of overly vigorous shoots and bend shoots to encourage formation of flower buds.
• Fight late frosts… this is almost impossible for the home gardener, but professionals have a whole range of tricks to raise temperature during late-spring frosts (braziers, warm-air blowers, helicopters flying above the orchard…).
• If your tree does not flower much, do as much landscaping as possible in your garden to attract pollinators : sow melliferous plants, put up insect hotels, leave a pile of dead wood near your tree, adopt or sponsor a hive…

Thinning of fruits (here on an apple tree) is carried out in June when fruits are about 2 cm in diameter