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Initiation and development of sufficient high quality flowers by the apple tree

Introduction

It is essential that apple trees produce sufficient flowers to set an optimum crop of fruit in relation to the size of the trees. It is also vital that these flowers are of sufficient quality to set fruits reliably and to produce large fruits of good texture and storage potential.

On young newly-planted trees the objective is to induce quality flowers on the trees in the second and third years after planting i.e. to improve their floral precocity. If orchards are to break-even rapidly and move towards profitability on the investment, cropping early in the orchard’s life is essential.

On mature trees, lack of abundance of flowering is a relatively rare problem and usually only occurs with varieties that are prone to biennial bearing. Fortunately, most of the traditional varieties suffering from this problem (e.g. Miller’s Seedling, Laxton’s Superb) are no longer grown commercially, but Elstar can become strongly biennial in its cropping. The varieties Cox and Braeburn are weakly biennial. However, flower quality is often a limiting factor in fruit set and retention on mature trees.

Occasionally, excessive flowering and inadequate new extension growth may be a problem with some varieties grown on dwarfing rootstocks such as M.27 and M9 on replant soils. Here the objective must be to reduce the abundance of floral spurs and increase shoot growth, so as to bring the trees back into a more optimal balance of fruit production with shoot growth.

 

Improving the initiation and development of high quality flowers by the apple tree

Flower initiation occurs early in the summer of the season prior to flowering and, during that season, many of the flower parts are formed. However, the final stages, which involve the formation of the pollen sacs and the ovules, do not occur until shortly before bloom in the subsequent spring.  High quality flowers are essential if fruit set is to be achieved in the often less than ideal weather conditions at flowering time in UK orchards. Flower quality influences whether fruits are retained on trees throughout the season and also their size and quality at harvest.

Adequate floral abundance and good flower quality are essential considerations for both mature and young, immature trees. Flowering and fruiting on young apple trees in the second and third year is particularly vital if the investment in the orchard is to be economically viable.

 

The importance of good flower quality

Flowers on apple trees differ in their potential to set and retain fruits and these differences are usually referred to as differences in flower quality. Flower quality is the ability of a flower to set and carry a fruit. The most common measurement of flower quality is the Effective Pollination Period or EPP.  

Types of flower buds

Spur buds:   Formed on spurs and short terminal shoots. These begin their initiation shortly after fruit set in the preceding season.

Axillary buds:  Formed in the lower (basal) leaf axils on extension shoots later in the season than spur buds, usually as shoot growth slows down in mid to late summer.

The aim must be to encourage the rapid production of high quality flowers on the spurs and short terminal shoots of young trees. Only with a few varieties, such as Gala and its sports, can axillary flowers be relied upon to contribute significantly to cropping on young trees. Even with Gala, axillary flowers are only useful if they are produced on strong wood of approximately 10 mm diameter.

Factors influencing flower quality

Anything that reduces the production of resources by the tree or diverts them away from the developing flower buds will reduce their quality.  An imbalance of hormone levels in the tree will also affect flower bud development. The main factors influencing flower quality are:

  • Crop load (too many fruits on the tree): Reduce flower bud numbers in the following year especially in varieties with a tendency to biennial cropping. Thin as early as possible to the correct crop load.
  • Too much vigour (length of shoots and numbers of shoots) will create an imbalance in the tree and reduce the quality of flower buds: Adopt practices to reduce vigour.
  • Lack of light and shading will significantly reduce flower bud quality: Prune to improve light penetration into the canopy.
  • Poor leaf quality; poor nutrition and disease (especially mildew) will impact directly on flower bud quality: Mildew should be controlled all the way through the season to prevent infection in developing buds which will carry over to the next season. Nutrient levels should be checked by visual and analytical techniques.  It is particularly important to avoid excess nitrogen levels and maintain adequate phosphorus and trace elements to ensure good flower bud development.

 

Improving flowering on young newly-planted apple trees

If apple orchards are to prove profitable it is important that they begin cropping as soon as possible after planting (i.e. exhibit good yield precocity) and then rapidly build up cropping to high and consistent yield efficiencies in the succeeding years.

The yields in the first few years of the orchard’s life are vital if the investment is to prove viable. Investment appraisals using techniques of Discounted Cash Flows or Net Present Values highlight the importance of yields achieved in the first five years of an orchard’s life.

To achieve these yields in the first few years following planting it is essential that the trees produce adequate numbers of flowers and that these flowers are of sufficient quality to set and retain fruits through until harvest. This precocious cropping must not, however, be at the expense of the trees continuing to grow and fill their allotted space within the orchard. Balanced growth and cropping should be the aim in these early years following planting, as much as in the subsequent years. 

When planning and planting young trees the importance and influence of the following should be considered:

  • Well-feathered trees
  • Choice of rootstock and/or interstock
  • Tree age/size and transplanting check
  • Soil and foliar nutrition
  • Pruning and training techniques
  • Plant growth regulator treatments

The importance of planting well-feathered trees  

A well feathered two-year old apple tree

Plant well-feathered trees with healthy, non-desiccated roots. Tree quality at the time of planting has a significant influence on how well the young trees establish and how quickly they begin to flower and fruit.

The onset of flowering is advanced and the abundance and quality of flowers improved if well-feathered trees are planted in good fertile soils.

Trees with 6 to 8 strong well-spaced feathers with wide branch angles should be chosen.

It is also important that these relatively large trees receive good water, nutrition and soil management to minimise the transplanting check to growth. Excessive transplanting check to growth will cause tree stunting and may lead to the production of branches with bare wood.

  • Various nursery techniques are used to improve tree quality
  • Where available choose trees guaranteed virus-free and true-to-type: Healthy trees establish more quickly and crop more abundantly.
  • Choose the most appropriate rootstock for the site/soil conditions, the chosen scion variety and the planned system of management: Irrespective of the vigour of tree desired by the apple grower, always choose  the rootstock, or rootstock/interstock combination, which is likely to induce  precocious cropping. Most dwarfing rootstocks (M.9 and its clones e.g. Pajam 1 and Pajam 2, M.27,  P.22 etc.) improve precocity of cropping. Unless soil conditions are excellent avoid use of very dwarfing rootstocks such as M.27 and P.22. Amongst the more vigorous rootstocks MM.106 and M.116 both induce good floral precocity in scions and are well suited to use in cider orchards. Use of dwarfing rootstocks as interstocks also improves floral precocity of scions.

Post planting management

Trees planted in the autumn into deep, highly fertile soils with adequate supplies of water throughout the subsequent season should establish very well and make abundant shoot growth. When planting small or poorly feathered trees this response is desirable. However, where larger than average trees are planted, such conditions could result in excessive shoot growth and the production of fewer, poorer quality flowers in the second season following planting.  Tree management after planting should aim to achieve balanced growth and precocious cropping.

Take steps to improve flower quality (i.e. ability to set fruits) on young trees. By being aware of the limitations on fruit set associated with poor flower quality on very young trees, it is possible to improve the situation by:

  • Applying appropriate fertilisers, as determined by soil and leaf analysis,  preferably through a fertigation system.
  • Bending branches down on upright varieties, or varieties with poor fruiting habit, can improve flower quality and subsequent fruit set. This is not necessary in very precocious varieties.

Ensure good tree establishment and avoid bare wood developing by:

  • Planting in good conditions, preferably before Christmas. Delayed planting can result in a planting check which is generally undesirable although in some circumstances may be beneficial e.g. stringfellowing
  • Providing adequate water and nutrients (both to the roots and by foliar feeding).

Growers should obtain analyses of the soil mineral and organic matter contents prior to tree planting. Based on these analyses and the current recommendations concerning optimum levels (RB209) any necessary base dressings should be applied.

Supplementary nutrition of trees in their first or second years following planting have been shown to improve crop levels in intensive plantings of 2000+ trees per hectare.

Supplementary irrigation should be applied, preferably using trickle systems, when soil moisture deficits reach 50 mm.

Prevent stress to the trees (e.g. wind desiccation).

Prune and train trees so as to establish ideal balance of growth and flowering:

  • Remove excess feathers, especially those with strong upright growth and poor branch angles. This will help produce better leaf and flower quality on the remainder. Aim for 6-8 good feathers. Remove low and unwanted feathers
  • Do not tip the leader except when poor quality trees with very weak or few feathers are used. Where good, well feathered trees are planted, leaving the leader unpruned will encourage the development of weak fruiting laterals and give natural growth control of the leader which is ideal in intensive orchards.
  • Tie down lateral growth towards the horizontal or below this angle on very strongly growing varieties.

Use low doses of chemical growth regulators where necessary to achieve reduced shoot growth, improved balance of growth and improved fruit set.

  • Low concentration treatments with Cultar may aid flower production on young trees that are growing too vigorously.

 

Improving flowering on mature apple trees

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.

 

Influence of increasing concentrations of Cultar on Bramley shoot growth

  • 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.

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

 

Flower quality on mature trees and its improvement

Growers should strive to produce flowers of high quality with an Effective Pollination Period (EPP) of 3 days or more.  For optimum flower bud quality:

  • Trees should be thinned early in the previous season to optimum crop loads.
  • Fruit harvesting should not be delayed too long, as late picking may reduce flower quality on varieties such as Bramley and Braeburn.
  • Maintain good tree health status by judicious use of pest and disease control  measures.
  • Use nitrogenous fertilisers sparingly in the spring and summer prior to harvesting so as to reduce shoot growth which competes with developing flower buds.
  • Consider foliar or ground (taking into account Nitrate Vulnerable Zone restrictions) applications of nitrogen in the autumn following harvesting, but before leaf fall, to improve the quality of flowers in the subsequent spring.
  • Adopt winter pruning techniques that improve light distribution through the tree but do not encourage excessive growth. This is best achieved by making a few larger cuts.
  • Summer prune to allow adequate light into the centres of the trees.
  • Bend shoots towards the horizontal or below, but this should not be carried out to excess or the optimal balance of new renewal shoot growth and flower bud production will be lost.
  • Growers should take note of the average maximum temperatures in February, March and April in their Cox orchards, as these have a significant bearing on flower quality: Where temperatures are higher than desired average maxima (i.e.10oC or higher), they are advised to intensify their efforts to secure good pollination and flower fertilisation. This can be achieved by supplementing bee populations, providing increased shelter and controlling early growth straight after petal fall using an approved growth regulator. Where temperatures are of the desired average maximum, or lower, growers will need to consider implementing appropriate thinning procedures.

Delaying flowering in the spring using plant growth regulators and other spray treatments

Pollination and fruit set are often reduced severely by spring frost damage to the flowers. One possible strategy that has been investigated in attempts to reduce this risk is delaying flowering in the spring using plant growth regulators or other spray treatments.

  • Although sprays of various chemicals can influence the time of spring flowering in the previous autumn or winter, none of these treatments are approved for use in UK apple orchards.
  • Further research may be warranted, examining misting techniques to delay flowering times of apple varieties.