Occasionally, insufficient flower numbers may be produced on varieties which are not usually prone to biennial bearing. This is usually the result of practices of tree management that lead to an imbalance between growth level and cropping.
- Excessive use of nitrogenous fertilisers and/or irrigation or severe pruning can all result in vigorous extension shoot growth and the production of too few flowers.
- Good tree management entails balancing the production of flowers with adequate new shoot growth.
Although, theoretically, trees should be able to produce sustained fruit production on a tree with a permanent spur system and minimal new shoot growth, many trials have shown that fruit size and quality slowly diminish on such trees.
Examples of this are trees planted on M.27 rootstocks, where insufficient renewal pruning is practised. Another example is trees producing no new shoot growth following excessive applications of growth regulators.
The first pre-requisite is to have some idea of the likely numbers of flowers developing on the apple trees. This assessment may be done pre bloom by dissection or in the orchard from blossom onwards.
Assess the correct numbers of fruit buds needed for an optimum crop
Take into account the variety, the growth habit of the trees and the cropping history. Assume 1 to 2 flower buds will be required to produce one harvested fruit. This is, however, dependent upon scion variety, tree vigour and site conditions
Cox, Bramley and Discovery
- In well managed, regularly cropping Cox orchards the average fruit set will be between 15% and 20%.
- Each fruit bud produces a cluster of 5 or 6 flowers. Therefore, assume one fruit will set from each flower cluster.
- If 100 fruits per tree are required, leave between 100 and 120 fruit buds.
- The fruit set in vigorous, shaded trees can be much lower and 2 fruit buds will be required to ensure one fruit at harvest.
Gala, Jonagold, Egremont Russet, Braeburn, Jazz, Kanzi and Rubens
- These varieties set more readily and also crop well on one-year wood.
- A setting level of approximately 200-250% can be achieved
- Therefore 80 buds should produce 100 fruits.
Increasing flower numbers on mature trees
Apple trees that are managed well should produce adequate numbers of quality flowers consistently from season to season. Where problems of insufficient flowering occur it is important to identify the cause of the problem and take the necessary remedial action. Where insufficient flowers are present it will be due to one or more of the following factors:
- Too much vegetative growth
- Lack of light in the tree canopy
- Too heavy a crop in the previous year
Poor flowering due to excessive vegetative growth
Where problems of excessive shoot growth and poor flowering on mature or semi-mature apple trees are experienced it is important to adapt pruning techniques in relation to the amount of fruit bud.
- Where insufficient fruit buds is the result of excessive vegetative growth pruning and tree management practices should be changed.
- When pruning in the winter prune only lightly, making fewer larger cuts.
- Concentrate on branch removal to achieve correct tree shape and let more light onto cropping branches.
- Delay leader pruning until the mid summer.
- Bend shoots to the horizontal (Gala, Braeburn and Jonagold) or below (Cox, Discovery, Bramley.)
- Increase use of plant growth regulators (such as Cultar and/or Regalis). Aim to stop growth before it becomes too strong, using winter and spring treatments. Timings and rates should be specific to individual orchards. Both materials can have unwanted side effects. Seek advice for specific recommendations.
- Only summer prune to remove strong upright shoots and those shading fruits within the canopy. When summer pruning, remove whole shoots back to their base. Do not head back shoots.
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| Influence of increasing concentrations of Cultar on Bramley |
If the above adjustments to management practices prove inadequate and flowering is still insufficient growers can:
- Consider girdling (bark ringing) of the trunk as a means of improving floral abundance and reducing excessive shoot growth. Care must be taken, however, when bark ringing varieties that are sensitive to diseases such as apple canker (Nectria galligena).
- Consider root pruning down one side of the tree in November or in the early spring six weeks before bud burst. Specialist root pruning equipment should be used, placing the blade between 50 and 75cm from the trunk depending on the degree of growth control required. Ideally supplementary irrigation should be available in dry summers to avoid any possible reduction in fruit size.
Poor flowering due to lack of light into the tree canopy
A lack of light within the tree will reduce flower bud numbers and their quality. As shoots tend to grow strongly towards the light, further shading can be induced.
Light levels within the tree canopy can be improved by:
- Changing pruning techniques to reduce shading. This is particularly important in the top of the tree to reduce the height and width of the upper tree canopy.
- Creating a well-defined ‘A’- shaped tree in spindle trees.
- Removing lower branches if they remain shaded.
- Removing strong upright shoots or bending them into a space.
- Reducing the height of windbreaks surrounding the orchard.
- Use of light-reflecting material on the orchard floor
Poor flowering due to excessive cropping in the previous season
The number of seeds in the fruit produced in the previous season will have a major effect on the numbers of flowers formed. To reduce this effect growers should:
- Thin as early as possible, to reduce the fruit numbers and hence the seed numbers causing the flowering inhibition.
- Not delay harvesting more than two weeks after the optimum date as this can reduce flower numbers, especially if other risk factors (e.g. vigour and shading) are also a problem.
Overcoming biennial bearing
With varieties that have developed a biennial pattern of cropping growers should:
- Prune away excessive spurs in the winter prior to an ‘on’ year.
- Thin blossoms on biennial varieties in the ‘on’ year.
- Thin, preferably at flowering time, using ATS.
No chemical treatments, other than thinning treatments are approved for use against biennial bearing