Flower numbers are influenced by:
- Light
- Tree vigour
- Food resources available within the tree
One of the main aims of winter pruning is to achieve the correct balance between fruiting and new growth. New growth is required to replace old fruiting wood and provide sufficient leaf area to support the crop.
Poor light levels in the tree will reduce both flower bud numbers and bud quality. Pruning should maximise light interception into all parts of the tree.
Tree shape
- The best tree shape is a well defined ‘A’-shape where the height of the tree is 1.5 times its width. (J.E Jackson et al). This shape is easily achieved in semi-intensive orchards but in wide spaced orchards the height will need to be reduced while still maintaining an ‘A’-shape.
- Within the tree, create a branch framework of evenly spaced branches which all point down to the edge of the tree space.
Treatment of the leader
Do not allow the leader to become too strong or dominant. This can be achieved by:
- Bending the leader into a gently ‘S’-shape.
- Not tying it too tightly to the stake and dead straight.
- Only pruning back into wood that will crop.
- Tearing off unwanted side shoots rather than cutting them off.
- Delaying any necessary pruning until early July.
- Keeping the leader narrow and with plenty of fruit bud.
A technique developed in Holland known as ‘klik’ (stub) pruning can be used in varieties where the leader naturally goes weak.
- In winter the leader is pruned back to a side shoot in the current year’s growth, and the side shoot is ‘klikked’ to 2 or 3 buds.
- In the following year the subsequent regrowth is then stubbed back again in the same way.
- The leader becomes stiffer and can be maintained at a constant height.
- In orchards with strong leaders the technique can also be used but the subsequent growth will be more vigorous.
- Although this technique does keep leaders to a standard height and is a simple method, it means that several cuts have to be made every year.
Treatment of lower branches
Improve light by:
- Removing strong upright shoots or bending them into a space.
- Removing heavy timber especially in the top of the tree.
- Where branches are overcrowded remove the dominant, vigorous ones first.
- Avoid ‘stubbing back’ branches as this promotes vigorous shoots further back into the tree, reducing fruit buds and creating more shade. If branches have to be shortened: Cut to 2 to 3 mm above the set of growth rings between one- and two- or two- and three-year-old wood. If a cut is needed further back along the branch always cut to a flat fruiting piece of wood and if this is not available crack the branch down to the correct angle to reduce subsequent growth.
The above techniques refer particularly to Cox. In Gala, Jonagold & Braeburn, cutting to upright shoots may be done without encouraging vigour, especially if the shoots have flower buds already present.
Where trees are too weak, where insufficient replacement wood is being produced or where too many flowers are being produced the following techniques can be adopted:
- Remove weak fruiting spurs and laterals particularly on the undersides or ends of branches.
- Reduce the number of branches, removing weak ones first.
- Stimulate the strength of branches by removing side branches and laterals.
- Check other causes of weak growth such as: poor soil structure, incorrect fertiliser regime, excess use of growth regulators, too heavy cropping.
Restructuring pruning of old and very large trees
Old and badly managed orchards are very difficult to get back into full production and it is usually better to grub the trees and replant. However, in a few cases it may be necessary to try and restructure old and very large apple trees.
- Reducing the large size of the trees by severe branch pruning is not to be recommended in isolation, as it will merely stimulate vigorous renewal shoot growth and minimal cropping.
- Hungarian research has shown that where severe branch pruning is essential to reduce the size of very large old trees, supplementary root pruning in the same season can help put the trees back into a better balance (see Brunner, 1990).
- Another alternative is to combine severe branch pruning with applications of plant growth regulators.
Application of plant growth regulator treatments
The application of approved plant growth regulators can be a valuable aid to tree management and can lead to a significant increase in flower numbers. Where continued reliance on growth regulators is made year after year, other tree management strategies should also be incorporated.
- It is generally accepted that treatments that retard shoot growth on apple trees usually result in an increase in flower bud initiation.
- The gibberellin-inhibiting chemical paclobutrazol (Cultar) can increase flower induction.
- Trials in the UK and abroad have demonstrated increases in floral abundance following use of Cultar (Shearing, et al., 1986).
- Providing the label recommendations are adhered to, no deleterious effects of the sprays on fruit set or fruit quality should be experienced.
The plant growth regulating chemical, prohexadione-Ca is also available for controlling shoot growth.
- In some, but not all, of the trials the treatments have also improved flower production.
- The product is less persistent than Cultar and the risks of overdosing are minimal.
- Where apple trees are making excessive shoot growth and are flowering poorly, treatments with Cultar will reduce subsequent shoot growth and increase flowering.
Trials over the years have shown the benefit of applying repeated low doses of growth regulators rather than fewer high doses.
- Suppressing growth early in the season before it gets away is also advantageous.
- Exact timings and rates of application are varied subject to individual orchard conditions and the need to reduce any potential side effects and should be discussed with a suitably qualified and experienced advisor.