Cherry production is characterised by rapid leaf development, rapid fruit development and a long post-harvest period. Without well-balanced crop loading fruit size is reduced. Critical periods include:
- 50 to 60 days from blossom to harvest for early to mid season varieties; and 100 days for late season varieties.
- 30 days after bud burst for full leaf development
- Shading occurs from 30 days after bud burst reaching a maximum at 60 days.
- 25% of final fruit weight is gained in the last week.
Leaves—the critical supporting act
Leaves, through photosynthesis, produce the structural carbohydrates and sugars that are critical for the growth of a cherry fruit. Not only do they produce the food but they also aid in mobilising nutrients and reserves from the rest of the tree and the soil.
While leaves are recognised for their pivotal role in growing bigger and better cherries, growers will need to know if there are enough, whether they are healthy and that the leafy canopy of a cherry tree is not limited in its ability to size the fruit. The leafy canopy can be viewed in a number of ways.
Light:
Cherry leaves become photosynthetically ‘competent’ earlier in the season than other species. This is what enables the cherry tree to size a mature crop in a very short growing season. This activity occurs despite a relatively small leaf area and cooler weather.
The young fruit are fed by the spur leaves next to the fruit, other spur leaves, some of the newly emerged leaves and reserves in the root system and trunks. Lack of light is very detrimental. Even a 10 to 15% reduction will reduce fruit set and fruit quality especially sugars and flavour.
Although fruit size is not directly affected, there are two indirect effects. First, the reduced fruit set results in fewer fruit, which can reduce the number of small fruit. However, the second indirect effect can negate this size improvement.
Orchard economics often demand a ‘once over’ harvest. Fruit in shaded areas will be picked at least five to seven days early and will not reach their full potential size or optimum sugars resulting in a tart or sour taste.
Note: ‘Shade leaves’ are larger and thinner. These are a good indicator of poor light, as are longer and thinner shoots. A dappled rather than a solid dense shadow under the trees indicates good light penetration.
Leaf:fruit balance:
There is no absolute figure for this ratio in cherry trees but one study in Washington in 1987 showed that by increasing leaf area up to 100 cm2 per fruit, fruit size continued to increase. In practical terms, that equates to two healthy leaves per fruit. One useful orchard guide is the visibility of fruit.
If you can see 20% of the crop among the leaves the balance is good but if most of the fruit is visible the tree is over-cropped and unbalanced and the cherries will not reach their full size potential.
Pruning—managing the leaf canopy:
Regular pruning is essential. It not only develops the optimum canopy but it also thins potential fruit buds to maximise fruit size. The following pruning suggestions will help:
- Four-year-old spurs and older produce smaller fruit. Renewal pruning and removal of old spurs will improve fruit size.
- Remove shaded wood on the inside of the tree.
- Thin out two and three year old well-spurred branches.
- Summer tipping after harvest is worth trying.
- Head two- and three-year-old branches to reduce buds.
Cultural management:
Establishing the right balance between leaves and fruit provides the potential for maximising fruit size. Managing water and nutrients will ensure the potential is fulfilled. Best practice principles are the foundation, with the following points being particularly relevant:
- Strategic water (such as deficit irrigation) can be used on later varieties in late districts. Summer rainfall can reduce the stress response but the strategy is still worthwhile.
- Fertilising after harvest will increase nutrition available to buds instead of leaves.
- Healthy flower buds produce larger flowers and potentially larger fruit.
Thinning:
As with all stone fruit, basic crop thinning is achieved by pruning. In fact, in a commercial situation that is the only way cherries are thinned as the short growing season, small fruit size and large number of fruit per tree has stopped any thought of fruit thinning by hand.
This may change with some developments in chemical thinning. Blossom dessicant chemicals used for other deciduous fruit crops may help in reducing crop load. Preliminary testing is promising particularly in removing fruit from around the growth ring at the base of the current season’s growth.
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