Fruit Set is affected by hours of cold

For any deciduous fruiting plants to set flowers and fruit they require a certain amount of winter dormancy. This is the total of hours the air temperature is between 0 - 7 degrees Celsius during the dormant period, or average winter, for a given area. This does not have to be consecutive days it is the total accumulation of hours during this period. However higher temperatures during this time can also offset chilling hours with temperatures above 15°C detracting from the total.

Temperatures below 0°C are not usually counted towards the total number of hours. Therefore an hour is subtracted for every hour below 0°C and for every hour above 15°C
 
 
Every region will have its own chill hours and so does every species and variety. Like many aspects of horticulture there are also variations in tolerances – some species with a preferred 1000 hours chilling requirement may still produce some fruit at 500 chill hours. However if a variety gets far more CUs (chill hours) then it needs, then it may never bloom and consequently it won’t set fruit. The opposite may occur when a variety that needs low chill hours receives far too many, this triggers a break in dormancy and early flowering - which may not be pollinated. Therefore cultivars should be selected to suit the chill hours of the region. Conditions can of course also vary from year to year a year and extremes can affect production. Unexpected hard frosts during the blooming stage will damage and often kill the flowers and that tissue damage may also occur, there will also be no fruit set.
 
 
Symptoms of Insufficient Chill Hours
Late, delayed and unusual emergence of new leaves - in spring leaves appear in small tufts at the tips of stems with bare spaces of around 30cm between the tip leaves and the lower leaves. Fruit set is compromised and so is the health of the tree.
 
Low fruit set – delayed or extended flowering leading to abnormal pollination and reduced fruit set. Some fruit may be produced but it will be small and often misshapen.
Poor fruit quality – firmness, colour and quality of fruit reduced, ripening delayed or fruit may not ripen at all. Small fruit is produced irrelevant of correctly applied cultural practices such pruning, fertilizing.