Deciding whether to reduce the crop load (by thinning or other methods)
It is important to maximise yields of apples in the grades most desired by the markets in order to sustain reasonable profitability.
- Maximising fruit set and total yields is rarely the best strategy, as this usually results in the production of too many fruits of small size and poor storage potential.
- In contrast, to maximise the yield per tree of top quality fruits it is important not to thin trees more than is necessary.
- Decisions on whether to thin or not are, therefore, not always easy to make.
For some crops, decision support models are being developed to aid fruit growers in making these decisions. One example is a model focused on crop loading of kiwi fruit vines developed in New Zealand (Mills and Atkins, 1992).
- No sophisticated crop loading models exist currently for apples grown in the UK and the grower must use experience of the orchard, knowledge of weather conditions and, often, natural intuition in making decisions on the need to thin.
The main factors which determine the need or not to thin an orchard are:
- The variety
- The rootstock
- The market requirements for the fruits
- The current season’s weather
- The condition of the tree
The variety
Small-fruited varieties, such as Gala and Cox and their various clones, are more likely to require regular thinning than larger fruited varieties such as Jonagold, Braeburn or Bramley.
- If the small-fruited variety is also prone to set heavily, as Gala is, then the need for seasonal and early thinning becomes even more acute.
The rootstock
Trees grown on very dwarfing rootstocks, such as M.27, need thinning in most years if adequate fruit size is to be achieved. This is particularly important when growing varieties such as Gala or Cox on this rootstock.
Average fruit size on the popular rootstock M.9 (EMLA, Pajam 1, Pajam 2, etc.) is generally better than on most other rootstocks and thinning severity will not need to be as great as for trees on M.27.
- However, where M.9 is planted on soils of lower than average fertility or where temporary drought conditions prevail the shallow roots of trees on M.9 will make them vulnerable to producing small fruit sizes unless irrigation and/or judicious thinning is carried out.
Trees on the rootstocks MM.106 and P.22 are also prone to overset and small fruit size on many sites and careful attention should be paid to adequate thinning.
The market requirements for the fruit
Most dessert apple varieties are grown for the fresh markets. Multiple retailers will now take a range of sizes and colour to satisfy their demand for consumer choice
Where fruits are marketed through alternative channels (farmers’ markets, farm shops etc.) different standards of background colour and fruit firmness may be acceptable.
- Care must be taken not to over-thin large-fruited varieties such as Jonagold, or fruit too large for the UK consumer markets may be produced.
- Bramley fruit, especially those grown for the fresh market, are usually required to be in the range 90-110mm. This requires careful crop load management and pruning
- Slightly smaller fruits and hence higher crop loads can be tolerated where the Bramley are focused on the processing markets.
The current season’s weather
This is often the most important factor to take account of when deciding whether to thin or not.
- Frost or cold, windy or wet weather during the flowering period usually equates with poor fruit set.
- When these unfavourable conditions prevail, growers are, understandably, reluctant to contemplate early (blossom) thinning.
- In these situations decisions on thinning are usually delayed until after fruit set, when the potential crop load can be assessed.
- Whilst this decision was sound in the past, as later thinning with Thinsec (carbaryl) was always an option, the withdrawal of carbaryl for use as a thinning agent, means that until BA is approved, hand thinning will remain the only option.
- Whilst declining to blossom thin will always give the best assurance of adequate fruit set should a frost occur in late bloom, it will be associated with a high cost for hand thinning if fruit set turns out to be bigger than required.
Temperatures during the cell division period (full bloom through to six weeks later) will influence fruit size.
- Where this period experiences higher than average temperatures, fruit size is more likely to be adequate.
- The exact relationship is not fully understood and fruit numbers per tree will override the temperature effect.
The condition of the tree
Trees in healthy condition with minimal shoot growth, dark leaves and many fruitlets that are clearly set should be thinned because such trees are likely to retain excessive numbers of fruitlets through to harvest.
- However, such healthy trees are also able to support and size up more fruits than less healthy trees.
Trees in less healthy condition or those lacking nitrogen, as often found in organic plantings, also need thinning.
- Often this needs to be more severe thinning, as the weakened trees are able to size up fewer fruits per unit tree size than healthy or well fertilised trees.