Breaking dormancy in sweet cherry, and the chilling hours question

Sweet cherries at fruit set, with the dried petals still on the spur — Fig. 1Fruit set on a cherry spur. On the same spur there are fruitlets of different sizes and flowers that did not set — the imprint of a bloom that was spread out in time. Whether they all set together was decided in winter, by the chilling that accumulated and by the way dormancy was released.

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

Sweet cherry is among the most chilling-demanding fruit crops grown in Greece, and at the same time it is increasingly planted in districts where chilling is not assured. This article sets out what dormancy actually is, why “chilling hours” are three different numbers that cannot be compared with one another, how much chilling cherry really needs, what the trials with dormancy-breaking agents show — and which of all this is permitted in Greece today.

Note on units and on scope Rates and concentrations quoted here come from published trials abroad and are given for an understanding of the physiology. Plant protection approvals cited are those of the Greek Ministry of Rural Development and Food and are valid in Greece only; they say nothing about what is registered in any other country.

58–84chill portions by cultivar, in a Mediterranean climate

70%the minimum share of chilling before any application

−0.5 °Cthe damage threshold at bud swell

0dormancy-breaking products authorised on cherry in Greece

Contents

What dormancy is and what releases it
Hours, units or portions — three different quantities
How much chilling sweet cherry needs
What happens when the chilling requirement is not met
How dormancy is measured when it cannot be seen
Dormancy breakers — two different jobs
The trial results, with the numbers
The 70% rule and the application window
The risks that do not appear in the brochures
What is permitted in Greece
What remains in the grower's hands
Sources

1. What dormancy is and what releases it

Dormancy is, in the classical definition, the suspension of visible growth of any plant structure containing a meristem. The same paper introduced the division into three types, and that division is not academic — it determines what works and what does not [1].

Type	What imposes the suspension	What lifts it
Paradormancy	Another part of the same plant — chiefly apical dominance.	Pruning, changing the angle of the branch.
Endodormancy	The bud itself, internally.	Chilling alone. No warmth, no fertiliser and no product lifts it fully if the chilling is missing.
Ecodormancy	External conditions — cold, drought.	Rising temperature. It lifts of its own accord.

The point that is constantly misunderstood Dormancy-breaking agents do not replace chilling. They act at the end of endodormancy and within ecodormancy, accelerating and synchronising a process that has already advanced. On a tree that has not received chilling, the result is poor or even negative. That is why the 70% threshold described below exists.

Sources: 1

2. Hours, units or portions — three different quantities

When it is said that “the cultivar needs 800 chilling hours”, the number means nothing without the model. Three are in circulation, and they give entirely different figures for the same tree in the same winter.

Model	How it counts	Where it falls short
Chilling hours (Weinberger)	One hour below 7.2 °C (45 °F) counts as one hour. Nothing else counts [2].	It subtracts nothing for warmth. In mild winters with warm spells it overestimates dramatically.
Utah chill units (Richardson)	Each hour is weighted by temperature, with an optimum around 6 °C, and units are subtracted when it is warm [3].	The subtraction can be excessive in very mild climates.
Chill portions (Dynamic)	Two steps: formation of an unstable precursor in the cold, and an irreversible conversion into a stable chill portion once a critical amount has accumulated [4].	More complex to compute. Requires hourly data.

Which one holds for Mediterranean conditions In a trial on ten sweet cherry cultivars in Apulia, southern Italy — a Mediterranean climate comparable to the Greek one — all four models were compared. The Dynamic model performed better than all the others, with the most consistent values between the two years [5]. The same conclusion emerges from other Mediterranean comparisons. The simple hours-below-7.2 °C model, which is the one most often quoted in discussion, is at the same time the least reliable in precisely those climates where the answer matters.

The practical rule: hours ≠ units ≠ portions. A number without its model is comparable with nothing. Before a value is copied from a brochure or a foreign article, the model it refers to has to be checked.

Sources: 2 3 4 5

3. How much chilling sweet cherry needs

The literature places the requirement of sweet cherry over a wide range, from 520 to 1,320 chilling hours depending on cultivar [6]. That range is not uncertainty — it is a real genetic difference between early and late cultivars, and it is the primary criterion when choosing for a new planting.

Cultivar or group	Requirement	Model and source
‘Burlat’ (early flowering)	58 portions	Dynamic, Apulia [5]
‘Ferrovia’ (late flowering)	84 portions	Dynamic, Apulia [5]
‘Royal Dawn’	400–500 hours	Weinberger, Chile [7]
‘Kordia’	at least 800 hours	Weinberger, Chile [7]
General range for the species	520–1,320 hours	Chilling hours, review [6]

These values are not comparable with one another when they come from different models. Dynamic chill portions and Weinberger chilling hours are different quantities, not different units of the same quantity.

The real criterion for choosing a site In Apulia all ten cultivars had their requirement satisfied under the conditions of the experimental site [5]. That is no guarantee for every Mediterranean location: the difference between 58 and 84 portions is the difference between a cultivar that crops every year and one that will disappoint in mild winters. On semi-lowland sites with warm spells through winter, choosing a late, high-requirement cultivar is a decision that no spray programme corrects.

Sources: 5 6 7

4. What happens when the chilling requirement is not met

The symptoms of insufficient chilling are specific, and they are often attributed to other causes:

Delayed and uneven vegetative budbreak (delayed foliation). The tree flowers but does not clothe itself — and this is the most characteristic sign [8].
Reduced budbreak overall, with whole sections of the branch left bare.
Extended bloom, spread over several weeks instead of concentrating.
Abnormal flowers and reduced fruit set, with low yields [6].
Smaller leaves and shorter pedicels, whereas adequate chilling accumulation gives more branching, better quality flowers, bigger leaves, longer pedicels and increased fruit set [6].

The confusion in diagnosis Delayed foliation is often attributed to nutrient deficiency, to root damage or to bacterial canker. The distinguishing sign of insufficient chilling is that the problem affects the whole planting of the same cultivar simultaneously, recurs in mild years and is absent in cold ones, and usually spares the lower-requirement cultivars in the same orchard.

Sources: 6 8

5. How dormancy is measured when it cannot be seen

The basic problem with dormancy is that it has no visible boundaries. The date of endodormancy release and the beginning of ontogenetic development — which starts weeks before anything is visible in the bud — cannot be recorded by phenological observation [9]. That is precisely why dormancy remains a “black box” in phenological models.

Two methods give an answer:

The method that is applied in practice

The forcing test. Shoots are cut on successive dates through winter, placed in a warm cabinet, and the number of buds that break and the time taken are recorded. While endodormancy is deep, budbreak is slow and partial; once it is released, budbreak becomes rapid and complete. This is the method by which the course of cherry dormancy under mild-winter conditions was modelled [8].
Biochemical markers. Global metabolite profiling of flower buds of the cultivar ‘Summit’ detected 445 metabolites, of which seven were judged suitable markers for the ecodormancy phase. Through endo- and ecodormancy the energy metabolism — glycolysis and the tricarboxylic acid cycle — is shut down to a minimum, while the beginning of ontogenetic development is closely linked to the up-regulation of carbohydrate metabolism [9].

The practical significance is simple: the only way to know where a planting actually stands is the forcing test, not counting hours below 7.2 °C. The hours are an estimate; the shoot in the cabinet is a measurement.

Sources: 8 9

6. Dormancy breakers — two different jobs

International practice recognises two categories, and confusing them leads to the wrong expectations.

Category	What it does	What it does not do
Hydrogen cyanamide	Advances the phenological stages. It is the strongest tool for bringing bloom forward [7].	It does not particularly even out bloom, and it is toxic.
Alternative “homogenisers”	Synchronise budbreak and bloom, with strong action on vegetative buds as well — more flowers accompanied by leaves [7].	They do not advance the stages to the degree that cyanamide does.

The best studied alternative is an organic nitrogen compound containing 4.7% calcium, 5.8% nitric nitrogen, 3.1% ammonium nitrogen and 6.1% urea, applied in combination with calcium nitrate [6]. Emulsified vegetable oil compounds are also under trial [10].

The mechanism of cyanamide has begun to be resolved. In an analysis of hormones within the flower buds, earlier budbreak was associated with a significant early elevation of the cytokinins dihydrozeatin and dihydrozeatin riboside, whereas changes in auxin and abscisic acid did not account for the acceleration [10].

Sources: 6 7 10

7. The trial results, with the numbers

The most useful comparison for Mediterranean conditions comes from the Central Valley of Chile, on ‘Santina’ grafted on ‘Pontaleb’. Treatments were applied at 520 chilling hours on the modified Richardson model [6].

Treatment	Harvest (days after full bloom)	Fruit weight (g)	Diameter (mm)	Pollen germination (%)
Control (water)	60	9.8	26.5	58.5
Organic nitrogen 6% + calcium nitrate 9%	56	11.5	28.6	62.1
Organic nitrogen 6% alone	59	10.2	26.5	44.4
Calcium nitrate 9% alone	60	10.7	27.5	41.5
Hydrogen cyanamide 2% + oil	56	11.7	27.8	43.2

These are the rates and concentrations of the trial. They are not a recommendation and do not transfer to a Greek planting, where the regulatory position is different — see section 10.

The finding that appears in no commercial brochure Pollen germination fell from 58.5% in the control to 43.2% with hydrogen cyanamide — a reduction of roughly a quarter. The nitrogen compound alone (44.4%) and calcium nitrate alone (41.5%) did the same. The only treatment that did not harm the pollen was the combination of organic nitrogen with calcium nitrate, which left it at 62.1% [6]. On a self-incompatible cultivar that depends on a polleniser, a reduction of that size in the germinability of the polleniser's pollen is damage that will never be traced back to a winter spray.

Two points on which the literature disagrees — stated openly

First, the advance in bloom. The abstract of that same paper states that two treatments “advanced bloom”. The results section, however, states that none of the treatments managed to advance and concentrate bloom, and that the difference was only 3 days for three of the treatments [6]. The abstract overstates what the data show; the figure from the results section is the one kept here.

Second, why the fruit gets bigger. In the Chilean trial the period from bloom to harvest shortened by 4 days and the fruit was larger. In an Australian trial, by contrast, the increase in size was attributed to earlier flowering and hence a longer growing period [10]. The two explanations are opposites. The safe conclusion is that the larger size results from the whole set of changes rather than from one cause, and that it is not predicted by the shift in bloom date.

The size distribution, not the mean The most practical finding of the Chilean trial is not the mean weight but the distribution. The combination of organic nitrogen with calcium nitrate shifted the curve towards the large sizes: 62.5% of fruit at 28–30 mm and 7.5% at 30–32 mm, that is 70% of the total above 28 mm. Hydrogen cyanamide, at a similar mean weight, concentrated 85% of the fruit in the 26–30 mm range [6]. In cherry pricing, that difference is the whole difference.

Sources: 6 10

8. The 70% rule and the application window

The most critical point in the whole business is not which product, but when.

The threshold For a dormancy breaker to be applied, the cultivar must have accumulated at least 70% of its chilling requirement [7]. Below that, the result is not merely smaller — it becomes unpredictable, and bloom can end up more uneven than it would have been with no treatment at all.

In experimental work, applications are made 35 to 40 days before expected natural budbreak [10]. In the Chilean trial the treatment was applied at 520 hours, and the first swollen buds appeared 48 days later [6].

One point overturns the grower's rough calculation:

An advance in bloom does not translate one-for-one into an advance in harvest.
10 days earlier full bloom → about 5–6 days earlier harvest
20 days earlier bloom, in a season with good chill accumulation → about 10–12 days [7]

The reason is that earlier bloom coincides with cooler temperatures, so fruit development proceeds more slowly. Anyone planning commercially on the assumption of a one-to-one relationship will end up with a harvest schedule that does not hold.

Sources: 6 7 10

9. The risks that do not appear in the brochures

Four risks, all documented

Frost. Advancing bloom exposes the planting to spring frost. The most sensitive stage is bud swell, with a damage threshold of temperatures below −0.5 °C for more than one hour. Significant damage has been reported between bud swell and the exposed cluster stage, in contrast to the more advanced white bud and full bloom stages, which tolerate more [7]. Without a frost protection system, advancing bloom is a gamble.
Death of the vegetative bud. On spurs, these products can cause death of the vegetative bud, so that the fruiting structure is not renewed [7]. The damage shows the following year, not this one.
Cracking and quality. In a trial with hydrogen cyanamide the fruit was larger, but there was a higher incidence of cracking and a reduced pack-out of A-grade fruit [10]. The extra millimetres are not paid for if they are lost on the grading line.
Pollen. The reduction in pollen germinability described in section 7 [6].

And operator toxicity Hydrogen cyanamide is highly toxic to humans: it causes severe irritation and ulceration of the eyes, skin and respiratory tract. It also inhibits aldehyde dehydrogenase, the enzyme that breaks down alcohol in the body; consumption of alcohol before, during or after exposure causes the acetaldehyde syndrome — vomiting, dyspnoea, hypotension and confusion [6] [11]. This is not a theoretical hazard; it is the reason the substance was withdrawn in Europe.

Sources: 6 7 10 11

10. What is permitted in Greece

Here the answer is narrow and has to be stated precisely, because the position differs from crop to crop.

On cherry there is no authorised dormancy-breaking product Cyanamide is a non-approved active substance in the European Union under Regulation (EC) 1107/2009, its inclusion having expired [11]. In the Greek Ministry approvals database, sweet cherry currently has 538 authorised plant protection uses; the plant growth regulators among them are gibberellic acid products and carfentrazone-ethyl, which have nothing to do with dormancy release. No authorised use is a dormancy-breaking product [12].

The 120-day emergency authorisation covers kiwifruit, not cherry Each year the Greek Ministry grants the product DORMEX 52 SL a 120-day emergency authorisation under Article 53 of Regulation 1107/2009, for limited and controlled use — and that authorisation covers the kiwifruit crop, in named regional units [13]. It does not cover cherry. Because these authorisations are issued separately each year and do not appear in the ordinary lists, the check has to be made against the current ministerial decisions before any intervention, not only against the database of standing approvals. Readers outside Greece must consult their own national register; emergency authorisations differ from country to country and from year to year.

And fertilisers carrying a dormancy-breaking claim Nutritional products and biostimulants are on the market with a label claim relating to dormancy release or uniform budbreak. They are fertilisers, not plant protection products: they are not assessed for efficacy under Regulation 1107/2009, and a label claim is not an authorisation for plant protection activity. Some of them, particularly those working through the nutritional status of the bud, have real support in the literature — the organic nitrogen compound of section 7 is itself a fertiliser. None of them substitutes for chilling hours, and their performance is judged against an unsprayed row in the same block, not by eye.

Sources: 11 12 13

11. What remains in the grower's hands

Six things that work without any product

Choosing a cultivar whose requirement fits the site. The difference between 58 and 84 portions [5] is the one decision that is never corrected. On a semi-lowland site with warm winters, a late high-requirement cultivar will disappoint repeatedly.
Altitude and aspect. A few tens of metres of elevation and a northern aspect change chilling accumulation substantially within the same district.
The forcing test. Ten shoots, a warm room and two readings in January give information that no estimate of hours can give [8].
Postharvest management. Increasing the tree's reserves of nitrogen and carbohydrates after harvest was studied precisely as a way of reducing the effective chilling requirement in mild-winter climates [8]. It is the most neglected operation of the year.
Frost protection before any thought of advancing bloom. The −0.5 °C threshold at bud swell [7] is not negotiable.
Record keeping. Full bloom date, length of bloom and percentage budbreak, every year, on the same row. After three seasons the planting answers for itself whether it has a chilling problem.

The essential point Chilling is not replaced; it is only topped up in its final part. A dormancy breaker is a tool for synchronising and advancing in a planting that has already received 70% of its chilling, not a remedy for the wrong site or the wrong cultivar. And on cherry in Greece today no such product is authorised — which returns the discussion to where it belongs: to cultivar, to site, to the tree's reserves and to frost protection.

12. Sources

The numbers in the text refer to the list below.

[1] Lang, G.A., Early, J.D., Martin, G.C. & Darnell, R.L. (1987). Endo-, para- and ecodormancy: physiological terminology and classification for dormancy research. HortScience 22:371–377. Source for the definition of dormancy and the distinction between the three types.
[2] Weinberger, J.H. (1950). Chilling requirements of peach varieties. Proceedings of the American Society for Horticultural Science 56:122–128. Source for the hours-below-7.2 °C (45 °F) model and for the experimental thresholds of 700, 900 and 1,100 hours.
[3] Richardson, E.A., Seeley, S.D. & Walker, D.R. (1974). A model for estimating the completion of rest for ‘Redhaven’ and ‘Elberta’ peach trees. HortScience 9:331–332. Source for the Utah model, the weighting of each hour by temperature and the subtraction of units in warm weather.
[4] Fishman, S., Erez, A. & Couvillon, G.A. (1987). The temperature dependence of dormancy breaking in plants: mathematical analysis of a two-step model involving a cooperative transition. Journal of Theoretical Biology 124:473–483. Source for the Dynamic model and its two-step mechanism with an unstable precursor and an irreversible conversion.
[5] Palasciano, M. & Gaeta, L. (2017). Comparison of different models for chilling requirements evaluation of sweet cherry cultivars in a Mediterranean area. Acta Horticulturae 1161:405–410. Source for the comparison of four models on ten cultivars in Apulia, for the superiority of the Dynamic model in a Mediterranean climate, and for the range of 58 (‘Burlat’) to 84 (‘Ferrovia’) chill portions.
[6] Ardiles, M. & Ayala, M. (2017). An alternative dormancy-breaking agent to hydrogen cyanamide for sweet cherry (Prunus avium L.) under low chilling accumulation conditions in the Central Valley of Chile. Acta Horticulturae 1161:423–430. Source for the treatment table, for pollen germination of 58.5% against 43.2%, for the size distribution of 62.5% at 28–30 mm, for the composition of the organic nitrogen compound, for the 520–1,320 hour range, for the symptoms of insufficient chilling and for the toxicity of hydrogen cyanamide.
[7] Management practices to advance and uniform bud break in cherry trees. Smartcherry, with technical input from C. Tapia (Avium), August 2025. Source for the 70% threshold, for the requirements of ‘Royal Dawn’ at 400–500 and ‘Kordia’ at a minimum of 800 Weinberger hours, for the distinction between advancing and homogenising agents, for the −0.5 °C threshold at bud swell and for the relationship between advance in bloom and advance in harvest.
[8] Kapp, C.J. (2008). Manipulation of the chilling requirement of sweet cherry trees. MSc thesis, University of Stellenbosch. Source for the symptoms of delayed foliation under mild winters, for the shoot forcing method, and for postharvest reserve management as a way of reducing the effective chilling requirement.
[9] Chmielewski, F.-M. & Götz, K.-P. (2022). Metabolites in cherry buds to detect winter dormancy. Metabolites 12(3):247. Source for the 445 metabolites and the seven ecodormancy markers in ‘Summit’, for the shutdown of glycolysis and the TCA cycle, and for the impossibility of recording dormancy boundaries phenologically.
[10] Bound, S.A., Foo, E., Gélinas-Marion, A., Nichols, D.S. & Nissen, R. (2022). The impact of dormancy breakers on hormone profiles, fruit growth and quality in sweet cherry. Agriculture 12(2):270. Source for application 35–40 days before expected budbreak, for the elevation of the cytokinins dihydrozeatin and dihydrozeatin riboside, for the non-involvement of auxins and abscisic acid, and for the higher incidence of cracking with reduced A-grade pack-out.
[11] Cyanamide — Pesticide Properties DataBase (PPDB), AERU, University of Hertfordshire. Source for the “not approved” status of cyanamide under Regulation (EC) 1107/2009 and for the expiry of its inclusion.
[12] Database of authorised plant protection products and biocides, Greek Ministry of Rural Development and Food (September 2026 edition; in Greek, valid in Greece only). Source for the 538 authorised uses on sweet cherry, for the plant growth regulators being gibberellic acid and carfentrazone-ethyl, and for the absence of any dormancy-breaking product.
[13] DORMEX 52 SL: grant of a 120-day authorisation for limited and controlled use on kiwifruit. Greek Ministry of Rural Development and Food, ref. 9442/276074, 20 December 2022 (in Greek). Source for the Article 53 emergency authorisation and for the fact that it covers kiwifruit and not cherry.

Photograph: Pastopoulos Agronomics archive.

Disclaimer This text is informative and is not a recommendation to apply anything. The rates and concentrations quoted come from published trials in other countries, are given for an understanding of the physiology, and do not transfer to any other planting. Plant protection products must be used solely in accordance with the approvals in force and the product label. The approvals cited here are those of the Greek Ministry of Rural Development and Food and are valid in Greece only; readers elsewhere must consult their own national register. The active substance cyanamide is not approved in the European Union, and there is no authorised dormancy-breaking product for sweet cherry in Greece; the annual 120-day emergency authorisation covers a different crop and is checked separately each year. Any foliar spray carries a risk of marking the fruit and scorching the foliage: a preliminary test on a limited number of trees, with the same product at the same rate, is required, followed by a wait of 5 to 7 days before any general application; spraying is done in the late afternoon or early morning, never in a heatwave and never on foliage under water stress. Every intervention is at the user's own responsibility, following an on-site assessment by a licensed agronomist. Pastopoulos Agronomics accepts no liability for the use of this information.