Deciduous fruit trees need sufficient winter chilling during dormancy for satisfactory bud break, fruit set and tree growth.
No noticeable growth takes place during dormancy—a physiological condition regulated by plant hormones. Dormancy is completed when trees have had sufficient hours of chilling.
Trees stay in a holding state (or ecodormancy) and buds will not break until temperatures increase in spring.
Many of our fruit trees originated in the temperate zones with warm summers and cold winters. The rest period enabled the fruit trees to survive the cold winters. However, when winters are too mild or too short, one of the serious consequences is delayed foliation.
This is characterised by an extended period of flowering which can lead to poor fruit size and poor quality of fruit. Buds and flowers also drop off, with buds developing poorly or not at all and vegetative growth is reduced or abnormal.
Vegetative buds have low vigour, may form rosettes and yellow leaves. Fruit buds that have had insufficient hours of chilling may have small styles or no styles at all, low pollen counts, small petals, early aging of flowers and no viable seeds in the fruit.
Chilling requirement
The hours of chilling needed by a tree to resume normal spring growth is referred to as its chilling requirement.
The chilling requirements of different varieties can be classified as low chill (sub-tropical), moderate chill and high chill.
Because we cannot measure what is actually happening in a fruit tree as it goes through winter, we try to relate winter temperatures to the response of the tree in spring. There are many possible methods of trying to do this including measurement of hours of chilling (or chill units).
Winter chilling index
The most common method used in cool to cold inland regions is to calculate number of hours of chilling between 0C and 7.2C (45F).
While this method is far from perfect, it provides a winter chilling index that fruit growers can relate to and use as a guide.
Richardson method
The Richardson method uses a mathematical equation and hourly temperatures, and is based on the accumulation of chill units where one chill unit equals one hour of exposure at 6C.
The chilling contribution becomes less than one as the temperature drops below or rises above the optimum value.
A negative contribution occurs at temperatures above 15C and a zero contribution below 0C.
The Richardson method has shortcomings in regions with mild winters.
Conversion chart
You could also relate the average temperature of the coldest month (July in Australia) to the chill hours in winter, which gives a good indication of the chilling of winter for many climates. The conversion chart (Figure 1) can be used as a guide to estimating chill units.
To use this chart, obtain the average maximum and minimum daily temperatures for the coldest month of the year. Apply this figure to the lower line of the chart (average temperature) and cross reference that figure to the graph line to work out approximate accumulated chill units for that area.
For example, if the average temperature for July is 11C, then the chill units would be about 750 hours. Local weather stations should be able to supply average monthly temperatures.
Dynamic Chill model
The fourth model was developed in Israel and is called the Dynamic Chill model. It converts temperature into units called chill portions.
The Dynamic Chill model incorporates many of the observed relationships between temperature and dormancy breaking when allocating chill portions.
Physiological method
There is another (physiological) method to determine the chill units (hours), by regularly collecting twigs or laterals from the orchard and putting them into a room where they can be kept at a constant 18C.
If the buds on the twigs or laterals develop within a 2-week period, they will be considered as having reached their required chill units. It is then a matter of checking the amount of chill units accumulated up to the point when that particular sample was taken from the orchard.
These methods or models show orchardists any potential insufficient winter chill, so that they can take appropriate steps to manage it.
Minimise the risk
When you can see symptoms of delayed foliation it is too late and the damage has been done.
(continued next issue)
See this article in Tree Fruit May 2018
(Taken from the manual, Apple on Open Tatura available here)