Most apple varieties are self-sterile and require pollen from another variety to achieve effective fruit set. Pollinating varieties, either other dessert or culinary varieties or ornamental crab apple varieties are planted in the orchard to provide this pollination. Take account of the following when making decisions on choice and abundance of pollinators required:
Summary of pollination requirements of the principal commercial varieties
Varieties differ in their self-fertility and their pollination requirements as noted below:
| Varieties | Pollination requirements |
|---|---|
| Cox and Queen Cox | Weak self-fertile but only at temperatures of 20-25ºC |
| Q Cox clone 18 | Fully self-fertile |
| Braeburn, Fiesta, James Grieve and Gala | Partially self-fertile, will set crops with their own pollen but a pollinator is recommended |
| Jonagold | Has the ability to set fruits without pollination (therefore, seedless) given suitable weather |
| Other varieties | Require a pollinator |
What constitutes an adequate supply of pollen therefore varies with the variety. It is also influenced by the season and the vigour of the orchard.
The importance of compatibility with the main commercial variety
It is wrong to assume that any two varieties of apple flowering at the same time will always be compatible with each other and capable of setting fruits when pollen is exchanged. Although, given favourable weather conditions during flowering this is mainly true, there are exceptions to this rule:
- Consider choice of pollinators carefully, avoiding unsuitable pollinators, especially on sites where conditions for pollination are less than ideal.
- Do not use triploid varieties, such as Bramley’s Seedling, Jonagold or Boskoop as pollinators for other varieties. They produce only small amounts of pollen most of which is sterile.
- Do not use Red Pippin as a pollinator for Elstar; unless climatic conditions are very favourable the varieties may prove incompatible.
- Do not use Falstaff or Greensleeves as pollinators for Gala, unless climatic conditions are very favourable the varieties may prove incompatible.
- Do not use Gala or Greensleeves as pollinators for Falstaff, unless climatic conditions are very favourable the varieties may prove incompatible.
Partial compatibility, where only 50% of the pollen is capable of growth down the style, may be experienced when using the following varieties as pollinators for Cox: –
Alkmene, Elstar, Gala, James Grieve, Falstaff, Red Pippin.
Partial compatibility, where only 50% of the pollen is capable of growth down the style may be experienced when using the following varieties as pollinators for Gala:
Alkmene, Arlet, Cox, Elstar, Golden Delicious, James Grieve, Red Pippin, Summered, Worcester Pearmain
Partial incompatibility is not a problem where ratios of pollinators is relatively high, blossom abundance on the pollinators and pollen supply is high and conditions for pollination are good.
Climatic influences on compatibility/incompatibility relationships
When weather conditions are very favourable during flowering some of the full and partial incompatibility mentioned above is overcome. Temperatures of more than 20 or 25oC will often be sufficient. Pollen quality and viability can clearly be affected by weather conditions.
- Pollen germination and growth down the style is greatly aided at temperatures of 15‑20oC.
- At higher temperatures (e.g. 25oC) pollination efficiency is improved with variety combinations, which are normally incompatible or show only partial compatibility.
- Winds cause pollen desiccation and often death.
- Frost causes death of pollen and the female parts of the flowers. The damage is not always visible.
Self fertility
A few varieties of apple show either full or partial self-fertility. This means that they can, given favourable climatic conditions during flowering, set fruits with their own pollen.
Self-fertile clones
Two varieties, Queen Cox Self-fertile Clone 18 and Cox Self-fertile Clone 8, both produced in the last 20 years, are fully self-fertile and can be planted without pollinators.
- Self-fertile Queen Cox clone 18, which is available from UK nurseries, gives more reliable cropping than the traditional self-sterile clones in years unfavourable for pollen transfer between varieties by bees or other insects.
- This self-fertile Queen Cox clone should not be used to pollinate other varieties as it produces insufficient viable pollen.
- Growers considering purchasing self-fertile Queen Cox clone 18 are recommended to obtain this only from a verified source.
Planting partially self-fertile varieties without pollinators
A few varieties show partial self-fertility, especially when temperatures at the time of flowering are high.
- Although several popular apple varieties, such as Red Pippin and Braeburn show a level of self fertility if climatic conditions at flowering time are favourable, this cannot be relied upon to ensure consistent and high yields of fruits on most UK sites.
- Although Braeburn planted without pollinators will set good yields of fruits, these will contain few seeds and will have low levels of calcium.
- The seeds are essential in the uptake of calcium into the fruits and the reduction in bitter pit incidence.
- Varieties such as Gala and Golden Delicious also often set fruits with their own pollen when weather conditions are particularly favourable.
- However, most of the self fertilised fruits usually drop off at the time of June Drop.
Pollination using ornamental crab apples or other Malus species
Several species of ornamental Malus can prove effective pollinators for commercial varieties of dessert and culinary apples.
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| Ornamental Malus as a pollinator |
- They have the advantage of taking up less space in the orchard than normal pollinating varieties.
- The tried and tested species/varieties are M. hillierii, M. aldenhamensis, M. Professor Springer M. Winter Gold and M. Evereste.
- Always plant several varieties of these ornamental crab pollinators in an apple orchard, not just one.
- Their winter chilling and spring forcing temperature requirements are different from those of the commercial apple varieties and this often leads to lack of synchrony in flowering times.
- Do not neglect the pruning and, where necessary, the thinning of ornamental crabs, or they may go biennial and fail to produce the required flowers in sufficient abundance.
Synchrony of flowering times
For effective pollination it is essential that the main and pollinating varieties flower at approximately the same time period in the spring.
- Choose pollinating varieties which, according to local records, have flowering periods that overlap by a minimum of six days with the main apple variety in the orchard.
- This overlap should be consistent and judged from records collected over a number of years at sites close to the intended site for the new orchard.
Production of adequate quantities of viable pollen by the pollinators
Pollinating varieties for use in orchards of self-sterile varieties of apple must produce adequate quantities of viable pollen.
- The quantities produced are influenced by the scion variety chosen, the rootstock used, the crop loading (on the fruiting crabs), the orchard climatic conditions, the density of pollinating varieties planted and their management.
Scion varieties and their production of viable pollen
Take account of the pollen producing potentials of the varieties chosen as pollinators.
- Varieties such as Golden Delicious produce copious quantities, whilst Cox and its clones produce much less.
- Triploid varieties, such as Bramley and Jonagold produce almost no viable pollen and should not be used as pollinators.
- Do not choose the self-fertile clones of Queen Cox or Cox as pollinators for self-sterile varieties; they produce too little viable pollen.
Rootstock influence on the production of viable pollen by scions
- Pollinating varieties grown on dwarfing rootstocks, such as M.9, produce more flowers per unit tree size than the same variety on a more invigorating rootstock.
- Trees on dwarfing rootstocks also take up much less valuable space in the orchard.
Crop loading and its influence on pollen production by scion varieties
- Manage the crop loads on the pollinating varieties so as to avoid overset and the establishment of a biennial pattern of cropping.
- Only if thinned well will the fruiting pollinating varieties produce abundant supplies of flowers and pollen on a consistent seasonal basis.
The orchard environment and its influence on pollen quality
- Avoid frost damage to pollinators by good site selection and, where possible, use of frost protection measures.
- In the event of frosts after green cluster, check the pollen viability using simple pollen germination tests.
The ratio between the pollinating variety and the main variety
Generally the pollination ratio should be 1 in 8 to 1 in 10, but in extremes i.e.
- Poor setting, cool sites with excess vigour 1 in 4 to 1 in 6 will be necessary .
- Weak trees of a partially self fertile variety on a good site 1 in 12 to 1 in 15 would be adequate.
Take advice from your local advisor before choosing a ratio of pollinator to main variety for planting in a new orchard. This ratio will be influenced by:
- How favourable the orchard location is, in terms of temperatures and shelter from winds.
- The populations of bees, either wild or introduced in the orchard.
- The pollen producing abilities of the pollinating varieties chosen.
- The propensity of the main commercial scion variety to set abundantly or lightly.
- The compatibility (full or partial) of the chosen pollinating varieties with the main variety.
- Avoid varieties exhibiting a degree of incompatibility as these will give poor results in marginal site/weather conditions.
The management of the pollinating variety [hyperlink 9] in the orchard
The management (pruning and training) of pollinating varieties should aim to stimulate renewal growth and adequate production of quality flowers.
- Apply water and nutrients to pollinating varieties so as to sustain their growth and flowering.
Providing ideal conditions for pollen transfer
Transfer is by insects and to a small extent by wind. Best practice is to achieve an orchard environment which:
- Encourages a wide range of natural insect vectors especially bumble bees by leaving (or creating) grassy sheltered banks and alternative food sources.
- Creates adequate shelter, reduces wind speeds to encourage insect flight.
- Does not reduce wind speed too low, creating stagnant air and no wind transfer of pollen.
Introducing bees where needed
Where these natural levels of insect activity are low, or where pollinator numbers are low and tree vigour is high, pollination can be supplemented by importing hives of honey bees or by encouraging the establishment of bees in the orchard.
- Honey bees will forage more successfully on clear days and when temperatures are above 12oC. Rent healthy, well- stocked (>15000 bees) hives and shelter the hives from cool winds.
- Introduce hive or bumble bees to orchards only when 20% of the flowers are open.
- Introduction earlier may lead to the bees seeking food supplies on other crops growing nearby.
- Once habituated to another crop it is often very difficult to attract the bees back into the apple orchard.
- Remove (by mowing or use of herbicides) weeds or other species that are flowering in the orchard at the same time as the apples.
- These may prove more attractive to the bees than the apple flowers.
- Avoid broad-spectrum insecticides during blossom
Improving conditions for pollen germination and pollen tube growth
Germination of pollen
The fertilisation process begins with the pollen falling on the stigma and germinating. Germination requires adequate but not excessive moisture as the pollen grains need to take in water in order to germinate, drying winds will reduce viability.
- Pollen grains lose viability rapidly once wetted so rain will significantly reduce pollination.
- Germination is temperature dependent with optimum temperatures being between 15ºC and 25ºC.
- Some laboratory tests have indicated that certain pesticide sprays can reduce pollen germination.
- The relationship between these tests and the effect in the orchard is not known at today’s spray volumes.
- Dinocap was the most damaging with Captan and sulphur also having some reduction on pollen germination.
- Sprays of penconazole (e.g. Topas) and chlorpyriphos (e.g. Equity and Lorsban) have been shown to reduce fruit set in some seasons if applied during flowering.
- Laboratory tests have shown that boron and calcium can aid pollen germination but field experiments have given very variable results.
- Various plant growth regulating chemicals have been trialled aiming to improve the fruit set and retention of apples, with varying levels of success.
Best practice to increase pollen germination
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| Pollen tube growth on stigma and style |
- Aim to create a sheltered orchard environment in order to lift average temperatures and reduce desiccation from drying winds.
- Avoid any sprays especially on those warm days when germination is the most likely to proceed.
- Avoid spray volumes which thoroughly wet the flowers.
- Provide plenty of pollen as there is evidence that increasing the number of grains on the stigma seems to stimulate germination.
- Ensure that tree nutrition is adequate and they are not deficient in boron or calcium.
Growth of the pollen tube
Following germination on the stigma the pollen tube must grow down through the style to reach the ovule in order to fertilise it.
- Pollen tube growth is almost entirely dependent on temperature.
- Where temperatures are low and the tube does not grow down the style within 2-4 days fertilisation may not occur.
- Some varieties, notably Falstaff and Redsleeves, have pollen that is able to germinate and grow at much lower temperatures.
Best practice for encouraging growth of pollen tube
- Create a sheltered warm environment in the orchard.
- Consider using Falstaff or Redsleeves as pollinators, especially in sites which have marginal spring temperatures.
Successful fertilisation of the ovule
The Effective Pollination Period (EPP) is the number of days after the flower opens during which time it can receive pollen and still set a fruit.
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| Growth of the pollen tube |
- Some varieties have shorter periods than others.
- Measurements of Cox showed it to be both shorter than other varieties and more variable from year to year.
- The EPP combines the time taken for the pollen to germinate, the pollen tube to grow and the time during which the ovule remains viable.
- How the EPP is influenced is not understood but aiming for best practice in all the above areas will induce stronger flowers, more viable pollen, better pollination conditions and a higher success rate at fertilisation.