Olive dormancy: what stops growth, what chilling does, and the disagreement over whether dormancy exists at all

Savvas Pastopoulos, Agronomist MSc, Pastopoulos Agronomics, Neos Mylotopos, Pella, Greece

The olive keeps its leaves all winter, and that alone changes what the word dormancy means. In a deciduous tree the winter is a clean pause: the tree closes, counts chilling and opens. In the olive there is no such pause, and in recent years even the existence of a true dormancy in the olive has been questioned. This text examines what stops shoot growth in the autumn, what chilling actually does to the buds that will carry flowers, where the scientific disagreement currently stands, and what all of this means for a Greek olive grove.

1. What stops shoot growth in the autumn

In most woody species of the temperate zone the signal that stops growth is the shortening day, not the temperature. Apple, pear and a few other genera are the exception and stop with temperature. For the olive the question was answered by measurement in 2020, by a study with three strands in southern Spain [1].

In the first strand, young Arbequina trees were split into five treatments, so that the effect of temperature could be separated from that of daylength. The rate of appearance of new leaf pairs was measured. Growth ceased only where the temperature fell. The plants kept above 18 °C went on producing leaves normally, even when the day was short [1]. Photoperiod, in other words, controls nothing in the olive.

Two simple models for the date of growth cessation were derived from the same data. Both work on hourly temperatures and on a single counter, which starts at zero in early autumn and rises for as long as the cold lasts. When the counter reaches the value required, shoot extension ceases.

The first model counts only the hours during which the temperature is below a threshold, one unit per hour, and never subtracts anything. Fitting to the data gave a threshold of 14.4 °C and a requirement of 160 hours, with a root mean square error of 3.1 days [1]. The 160 hours correspond to roughly six full days of total time below the threshold. In the field they are spread over several weeks, because on autumn days only part of the twenty four hours, usually the night and the first morning hours, lies below 14.4 °C.

The second model adds a second term: every hour at a temperature above the threshold subtracts 0.32 of a unit from what has already accumulated. Fitting gave a threshold of 14.5 °C and the same 160 hours, with a root mean square error of 2.5 days [1]. This coefficient is the quantitative expression of reversibility: about three warm hours cancel one cold hour, so a warm spell in mid autumn can turn the counter back and postpone the cessation of growth. It is precisely this term that improved the accuracy over the first model.

The test was carried out against independent data, that is on measurements that had not been used in the fitting. For Arbequina the second model predicted the date of cessation with a root mean square error of 3.16 days and a mean absolute error of 2.67 days. Once the other four cultivars were included, the corresponding errors were 4.79 and 4.00 days [1]. This accuracy applies to the particular site and the particular cultivars of the study, and is kept as an indication of the order of magnitude, not as a guarantee for a Greek olive grove.

Two limits of the model are stated by the authors themselves. The first is that the requirement of 160 hours is not a chilling requirement for flowering but for the cessation of growth, and the two processes are separate. The second is that the calibration was carried out on young potted trees, one to three years old, in the area of Córdoba, so the transfer of the figures to mature trees of another cultivar and another climate is not a given [1].

Two details from the same work have practical value. The first is that the base temperature for leaf appearance was found to be 11.1 °C, appreciably lower than the 14.4 °C threshold that stops growth: the tree therefore stops before it reaches the point at which it can no longer grow. The second is that the five cultivars tested, among them the three largest in the world by area, differed very little in the date of cessation [1]. Location mattered more than cultivar, with differences of one to three weeks between the two sites of the study.

Sources: 1

2. What chilling does to the buds that will flower

The cessation of growth is one half. The other, and the economically critical one, is what happens inside the axillary buds, from which the inflorescences of the following spring will emerge.

The sequence of events, as described by the Cordoba group after decades of observations and measurements on explants, is as follows [2]. The axillary buds form over a long period within the growing season and pass from undefined to potentially reproductive, unless the presence of fruits on the tree prevents it. Throughout that period they are held closed by the leaves and the fruits themselves, through a mechanism called para-dormancy. When the autumn temperatures fall, dormancy becomes generalised in all the buds. From that point on, what releases them is the accumulation of chilling.

The point that overturns intuition is the next one: the morphological differentiation of the inflorescence begins after the winter rest, not before. In the bud sections they examined, on 9 January only a few buds had met their chilling requirement and a zone of cell activity was just becoming visible. On 7 February, when all of them had met it, the branch of the inflorescence was already formed [2]. During the winter, therefore, a bud that will carry flowers and one that will carry a shoot cannot be told apart under the microscope.

Which temperatures count

For the flower buds, chilling is not counted with the same threshold that stops growth. Measurements on explants showed that 12.5 °C is an efficient accumulation temperature, and indeed allows slow budburst once enough chilling has accumulated. At 30 °C, by contrast, natural chilling accumulation is nullified [2]. The International Olive Council gives the same range more precisely: chilling accumulates from 7 to 15 °C, with an optimum of 12.5 °C, above 16 °C it does not accumulate at all, and still higher temperatures cancel chilling that has already accumulated [4].

The comparison of the two thresholds is worth making explicitly, because they are not the same quantity. The 14.4 °C threshold concerns the cessation of shoot extension and was measured in hours, with a requirement of 160 hours [1]. The optimum of 12.5 °C concerns the release of the flower buds from dormancy and requires many times longer [2, 4]. These are two different processes that happen to occur in the same tree at the same season.

The hypothesis the authors put forward for the role of chilling is that it works as a synchroniser. The buds formed over a period of months, at different ages and positions along the shoot. The winter rest imposed by chilling prevents them all from starting early and separately, and once the requirement is met it lets them start together [2]. This also explains the reverse: when the winter is warm and chilling is insufficient, budburst, flowering and ripening come out asynchronous and prolonged, with stages of different age on the same tree [4].

Sources: 1 2 4

3. Does the olive have a true dormancy

The term dormancy is used for three different things, and the confusion is not academic. Endo-dormancy is an internal lock: the bud does not start even if ideal conditions are given to it. Para-dormancy is suppression by another organ of the same plant, by the leaves or the fruits. Eco-dormancy is simple arrest by the environment: as soon as the limiting factor is removed, growth resumes without delay.

In 2024 a viewpoint was published in the Journal of Experimental Botany arguing that the olive does not pass through a true endo-dormancy [3]. There are two arguments. The first is that, when buds or explants are moved to favourable conditions, the percentage of budburst is very high, even in midwinter. The second is that removing the leaves in combination with warming gives one hundred per cent vegetative budburst. If, that is, para-dormancy and eco-dormancy are removed, nothing is left to hold the bud back, so there is no internal lock [3].

The same work also restates the role of chilling. The cold does not break a lock, but induces the expression of the OeFT gene, which is the flowering gene. The evidence it invokes is that cold induces OeFT, that warm treatment of buds during the winter completely suppresses flowering, and that transgenic plants overexpressing OeFT flowered out of season [3]. The conclusion is that chilling is not needed as such for the olive to flower, but in order to activate the gene. This agrees with an observation reported by both of the other papers: buds that burst before or early in the winter give shoots, not inflorescences.

Practical significance of the above

The difference is not theoretical. If the olive is in eco-dormancy, that is held back only by the cold, then a warm spell in midwinter is enough for growth to resume. The study shows this clearly: plants moved into a greenhouse above 18 °C on eight different dates through the winter all started, irrespective of how much chilling they had received up to that moment, usually within one to two weeks [1]. There is, in other words, no internal safeguard keeping the tree closed until March.

The practical consequence is that a warm spell in January or February does not pass without cost. On the one hand it may start growth that will then be exposed to the next frost. On the other, and more seriously, high temperatures cancel chilling that had already accumulated [2, 4], so the tree ends up behind where it was.

Sources: 1 2 3 4

4. What all of this means in a Greek olive grove

The figures above were measured on Arbequina, in pots, in Cordoba and in Espiel, with trees one to three years old [1]. The Greek question is a different one and is answered by Greek sources: which Greek cultivars need how much chilling, and where that is secured.

The Greek cultivars are not all alike

In the teaching material of the Agricultural University of Athens the Greek cultivars are split into two groups according to their chilling requirement, and the same distinction is repeated in technical writing of the sector [5, 6].

The values are given as an upper limit below which chilling counts, not as a requirement in hours. The second source gives 12 °C and the first 10 °C for the same group, a difference which shows that these values are rules of thumb and not calibrated thresholds.

The practical consequence is clear and it concerns siting. Chondrolia of Chalkidiki and Konservolia need prolonged winter chilling and perform poorly in areas with a mild winter, such as the coastal parts of Crete [6]. Koroneiki and the rest of the first group fruit normally in areas where the winter mean stays above 10 °C [6]. From this point of view, the choice of cultivar for a new orchard is also a decision about whether the area supplies enough chilling.

As to the total requirement, the Greek teaching material gives 10 to 13 °C for 1,500 to 2,000 hours [5]. Technical writing of the sector reports at least ten weeks below 16 °C, and refers to older research in which trees with 1,200 hours below 7.2 °C flowered and set fruit normally, while trees with no hours at all below that threshold formed no flowers [6].

What the measured Greek data show

The only extensive Greek series that combines phenology with chilling accumulation is from Crete. It covers about 1,200 observations of Koroneiki flowering from 1984 to 2018, at four sites from 45 to 624 metres of altitude, using the Dynamic model and chill portions [7].

Accumulation ranged from 50.6 chill portions at Gazi, at 45 metres, to 97.3 portions at Aghia Varvara, at 624 metres, with Dafnes and Kastelli, at 315 and 340 metres, giving intermediate values [7]. The difference between the coastal and the mountain site is almost twice the accumulation, within the same island. The projections of the same work show a reduction in chilling of 12.0 to 28.3 per cent for the period 2021 to 2060 and of 22.7 to 70.9 per cent for the period 2061 to 2100, depending on site and scenario. In the worst scenario and for the far future, at Gazi the accumulation is practically nullified in some years, and the authors warn explicitly that such low accumulation may not be adequate for the induction of flowering in Koroneiki [7].

What changes in management

Four points follow directly from the above and concern management.

• Siting Chondrolia and Konservolia is a chilling decision. In an area with a mild winter the orchard will underperform every year, and that is not corrected by fertilisation or by irrigation.
• A warm winter costs twice. It cancels chilling that had accumulated and at the same time it may start premature growth, because there is no strong internal lock.
• Asynchronous flowering is a symptom, not a quirk. When inflorescences of different age coexist on the same tree, the most likely cause is insufficient chilling, and this is worth recording season by season.
• Alternate bearing starts the previous summer. The presence of fruits prevents the conversion of the axillary buds into reproductive ones [2], so whatever concerns this season's crop load also decides the flowering potential of the next.

For the way chilling is measured, and for the reason chill hours and chill portions are not the same quantity, there is a separate text: Chill hours: the three models and when each one tells the truth, available in Greek only. The accumulation for five areas of Northern Greece is computed live on the page Chill hours and chill portions, also in Greek.

Sources: 1 2 5 6 7

5. Sources

The numbers in the text refer to the list below.

• [1] López-Bernal, Á., García-Tejera, O., Testi, L., Orgaz, F. and Villalobos, F.J. (2020). Studying and modelling winter dormancy in olive trees. Agricultural and Forest Meteorology 280:107776. Source for the role of temperature rather than photoperiod, for the growth cessation models with thresholds of 14.4 and 14.5 °C and a requirement of 160 hours, for the subtraction coefficient of 0.32, for the validation errors of 2.67 and 4.00 days, for the base temperature of 11.1 °C, for the reversibility within one to two weeks and for the small difference between cultivars. Read in full.
• [2] Rallo, L., Ramos, A., Rubio-Valdés, G. and Rapoport, H.F. Integrated Overview of Olive Reproductive Bud Dormancy and Biennial Bearing. Acta Horticulturae, International Society for Horticultural Science. Source for the sequence of events in the axillary buds, for the role of leaves and fruits in para-dormancy, for the differentiation of the inflorescence after the winter rest, for the sections of 9 January and 7 February, for 12.5 °C as an efficient temperature and for the nullification of accumulation at 30 °C. Read in full.
• [3] Arias-Sibillotte, M., Considine, M.J. and Signorelli, S. (2024). Reinterpreting olive bud dormancy. Journal of Experimental Botany 75(19):6017-6021. Source for the position that the olive does not pass through a true endo-dormancy, for the distinction between endo-dormancy, para-dormancy and eco-dormancy, for the one hundred per cent vegetative budburst after defoliation and warming, and for the role of chilling through the OeFT gene. A viewpoint, not a new study. Read in full.
• [4] International Olive Council. World Olive Catalogue, Module 2, Chilling requirements. Source for the accumulation range of 7 to 15 °C, for the optimum of 12.5 °C, for the absence of accumulation above 16 °C and the cancellation at higher temperatures, and for the asynchronous budburst and flowering when chilling is insufficient.
• [5] Roussos, P. The annual growth cycle of the olive, bud differentiation. Teaching material, Agricultural University of Athens, in Greek. Source for the requirement of 10 to 13 °C for 1,500 to 2,000 hours and for the split of the Greek cultivars into low and high chilling requirement.
• [6] Bartzokas, N. The role of chilling in olive fruiting. SEDIK, in Greek. Source for the Greek upper limits of 16 and 12 °C by cultivar, for the ten weeks below 16 °C, for the 1,200 hours below 7.2 °C and for the poor performance of the high requirement cultivars on the coast of Crete. Technical writing of the sector, not peer reviewed, and used only for the Greek context.
• [7] Grillakis, M.G., Kapetanakis, E.G. and Goumenaki, E. (2022). Climate change implications for olive flowering in Crete, Greece: projections based on historical data. Climatic Change 175:7. Source for the roughly 1,200 observations of Koroneiki flowering from 1984 to 2018, for the chill portions of 50.6 at Gazi and 97.3 at Aghia Varvara, for the projected reductions of 12.0 to 28.3 and 22.7 to 70.9 per cent, and for the warning that accumulation may not be adequate for Koroneiki. The abstract and extracts of the full text were read.