At the end of natural fruit drop in December the number of fruitlets remaining on the tree will provide a good indication of the final crop load. At this point it is important to do a crop load estimate to allow you to make decisions about any further thinning that may be required.
Hand thinning is the final management tool available to manipulate crop load and increase fruit size before harvest. Once the cell expansion stage is reached (Jan - May) the cells inside the fruit stop dividing or multiplying. This means the fruits final size may be predetermined as early as January providing the trees are not stressed.
Hand thinning should be done as early as possible after fruit drop to achieve the greatest size benefit. Hand thinning varieties that continually produce large crop loads with fruit clusters (e.g. some mandarin varieties) will result in increased fruit size and more marketable fruit. The competition for nutrients between fruits, particularly those in clusters, is reduced by thinning resulting in an increase in the supply of carbohydrates to the remaining fruit.
The option of hand thinning gives growers the ability to manipulate crop load and remove poor quality fruit from the tree in one operation.
The objective of hand thinning is to:
- remove damaged fruit first
- remove small fruit
- thin clusters to one or two fruit
- thin to prevent fruit touching
- remove fruit hanging close to the ground
- leave terminal fruit on Imperials
For varieties sensitive to sunburn and sun bleach, leave extra fruit on the tree at the first thinning to allow for the removal of sunburnt fruit after the hazardous period. This is especially important where many fruit are exposed and will depend on the amount of protection offered by the tree canopy.
Hand thinning is limited by factors such as the high cost of labour and the limited extra growth achievable in years with moderate crop loads. It remains an option if crop load is excessive and no other method of crop manipulation has been implemented.
Harvest timing can significantly impact on flowering and fruit set in the following season. Where fruit of a variety is left to hang on the tree late (to obtain a better market window or improve fruit eating quality) a reduction of flowering the following season can often occur. This is particularly true of many mandarin varieties.
Harvest management can also increase the current seasons fruit size. By carrying out multiple picks the largest and most coloured fruit can be removed on the first pick allowing smaller and less mature fruit more time to size. GA sprays are often used to control rind ageing but will have the effect of prolonging the harvest season. If applied too early GA will cause fruit to retain green colour and may make degreening more difficult.