What Are ‘Chill Hours’ And Why Do They Matter?

The Apprentice House Orchard, Quarry Bank, Cheshire, December 2020

In this article, I’ll be discussing ‘chill hours’ (or ‘chill units’) and the ‘chill (or chilling) requirement’ of fruit trees, under the following topics: what are chill hours, why is chilling important, and why does chilling matter to me?

TL;DR Version: fruit trees need a certain number of hours’ worth of low temperatures over the winter period in order to grow back strongly in spring. Knowing the ‘chill requirement’ of a fruit species or cultivar helps gardeners and orchardists avoid buying and planting trees that are unsuitable for their local winters.


What Are ‘Chill Hours’ (or ‘Chill Units’?)

If you’re a keen fruit-grower, with a particular interest in apples and/or pears, you might have see the occasional mention in a growing guide or online article of ‘chill hours’, ‘chill units’ or ‘cold hours’, usually as a requirement for flower and fruit production in the following spring.

Here’s a summary from harvesttotable.com:

“Chill hours are the number of cold hours or days that a deciduous fruit tree (or nut tree) requires for flowering and fruit production each year. Every fruit tree variety has its own number of hours of chill needed for fruit production. Some fruit trees need as few as 100 chill hours, others need as many as 1,000 chill hours or more.”

Not to split hairs, but technically what they’re discussing there is a ‘chill (or chilling, or chill accumulation) requirement’ rather than a ‘chill hour’ (or ‘chill unit’).

A single ‘chill hour’ is as an hour during which the temperature of the air surrounding the dormant tree is within the appropriate ‘chill range’. The ‘chill accumulation’ is the total number of chill hours a tree needs to spend at that temperature during the ‘chill period’ in order to meets its ‘chill requirement’.

There are various models for calculating the chill range and the chill accumulation during the chill period (usually December and January). In their 1992 paper ‘Chilling Accumulation: its Importance and Estimation’, David H. Byrne and Terry Bacon detailed three models, which work by counting the number of hours of temperatures <45°F (7°C); or between 32°F and 45°F (0°Cand 7°C); or in the ‘Utah model’, assigning varying chill units on an hourly basis, within a temperature range from 34°F to 60°F (1°C to 15°C), because “[m]ost people agree that temperatures below freezing or above 60°F are not effective for [chill] unit accumulation.” Those ranges do vary a bit – for instance, that upper limit has been estimated to be closer to 12°C in later studies as you’ll see below – but they’re all in the same ball-park.

The simplest way of working out how many chill hours there are, were, or will be in a day, is to take a look at your local weather forecast and tot up the number of hours in the appropriate °C range. Recent temperature records would be a bit more accurate, but for a quick example, today’s BBC website forecast (screen-grabbed yesterday) for my neck of the woods will do nicely:

So according to Byrne and Bacon, the trees on our allotment plot should have totted up 18 chill hours between 6 a.m. and midnight, and 24 hours if you include the period between midnight and 6 a.m. which is, of course, the maximum daily contribution.

The typical apple or pear tree’s total ‘chill requirement’ has been estimated to be 1,200 to 1,500 hours (at 5°C – 7°C) by Ryugo in 19881 and circa 1,000 hours (at 6°C – 9°C) by Heide & Prestrud in 20052.

Generally speaking then, that means another 41 days like today, between the start of December and the end of January, ought to be enough to roughly meet our orchard trees’ chill requirement for the season. And although the recent cold snap we’ve had, with its overnight sub-zero temperatures, meant a contribution of fewer chill hours per day to the accumulated total, I think our recent weather puts us well on course, which is great news.

But why is it great news?

Why Is Chilling Important?

Frost on apple buds – a sign of accumulating chill hours

In a series of experiments carried out between 2000 and 2003, O. M. Heide and A. K. Prestrud tested the re-growth response of a number of different apple and pear rootstocks (which were micro-propagated and then allowed to grow into trees) to a range of temperatures low, in order to make a comparison with the same rootstocks’ response to photoperiodism (varying daylight lengths).

The key conclusions they drew were:

  • “[T]emperatures below 12°C consistently induced growth cessation and formation of winter buds in all tested cultivars regardless of photoperiodic conditions.”3
  • “Chilling at 6 or 9 °C for at least 6 weeks (about 1,000 h) was required for dormancy release and growth resumption, whereas treatment at 12 °C was marginally effective, even after 14 weeks of exposure.”4

Byrne and Bacon – who were studying commercial peach trees in Texas – gave details of “insufficient chilling symptoms”5:

  • Delayed Leaf Development or “small tuft of leaves near the tips of the stems [but no] leaves for 12 to 20 inches below the tips” leading to poor fruit set, suckering from the rootstock, and poor development of next year’s fruit buds.
  • Reduced Fruit Set and Poor Fruit Quality. “Bloom is delayed, extended, and due to abnormalities in pistil and pollen development, fruit set is reduced”, leading to small, mis-formed fruit (“buttons”) that develop late, aren’t worth harvesting, and can harbour pests and diseases; or larger fruits that are misshapen and poorly coloured, therefore un-sellable.

In other words: apple and pear trees will enter a state of dormancy when the temperature drops below 12°C. But for them to re-grow strongly in the spring, six weeks of suitably cold weather gives much better results than 12 weeks of mild-to-cool weather. And if peach (also apple, pear and other fruit) trees don’t get to enjoy this short, sharp period of chilling, their leaves will grow back more slowly in spring, and they may produce fewer and poorer quality fruit, not just this season, but next season as well.

It’s worth bearing in mind that chilling is far from the only factor affecting the break of dormancy, the vigour of a tree’s spring growth, the breaking of buds, or the successful setting and development of fruit. A fruit tree is an incredibly complex biological system of checks, balances, signals and responses, and is affected by a huge range of factors and variables.

But the law of limiting factors always applies: is any one of those variables – such as the tree’s winter chill accumulation – is seriously deficient, then it can have a significant ramifications, because increasing or optimising any of the other inputs won’t improve the tree’s overall performance whilst the limiting factor is in effect.

Why Does Chilling Matter to Me?

Apple tree in full blossom – must have been properly chilled the previous winter

You may be wondering why on Earth the calculation of accumulated chill hours is something you should worry about. Especially if you’re a home, allotment or small orchard grower who just wants to grow a decent amount of fruit that’s good to eat.

The answer is: no, you probably shouldn’t worry too much, once your trees are in the ground. There’s really not a lot you can do to influence winter weather patterns, whilst there are actions you can take to try to help improve fruit set, fruit quality and the tree’s re-growth in spring, such as thinning out fruitlets, and others which I’ll talk about in future articles (and try to remember to link to here).

But still, knowing what chill hours are and why they matter really is rather useful information to have before you decide to buy, graft or transplant a fruit tree to plant it in your garden, plot or orchard.

The harvesttotable.com article I linked to earlier points out that different top fruit species have different chill requirements. It also stresses that chill requirements can vary widely between cultivars of the same species. And from the point of view of the orchardist or gardener who just wants to grow good fruit trees, that variation between cultivars is very important.

A tree with a high chilling requirement probably won’t produce flowers and fruit as reliably in a location that averages warmer winter temperatures, than a tree with a lower chill requirement. So an apple that thrives in Northern English or Scottish winters probably won’t flower and fruit as reliably in, say, the balmy South of France.

A fruit tree is a serious investment in time and growing space as well as, potentially, financially. So having an idea of whether or not that cultivar you like the sound of has a particular chilling requirement, and if the winters in your part of the world usually match up, might just stop you from planting a poorly-fruiting dud.

So it might be surprising how infrequently chill hours are mentioned when books and websites list the key factors to consider when selecting fruit trees for planting. There’s no mention of them in a couple of website guides and otherwise generally excellent books that I checked, although they do (quite rightly) go into detail about the importance of matching pollination groups and selecting the right rootstocks. Checking the catalogue pages of some of the major fruit tree nurseries and suppliers, I found no mention of chilling requirement alongside the useful details they do provide, such as pollination group and whether the tree is a partial tip-bearer or a predominantly spur-bearing type.

Even the impressively comprehensive OrangePippinTrees.com – the website of an American nursery, with a vast database of fruit cultivars – doesn’t mention a chilling requirement amongst the huge amount of other information in the ‘characteristics’ section of their cultivar pages (i.e. ‘Bramley’s Seedling‘ – scroll down and you’ll see what I mean).

Although they do have an article on their site that discusses the minimum chill requirement for apple trees, that finishes by saying:

“As a final word it is worth noting that little research has been done on this subject, and it is perhaps best not to get too precise about the exact number of chill hours, but simply categorise varieties as high, medium, or low chill. In addition, chilling hours is not the only factor in determining whether a particular apple variety will do well in hot climates, but is certainly worth taking into consideration.”

Perhaps this is the reason why so few information sources mention or list chill hours: too little research has been done to identify which cultivars have which requirements, so the information isn’t readily available. Perhaps nurseries have decided it’s better to not mention chilling requirement at all, rather than worry potential buyers by talking about it, without being able to provide the information at point of sale?

Nevertheless, I would strongly advise anyone who is thinking of planting a fruit tree, and particularly an orchard’s worth, to do the research on chill hours before you buy your trees. Pick up the phone and speak to your local, specialist fruit tree nursery. Ask them about chill requirements and whether the tree or trees you’re thinking of buying are suitable for the prevailing local climate and the sort of winters you usually get.

They probably won’t be able to give you a precise figure for an individual cultivar’s chill requirement, but specialist nurseries are usually staffed by knowledgeable, experienced fruit tree enthusiasts. These folks will have a good idea of whether a particular cultivar is, for example, bred and adapted for growing Up North or in Scotland, or whether it’s better suited to the milder climates of Kent or the West Country.

As a general rule-of-thumb the best course of action is always going to be to buy healthy, established trees from your nearest specialist nursery. This gives you a much better chance of the tree having spent at least a couple of seasons adapting to the local or at least regional conditions.

Beyond that, the precise number of chill hours your trees accumulate every winter are always going to be in the lap of the weather gods. Fingers crossed, and good luck!


Footnotes:

  1. Ryugo, K. 1988. Fruit culture: its science and art. John Wiley and Sons, New York, 344 p. [back]
  2. O. M. Heide, A. K. Prestrud, Low temperature, but not photoperiod, controls growth cessation and dormancy induction and release in apple and pear, Tree Physiology, Volume 25, Issue 1, January 2005, Pages 109–114, https://doi.org/10.1093/treephys/25.1.109 [back]
  3. ibid [back]
  4. ibid [back]
  5. Byrne, D. H., and T. A. Bacon (1992). Chilling estimation: its importance and estimation. The Texas Horticulturist 18(8):5 [back]

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