Most deciduous fruits commercially grown in the world today come from the family of plants Rosaceae. Mostly they come from the north temperate zone which includes Europe, Asia and North America. Many commercial fruit species today were developed over the centuries from native populations in colder country than Australia. The way they coped with winter was to drop their leaves in autumn and for the buds to break in spring as temperatures became warmer. One of the ways to overcome a warm spell in winter was to develop a requirement of a certain time at low temperatures before the buds would break. This stopped bud break coming too early, and the buds being destroyed when the cold weather returned.
The last few weeks have been a good example of a warm spell too early. At Orange it had been quite a cold winter until the second week in August. Like the rest of the state, those three weeks of August were extraordinarily warm for the time of year, three days we had 200C and much of NSW was in the thirties as many of you know. This first week in September went back to cold with snow and -20C or more frosts. This is likely to have done damage to blossoms especially peaches, nectarines and apricots, which would have been at full bloom or later. Hopefully plums, pears and apples may be not badly affected. This is one of the reasons for this natural development of plants against cold.
Most apples and pears need many hours of winter chill (below 70c, tablelands winter in NSW) to break dormancy in spring and then flower. The major varieties grown in the world need 800 to 1500 hours of chill. Lack of chill makes buds slow to break in spring or not at all. The flowering period may be 4-6 weeks rather than 1-2 weeks, with very irregular bud break all over the tree. This may mean that the cross pollinating variety's flowers are over or not yet in bloom as different varieties have different requirements. Also fruit quality and maturity will be non-uniform on the tree, and harvesting and storage will be difficult.
Winter chill hours used to be calculated from thermographs in the field on the number of hours below 70C. More is known now and a number of models have been developed to more accurately calculate winter chill units. Very low freezing temperatures are thought to not count. Most varieties require a period of heat afterwards to complete the bud break. In a 1500 hour chill period on apple MM.111 rootstock, 150C in the last 500 hours had a positive effect on bud break (Young 1992). 15, 20 and 300C in the first and second 500 hours had a negative effect. Ashcroft et al. in 1977, published a method determining chill units and growing degree hours from temperature data and observed dates of full bloom. Figure 1 is a simple ready reckoner to calculate the possible winter chill occurring in a warmer area (not developed by author).
In the last decade or so, moves have pushed ahead to grow these fruit in warmer climes of Australia, that do not have sufficient winter chill any winter, such as around here on the North Coast. Other areas have enough chill most winters but others are too mild, such as at Stanthorpe in Queensland, parts of Southern Victoria and Western Australia.
How much winter chill is required for a variety or species is often decided by where that plant grew originally. Varieties grown in high latitudes and cold climates are likely to require high chill. Others grown in lower latitudes and warmer winter temperatures may require lower chill.
More recently, perhaps in the last 30 or more years, many warm climate countries have wanted to grow their own apples, including some tropical or sub-tropical countries. In these countries, new plantings were put in up in the high hills or mountain areas for more chill added by the altitude. Some varieties have been bred or selected requiring less hours of winter chill, mostly in these warmer districts of the world, eg. Florida, West Australia, South Africa, Israel, Indonesia, or Brazil. Quite often varieties that flower early are fairly low chill, whilst late flowerers are more likely to be high chill.
In Java, in the early 1970s, apples were grown in the highlands (Janick 1974). They grew two crops per year, one harvested in April and one in October. To do this the fruit was picked, the trees were left alone for a month before all the leaves were picked off. They would flower in a month, have three months of growing before being picked again. Tip bearing varieties like Rome Beauty were best for this system and they have selected new varieties to suit the system.
Apples species are much more complex in their ancestry than pears and more mixed up. Selections were grafted from at least 600 BC from Asia Minor and Eastern Europe. No particular species has been grown in a definite area of the world, so apples commercially are the same the world over, with some certain varieties that may be grown in a particular country for their taste and quality under local conditions.
Quite often low chill varieties flower early and high chill varieties flower late. This tendency to chill hour requirements is often carried into the progeny in a breeding program.
Table 1. Apple varieties of different maturity requiring different chill hours.
| High chill | Medium chill | Low chill | |
| Early maturity | Gala | Akane Gravenstein | Dorsett Golden (250) Anna (300) Ein Scheimer (400) 60-39 (400) 88-20 (375) Tropic Mac (300) Tropic Sweet (300) |
| Medium maturity | Delicious Golden Delicious Jonagold | ||
| Late maturity | Fuji Braeburn | Granny Smith Sundowner Pink Lady |
Pear species commercially world wide are :-
Most of the first three grew naturally in much colder climates than Australia and so need a lot of winter chill to break dormancy. The last species is more from Southern China and some of these do not require so much chill. Varieties in Australia are mostly P. communis, pyrifolia and bretschneideri or hybrid crosses. It is difficult to find in the literature what varieties require, whether apple or pear, but there are a few references (Spiegel Roy & Alston 1979, Ghariani & Stebbins 1994).
Some of these have European pear ancestry or are European pear crossed with another species, which may have given the new variety a low chill characteristic.
Table 2. Pear varieties with certain species ancestry and different chill requirements in Australia.
| High chill | Medium chill | Low chill | |
| Pyrus communis | Comice Dr Jules Guyot Williams' Beurré Bosc | Packham's Josephine Conference | Flordahome Hood Moonglow Magness |
| Pyrus pyrifolia | Nijisseiki Shinsui Kosui Hosui Dan Bae | - | - |
| Pyrus x bretschneideri | - | - | Tsu Li Ya Li |
The chilling requirement of a rootstock can have an effect on the chilling requirement of the variety. Most of the clonal apple rootstocks used commercially were selected and bred in much colder climes, so how they work in warm districts is not been fully tested. Granny Smith seedlings are not so high chill may be used but the tree vigour tends to be too vigorous. Pear rootstocks have more choice as P. calleryana rootstock grown all over Australia. Williams' on their own roots required over 1100 hours, P. calleryana alone required 400 hours and Williams' on P. calleryana required an intermediate number of hours (Westwood & Chestnut 1964). Where a rootstock is high chill (eg. MM.111 1100 hours, Young 1992), the effect on a low chill scion variety may be less than the other way about as with pears, as the buds of the scion have had enough chill.
Dormancy can be broken with chemicals such as Cyanamide (Dormex), thiourea dinitro-ortho-cresol (DNOC), potassium nitrate, but suitable varieties for the chill hours of the area are more effective. 1% and 2% cyanamide applied 20 days before bud burst and one month later were effective in breaking dormancy 78-81%, but 2% was more effective (Finetto 1993).
Although varieties needing high chill can be grown in warmer districts with the aid of chemicals like Cyanamide, the use of suitable low chill varieties of apples, pears and nashi can be more effective. If one is wanting to grow pome fruit in ones area, the first thing to find out would be the average winter temperatures for the area before deciding what to grow.
Ashcroft, G.L., Richardson, E.A. and Seeley, S.D. (1977). A statistical method of determining chill unit and growing degree hour requirements for deciduous fruit trees. HortScience 12(4):347-348.
Ghariani, K. & Stebbins, R.L. (1994). Chilling requirements of apple and pear cultivars. Fruit Varieties Journal 48(4):215-222.
Janick, J. (1974). The apple in Java. HortScience 9(1):13-15.
Finetto, G.A. (1990). The effect of hydrogen cyanamide on breaking endo-dormancy of mid-chilling apple cultivars in Yemen A. R. during 2 years. Acta Hort. 329:268-270.
Linvill, D.E. (1990). Calculating chilling hours and chill units from maximum and minimum temperature observations. HortScience 25(1):14-16.
Spiegel Roy, P. and Alston, F.H. (1979). J. Hort Science 54(2):115-120.
Westwood, M.N. and Chestnut, N.E. (1964). Rest period chilling requirements of Bartlett pear as related to Pyrus calleryana and P. communis rootstocks. Proc. Amer. Soc. Hort. Sci. 84:82-87.
Young, E. (1992). Timing of high temperature influences chilling negation in dormant apple trees. J. Amer. Soc. Hort. Sci. 117(2):271-273.