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A feature of all deciduous woody fruiting crops is that during the winter they drop their leaves and go into a resting stage which we call dormancy.
The process itself commences at time of leaf drop or close to it, and comprises of two distinct phases. During the first phase a strong growth inhibitor is present, and while this inhibitor is active the dormant buds cannot be forced into growth by placing the dormant plant or shoots into a warm temperature situation such as a glasshouse. The grip of this growth inhibitor can be broken by a period of chilling, which in places with a distinct stable winter climate it usually takes 700 to 1000 hours below 70C, depending on species and cultivar, to satisfy the chilling requirement.
Once the chilling requirement has been satisfied, the second phase of dormancy commences. During this period, buds lose their cold hardiness and move into a rapid development phase which ends in bud break. This stage is a heat driven process. Under cool spring conditions it is a slow process, which, if earlier winter chilling has been marginal will result in a drawn out bud break. If spring temperatures are high, bud break will occur quickly and the period of bud break will be compressed.
In fruit growing areas which have mild winters such as in our milder coastal districts, South Africa, much of South America or Israel, the number of hours below 70C has not given a good estimation of chilling. For instance, in Auckland in mild winters the hours below 70C indicate chilling levels of around half or even less than the level reported in the literature for apples, yet we see reasonable bud break performance.
To try and overcome this weakness of the old hours below 70C theory, chilling unit models based on a wider temperature range have been developed.
The first of these was the Utah or Richardson Chill Unit model. This model assumes that the most biologically active temperature range to satisfy dormancy breaking occurs over the 2.5 to 9.10C range, to which one chilling unit is given. Between 1.5 and 2.40C, or between 9.4 and 12.40C, half a unit is given. Below 1.40C and between 12.5 and 15.90C zero units are accumulated, with negative units above 160C. Between 16 and 180C half a unit is subtracted for each hour within the temperature range, while above 180C one unit is subtracted.
This model fits better for milder climates and also blows away the myth that frost is necessary for dormancy breaking. There are, however, some studies which suggest that winter chilling can be helped by a sudden frost or cold snap during the leaf fall period to precipitate the plants rapidly into full dormancy.
In climates that experience short periods of high temperature during the winter, such as we often experience from warm north-westerly wind events on the east coast of New Zealand, there are weaknesses in the Richardson Chill Unit model. To overcome the problem of short periods of high temperatures, research carried out in South Africa and Israel has lead to a modified model which assumes that after a certain period of cold a stable chilling unit is formed which cannot be broken down by high temperatures.
The model assumes fastest accumulation of chill units at night temperatures of 6 to 80C alternating with day temperatures of 13 to 150C, with negative scoring at temperatures above 190C. This is a complicated model and has been replaced by a system of daily positive chill units which can be read off a simple table showing a grid of maximum and minimum temperatures.
In a typical Hawke's Bay winter, average daily accumulation for June is estimated to be 18 units, and for July 19 units. Using this data, we can expect about 900 chill units to be accumulated by the end of July.
Once 900 units have been accumulated, the initial growth inhibition phase of dormancy is probably over, and the buds will now respond to daytime heat accumulation and develop towards bud break. After 300 to 500 growing degree hours at base 100C have been accumulated, the buds reach the stage when they become responsive to the dormancy breaking compound hydrogen cyanamide.
Noting time of leaf fall or the first significant air frost during leaf fall will provide the starting point for the commencement of your winter dormancy period.
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