Olive Tree Dormancy: The Role of Cold Weather in Fruit Production

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According to scientific studies, dormancy is essential for a bountiful olive harvest. If you’ve been wondering how to improve fruit production on olive trees, you may be interested in this. 

Here is some information on olive tree dormancy and why it may affect your yields.

Key Takeaways:

  • Olive trees need cold weather (5°C – 7°C) for 1-2 months for productive dormancy.
  • Dormancy in olive trees has three phases: para-dormancy, endo-dormancy, and eco-dormancy.
  • Cold temperatures during dormancy positively impact olive tree harvests and bud formation.
  • Olive trees require varying chilling hours for dormancy, affecting fruit production.
  • Climate and wind protection are crucial factors influencing olive tree chilling requirements.

What Is Olive Tree Dormancy?

olive tree in a dormancy state

Although olive trees are associated with sunny regions such as the Mediterranean, they need some cold to produce lots of fruit. This is where dormancy comes in.

Dormancy is a period of one to two months of cold weather during which olive trees rest before flowering and bearing fruit. This period is usually in winter when temperatures are between 5°C – 7°C. 

It is also called the chilling period and is measured in chilling units. These units are calculated as the hours necessary for an olive tree to rest for maximum productivity.

Olive trees go through dormancy from winter to early spring. During this time, nutrients are assimilated and stored in the olive tree for fruit growth through photosynthetic action. If the olive tree goes through a successful period of dormancy, it uses these nutrients to produce many fruits.

Phases of Olive Tree Dormancy

Olive tree dormancy is classified into three phases. These are para-dormancy, endo-dormancy/chilling, and eco-dormancy/forcing.

Dormancy phases explanation:

  • During para-dormancy, signals from the olive tree encourage buds to start developing slowly. This process is controlled by internal physiological factors.
  • In endo-dormancy, the arrest of dormancy is controlled by the bud as it lowers metabolism to slow down the growth of foliage and flowers. This is to fulfill chilling requirements and ensure the healthy development of floral buds and maximum olive yield after dormancy.
  • During eco dormancy, olive tree growth is inhibited by external factors such as temperature to further aid in healthy bud formation. 

These dormancy periods end when temperatures get higher, and olive trees resume growth with the axillary buds developing into fresh foliage and flowers.

Why Olive Trees Need Cold Weather For Fruit Production

olive tree in dormancy period

Although extreme cold weather can damage olive trees, scientific studies suggest that a period of cold temperatures during dormancy can positively affect an olive tree’s harvest.

Olive tree fruits develop from the flowers that come up from axillary buds. These buds emerge from the leaves of the previous year’s shoots to initiate future flower growth.

The growth of axillary buds usually starts in autumn and through to the winter months. Therefore, olive production may drop due to poor axillary bud formation if winter temperatures do not decrease enough for proper dormancy.

The duration of olive tree dormancy gives the tree enough time to form viable axillary buds. These buds remain on the branches until temperatures get warmer when they bloom into flowers and continue to fruit formation.

Dormancy also benefits olive trees in other ways:

  • It reduces the population of olive fruit flies, as their vitality is drastically reduced due to low temperatures.
  • It prevents fungal diseases such as Peacock spots disease-causing fungi contained during the cold weather.
  • The freezing of water due to extreme cold temperatures causes micro-cracks in the soil that improve aeration, which benefits the olive tree root systems.
  • Once temperatures increase after dormancy, the thawed water becomes an excellent reserve for olive trees to thrive.

Chilling Hours

Chilling hours are the hours it takes for an olive tree variety to undergo a period of successful dormancy. This is when olive trees stop growing, renew their leaves, and store energy for future fruit production.

chilling hours map USA
Source: https://mrcc.purdue.edu/VIP/indexChillHours.html

The amount of cold hours an olive tree goes through during this chilling time must meet the minimum requirements for the olive tree variety.

If this does not occur, no flower buds will appear on the olive trees. Instead, only branches or leaves will grow, resulting in a low olive fruit harvest.

Early flowering cultivars that thrive in regions with short winters, such as the Arbequina variety, require a minimum of 300 hours of dormancy to produce mature buds for optimal fruit production.

Tardy/ late flowering olive trees such as the Leccino and Frantoio olive varieties need long winters to get the over 600 hours of dormancy necessary for optimal maturation of flower buds.

There are also low-chill unit olive trees that require even fewer dormancy hours than the 300 – 600 hours mentioned above. An example is the Chemlali variety, which only needs 125 chill units.

Factors Influencing Chilling Requirements In Olive Trees

The factors influencing chilling requirements for olive trees surround climate. The intensity and length of cold temperatures during the olive tree dormancy period depend on the climatic conditions where they are planted.

Those with low chilling requirements, also known as early flowering cultivars, are ideal for regions with short winters, such as Catalonia in Spain.

Places with long winters, such as Central Italy, require olive tree varieties with high chilling requirements called tardy/ late flowering olive tree varieties.

While light winds are good for olive pollination, harsh winds may damage olive trees during dormancy. Therefore, for olive trees to have a successful dormancy period, they should be situated in areas with suitable wind protection.

This is a West or South facing direction where they are safe from easterly and northern winds.

Farmers wrap olive trees in horticultural fleece when the winds are too harsh to prevent freezing. This keeps the olive trees a degree warmer than the outside temperatures, preventing frost issues during dormancy.

How To Monitor Olive Tree Dormancy

Essential factors to monitor when maximizing olive tree yields are;

  • The onset of axillary bud dormancy
  • The length of the dormancy
  • The dormancy release period

Many olive-producing countries have a chill unit tracking system where farmers can get accurate temperature readings and computed chill unit count. This helps in determining the necessary dormancy period for different olive tree varieties.

These chill unit tracking systems also help determine the ideal dormancy release temperatures for reproductive budburst.

For example, 20°C is a suitable temperature for olive trees to produce axillary buds. However, higher temperatures can inhibit this vital part of olive production.

Farmers should monitor their olive tree dormancy using chill unit tracking systems to avoid problems like frost that can compromise their harvest.

The Impact of Climate Change on Olive Tree Dormancy

The high temperatures associated with global warming interfere with the olive tree fruiting cycles.

Olive shoots require average temperatures of 12°C to 20°C for optimal growth and development. Unfortunately, global warming effects can push these temperatures to 35°C.

This increase in temperatures extends olive fruit maturation, lowers the quality of olive fruits, and delays harvests.

These effects are due to the dryness and heat occasioned by global warming that increases temperatures, thereby causing warm winters.

Such warm winters reduce the set amount of chill hours necessary for olive trees to produce axillary buds during dormancy. This usually prevents olive trees from bearing enough fruit in the next season.

The shift in temperatures due to global warming can also cause a cycle of thawing and freezing during winter that destroys olive tree roots. This, too, can affect the productivity of olive trees.

Therefore, learning how to mitigate the effects of global warming on olive farms, especially during dormancy, is essential to prevent such problems.