The pollination of apple trees

apple blossom and pollination
Photo by Artur Luczka on Unsplash

You think your sex life is complicated? Well, think again, because fruit trees have it much worse than you. Say, for example, you are a good-looking Lord Lambourne apple tree in a garden, and you’re blossoming, and you really like the look of the Blenheim Orange next door. But this particular love story is doomed from the outset because the Blenheim Orange is a triploid variety, with three sets of chromosome while you just have two. So you decide to think about the Suntan tree a few gardens down. Unfortunately there is a problem here as well, because Suntan will only flower after all your petals have fallen. So you’ll have to make do with the Granny Smith (age is not a problem here) tree on the other side of the road. Granny Smith flowers a bit later than you, but there is some overlap and so there may well be the chance of some action.

Quick links:

Apple blossom anatomy

How bees pollinate apple trees

Frost and its effect on pollination

Crab apples and apple pollination

Apple pollination chart

How to select pollinizing apple varieties

Night shift – moths and fruit tree pollination

Bees pollinating apple trees

But – and there are a lot of buts when it comes to the sexual reproduction of fruit trees – it’s not just a relation between you, Lord Lambourne, and your partner Granny Smith, on that momentous morning. You have to have a go-between, someone to transport your pollen to the flower on the Granny Smith tree across the street. That someone is a bee, and if the weather is very cold and wet, or there aren’t any bees about during the two weeks that you are in flower, there’s not going to be any fruit growing on your branches to keep you company over the long, hot summer.

Apple tree pollination

So, let’s take a closer look at the apparatus, and consider the apple tree as an example of how fruit tree pollination works. Apple blossom is a “perfect” flower, because each blossom includes both male and female reproductive organs. So potentially, apple blossom could be self-pollinating, but in actual fact, many varieties are self-sterile, meaning that cross-pollination is necessary to set a crop, and even self-pollinating varieties set better when they are pollinated by different varieties.

“Cox trees think about sex every 25 seconds!” (Dan Neuteboom)

The reason for this is that, left to their own devices, apple trees use sexual reproduction to generate a high degree of genetic diversity. In this respect, they differ from us human beings. While we are happy to see our children inheriting something of our character and appearance, if you eat an apple and plant one of the pips, the fruit that you will get on the resulting tree will always be totally different to the apple that you ate. This policy may have evolved because overall it decreases the species’ vulnerability to pests. Whatever the reason, it means that whenever someone discovers an apple tree with a particularly tasty apple, the only way to make more trees producing the same type of apple is to clone it by grafting small pieces of twig, or a single bud, onto a rootstock.

Apple blossom anatomy

But I digress. Back to the apple blossom. The female organ, the gynoecium, has five carpels, each comprising a stigma, a style and an ovary. The ovary usually contains two ovules. The male organ, the androecium, consists of a series of filaments each ending in a pollen-producing anther. At the base of the flower, the nectary contains nectar, which is what the bee is looking for. Nectar is simply the sap produced by photosynthesis in the leaves, a sweet liquid consisting of 80% water and the rest complex sugars (maple syrup is a condensed version of sap) that all parts of the tree use as fuel for its daily life.

apple blossom and pollination

Bees pollinating apple trees

bee pollinating apple blossom
Photo by Capri23auto on Pixabay

The bee arrives at the flower in its quest for food, and uses its proboscis to suck up the nectar. To get close to the nectaries it has to push past the pistils (the female part ending in the stigma) and the stamens (the male part with the pollen). In the process, its abdomen brushes against the anthers and accumulates some pollen, which adheres as a result of the static electricity that the bee’s hairs have accumulated during flight. When the bee gets to the next tree, some of this pollen comes off onto the stigma. This is pollination, but the process of sexual reproduction has not occurred yet.

Before we get to fertilization, there are a few more interesting details about the bee’s activity. Many bees are “generalist” pollinators, meaning that they are not restricted to a single type of flower but can gather nectar and pollen from many plant species. On the face of it, this sounds like bad news for fruit trees, because one can imagine the bee visiting an apple tree, and then getting side-tracked by all the flowers in a wildflower meadow and never getting along to another apple tree in order to cross-pollinate. But in actual fact, studies show that each single bee tends to concentrate on one particular species, so that a bee who has been delegated to apple trees will generally visit mainly apple trees on a route that it learns and then repeats from day to day.

This technique, called “traplining”, helps the bees because this “voluntary specialisation” reduces competition for that particular food source, and enables the bee to concentrate on flowers that, at that particular moment in time, are producing large amounts of nectar. It reduces the energy that it expends, because it doesn’t have to search for new sources every day. Colonial bees can teach their entire route to other bees, which can be useful in the case of plants with lots of blossom like apple trees, that may require a fair number of bees to transport nectar and pollen back to the hive.

Honey, a pollination by-product

A photo showing the pollen baskets – corbiculae – on the bee’s legs. Image by Julia Schwab on Pixabay

Forager worker bees that collect the nectar for honeybee hives store it in a special organ, the honey stomach. After from 50 to 100 flowers, the honey stomach is full and the bee heads back to the hive. Enzymes in the honey stomach are already breaking down the complex sugars into simpler molecules. Back in the hive, the forager bee transfers the nectar to a younger house bee, who uses the same organ to continue processing it and reducing its water content. This is repeated several times until it has reached a 20% proportion of water.

Then the last house bee in the chain puts the nectar into a cell of the honeycomb. The hive worker bees fan the nectar by beating their wings, and it thickens to form honey.

Once the cell is full, the house bees seal the cell with beeswax from their wax glands. The store of honey built up by bees provides food for the times of year when there is no food around. Honey is incredible stuff: it has been found in Egyptian pyramids, and a sample dated at about 3,000 years old is still edible and tastes like any honey. Apparently its long shelf life is determined by a low water content, hydrogen peroxide, and acidity.

Pollen is actively collected by some bee species, which have special pollen baskets (corbiculae) on their legs, which they fill by wiping them over their pollen-strewn furry coats. While nectar contains mainly carbohydrates, pollen has a high content of protein and fats, used in the hive for nourishing the developing larvae. Bumblebees and solitary bees gather nectar and pollen but don’t store it like honey bees, because they have a different life cycle and don’t need to accumulate massive stores of honey for the winter.

Pollination – pollen tube germination

Once the pollen particles have reached the stigma, they adhere to the sticky secretion on the top of the stigma, which hydrates the pollen grain, enabling it to germinate. A pollen tube starts growing downwards through the style, eventually reaching the ovaries after two or three days.

During this period, in many varieties the style tissue performs a filtering “self-incompatibility” function by producing toxic proteins that can stop the growth of the pollen tubes. If the pollen tubes are genetically identical to the style, their growth is halted. Only when the pollen tubes are genetically different can they deactivate the style’s toxic proteins and continue their growth.

Once the pollen tube has reached the ovary, the ovule can at last be fertilized. This is a dual process: the first fertilization between the male gamete and the egg creates an embryo, the part of the seed that will generate the new plant; the second fertilization creates a triploid endosperm, which provides the initial food supply for the developing embryo, and represents the bulk of the white tissue inside an apple seed. The flower’s ovary begins its development into a fruit, and the ovules become the seeds.

All this has to occur ideally for all five stigma, in order to fertilize all ten ovules. This does not always happen, and apples with fewer than its complete set of seeds are generally smaller and perhaps misshapen. Sometimes they fall off the tree early in the fruit’s development.

The nectary, positioned around the developing fruit, helps supply the growing apple with nutrients.

Frost and its effect on pollination

apple blossom frost irrigation protection
Frost irrigation is sometimes used by commercial growers for protection. Photo by Cydonia on Pixabay

Frost can have a devastating effect on pollination and fruit set. From when budding begins after dormancy, right through to full bloom and post bloom, a proportion of buds are killed after just half an hour of temperatures a few degrees below freezing. Apple trees have developed their own way of compensating for a failure to set fruit caused by frost. The biggest and best flowers are formed on two-year old wood and young spurs. The flowers forming on three-year old wood are smaller and develop later, and they are usually abandoned by the tree when fruit set is good on the two-year-old wood blossom. But if a frost kills most of the best flowers, the tree has another chance: it nurtures the flowers on the three-year old wood, and they open a little later and can set a crop.

The very last flowers to open and develop on the tree are those on one-year-old wood, which are the smallest of all. They open about 7-10 days later than all the other flowers on two, three and four-year old wood. So the flowers on the one-year-old wood are the tree’s very last chance to set a crop in case of adverse weather conditions.

Cold and wet weather can also negatively affect pollination. There are a few totally dependable varieties that will produce fruit whatever the weather – cold, wet or frost – during pollination time: Red Falstaff, James Grieve, Pinova and Fiesta.

Pollination requirements of apple trees

Some apple tree varieties don’t possess the self-incompatibility mechanism and can be pollinated with their own pollen. They may be “self-fruitful”, when another tree of the same variety is required, or “self-pollinating”, when just the one tree is sufficient (even though, as mentioned above, fruit set is better with cross-pollination). Varieties that are self-incompatible or self-sterile require pollen from another variety.

In very favourable years, given the right temperature and other environmental conditions, some self-sterile varieties can set a crop without cross-pollination.

In a garden situation with a small number of trees, when selecting the varieties to be planted, it is important to ensure that they are compatible for mutual pollination. A few varieties cannot be used as pollinizers, because they are triploid, such as Bramley, Suntan, Blenheim Orange or Jonagold. Triploid varieties require two diploid varieties for pollination, because both these varieties have to be able to pollinate each other. Alternatively, with a triploid, you can plant a single self-fertile pollinizer.

Crab apples and apple pollination

crab apple blossom
Crab apple blossom. Image by Sue Krebs from Pixabay

Crab apples, Malus sylvestris, are excellent pollinizers because they generally have longer bloom times than apples, produce lots of pollen, and are easy to look after. They are a particularly practical option for a garden orchard. Crab apple is one of the ancestors of Malus domestica, created by selection and cultivation by man in Central Asia from about 4,000 years ago. So the two species are genetically close enough to be compatible.

We have included some crab apple varieties in the table below. Malus hupehensis (Tea crabapple) is interesting because it is the only Malus variety that covers the very late apple varieties such as Suntan and Winter Wonder. Malus x zumi Golden Hornet is perfect for pollinizing early apple varieties. Malus robusta is great for mid-season varieties. Malus aldenhamensis Wine Red Crabapple is another variety good for early-mid season apple pollination. Needless to say, a crab apple is in itself a spectacular addition to any garden, with its beautiful blossom, copious fruit, and the added extra of crab apple jelly.

Apple pollination and variety selection

The chart below shows compatibility data for some UK apple varieties. Of course, it is not complete: there are over 2,000 varieties officially registered in the UK. Your choice of varieties will begin from a tree that particularly interests you, which will then determine the companion trees. Remember to consider the distance between the pollinating varieties: the two trees should be no more than 8 metres apart.

I would like to provide a few indications as regards variety selection. In my experience, most late and very late varieties (D, E, F) produce fruit of better keeping quality and excellent flavour. Winter Wonder, a natural mutation of Suntan, has proved to be a favourite amongst customers at the farmers’ market at we are still present. Another interesting variety is Idared, which originated from the United States. It flowers mid-season, it is inedible before Christmas, but it keeps incredibly well and is the best English eating apple in the months of March, April and early May!

Some UK varieties have been negatively affected by climate change. The best example is Cox’s Orange Pippin, which was without doubt the best eating apple and the variety planted most until about 1990. In today’s UK climate, it has lost its superb flavour and regular cropping characteristics.

Apple pollination chart

The compatibility of varieties depends on their capability of self-fertility, whether they are diploid or triploid, and on the blossoming time. Apple varieties are generally classified into six blossoming periods, from A (early) to F (late), or from 1 to 6. These categories are shown in the table below, which includes a selection of predominantly British apple varieties.

A variety in the same flowering group, or in the group immediately before or after, will be able to pollinize the other variety. For example, if your variety is Beauty of Bath, flowering group C, it can be pollinized by Adams Pearmain (C), or by Bountiful (B), or by Braeburn (D). Scroll down below the table for more tips on selecting varieties for your garden orchard.

Some crab apple varieties are shown at the end of the list. We have done our best to indicate approximately which apple flowering groups they cover.

Variety Flowering group Self-fertility
Adams Pearmain C Not self-fertile
Annie Elizabeth D Partially self-fertile
Arthur Turner C Not self-fertile
Ashmead Kernel D Partially self-fertile
Beauty of Bath C Not self-fertile
Blenheim Orange D Triploid, needs 2 pollinizers
Bountiful B Not self-fertile
Braeburn D Self-fertile
Bramleys Seedling C Triploid, needs 2 pollinizers
Camelot (cider) B Not self-fertile
Charles Ross C Partially self-fertile
Cheddar Cross A Not self-fertile
Chiver’s Delight D Not self-fertile
Claygate Pearmain D Not self-fertile
Court Pendu Plat F Not self-fertile
Cox Orange Pippin C Not self-fertile
Cox Self Fertile B Self-fertile
Crawley Beauty F Not self-fertile
Crispin C Triploid, needs 2 pollinizers
Dabinett (cider) F Self-fertile
D’Arcy Spice D Partially self-fertile
Discovery C Not self-fertile
Doctor Harvey C Not self-fertile
Duke of Devonshire C Not self-fertile
Edward VII F Not self-fertile
Egremont Russet B Partially self-fertile
Ellis Bitter (cider) E Not self-fertile
Ellison’s Orange D Partially self-fertile
Elstar C Not self-fertile
Exeter Cross C Partially self-fertile
Fiesta (Red Pippin) B/C Partially self-fertile
Golden Delicious D Partially self-fertile
Golden Noble E Not self-fertile
Granny Smith C Self-fertile
Greensleeves C Partially self-fertile
Grenadier C Partially self-fertile
Hereford Redstreak (cider) E Not self-fertile
Herefordshire Russet C Not self-fertile
Howgate Wonder C Partially self-fertile
Ingrid Marie D Not self-fertile
James Grieve C Partially self-fertile
Jonagold D Triploid, needs 2 pollinizers
Jupiter D Triploid, needs 2 pollinizers
Katy C Not self-fertile
Kidds Orange Red C Not self-fertile
Kingston Black (cider) D Not self-fertile
Lanes Prince Albert D Not self-fertile
Laxton’s Epicure B Partially self-fertile
Laxton Fortune C Partially self-fertile
Laxton Superb D Partially self-fertile
Lord Lambourne B Partially self-fertile
Newton Wonder D Partially self-fertile
Norfolk Beefing D Not self-fertile
Orleans Reinette D Triploid, needs 2 pollinizers
Peasgood Nonsuch C Partially self-fertile
Pinova D Not self-fertile
Pixie D Not self-fertile
Princesse C Not self-fertile
Red Falstaff C Not self-fertile
Red Pippin (Fiesta) B/C Partially self-fertile
Rev. W. Wilks B Self-fertile
Rosemary Russet C Not self-fertile
Royal Gala D Partially self-fertile
Saturn C Not self-fertile
Scrumptious C Self-fertile
Scotch Dumpling B Not self-fertile
Sops in Wine C Not self-fertile
Spartan C Partially self-fertile
Star Apple (Dutch variety) C Not self-fertile
St. Edmunds Russet/Pippin C Partially self-fertile
Sunset C Self-fertile
Suntan E Triploid, needs 2 pollinizers
Tom Putt (cider) C Triploid, needs 2 pollinizers
Tydeman’s Late Orange D Not self-fertile
Warner’s King B Not self-fertile
Winston D Self-fertile
Winter Wonder E Not self-fertile
Worcester Pearmain C Partially self-fertile
Malus aldenhamensis (Wine Red crab apple) D-F A good late season pollinizer
Malus Chestnut Crab (crab apple) A-C A good early-midseason pollinizer
Malus evereste (crab apple) A-C A good early-midseason pollinizer
Malus floribunda (crab apple) A-C A early pollinizer
Malus x zumi Golden Hornet (crab apple) B-D A midseason pollinizer
Malus Harry Baker (crab apple) B-D A midseason pollinizer
Malus hupehensis (Tea crabapple) E-F A good pollinizer for late varieties
Malus John Downie (crab apple) B-D A early-mid-season pollinizer
Malus × moerlandsii Profusion (crab apple) A-C An early-midseason pollinizer
Malus Pink Glow (crab apple) A-B An early pollinizer
Malus x robusta Red Sentinel (crab apple) A-C An early season pollinizer
Malus Royalty (crab apple) B-D A early-mid-season pollinizer
Malus × scheideckeri Sun Rival (crab apple) D-E A late-season pollinizer
Malus Van Eseltine (crab apple) B-D A mid-season pollinizer
Malus Wisley Crab (crab apple) C-E A mid-late season pollinizer

 

How to select pollinizing apple varieties

When selecting varieties for your garden orchard, these are the things to remember:

        1. A variety in the same flowering group, or in the group immediately before or after, will be able to pollinize the other variety.
        2. Varieties marked as self-fertile or partially self-fertile will set a crop without another pollinizer, but in any case, both will perform better with a pollinizer. In some years, with very warm and favourable weather conditions during blossom time, self-fertile or partially self-fertile fruit trees can produce a crop without the help of another variety. However one cannot rely on this characteristic, if you want to have fruit every year.
        3. A triploid variety needs a diploid pollinizer chosen according to point 1 above, but it cannot pollinize the diploid, so in practical terms, a triploid variety needs two diploid pollinizers.
        4. Crab apple varieties (Malus sylvestris) are excellent pollinizers for domesticated apples (Malus domestica), because Malus sylvestris is closely genetically related, and an ancestor of Malus domestica. It produces lots of blossom that spreads over several flowering groups.
        5. You may have seen a lone apple tree in a garden that sets a crop regularly every year, and so you may wonder why there is all this bother about pollinizers of the right type and category? The explanation is simply that in this case there is undoubtedly another apple or crab apple tree in the vicinity and on the bees’ route. Studies have shown that bees can travel even 23 km in their daily quest for nectar and pollen.
        6. When planning your garden orchard, remember that the maximum distance between an apple tree and its pollinating variety is 8 metres.
        7. In variety selection, there are other factors involved, above all geographical location, to be considered in order to ensure good fruit set, and so we always recommend seeking expert advice in variety selection.
        8. A wild flower bed or a wild flower meadow can help with fruit tree pollination. Read more about how to grow a wild flower meadow.
      1. Night shift – moths and fruit tree pollination

        moths and pollination
        Photo by Sandid from Pixabay

        What goes on in an orchard after sunset? A recent study titled “Nocturnal pollinators strongly contribute to pollen transport of wild flowers in an agricultural landscape”, has revealed the possibility that moths may play a part in the pollination of many plant species, including the Rosaceae, the family that includes many fruit trees such as apples, pears, quinces, apricots, plums, cherries, peaches and almonds.

        In a research project conducted in Norfolk by Richard E. Walton, Carl D. Sayer, Helen Bennion and Jan C. Axmacher and published by The Royal Society on 13 May 2020, nine ponds within an agricultural habitat were observed, identifying the plant species in flower, and the daytime and night-time pollinating species, principally bees by day and moths by night. Moths were captured and the pollen grains adhering to their furry thoraxes were identified.

        The researchers found that 103 moth species carried pollen from 47 plant species (amongst which blackthorn, Prunus spinosa) on their bodies, amongst which 7 plant species that seemed to have not been visited by any daytime pollinators at all. Particularly important plants for the nocturnal pollinating moths were blackberry and two clover species.

        The authors point out that while moths were observed to settle on flowers, and that pollen was found on their bodies, this is not direct proof of pollination. Nonetheless, their observations suggest that nocturnal moths could be involved in the pollination of wild plants, and may help in the pollination of agricultural crops, particularly at the present time in which daytime pollinating insects are declining in numbers. But this possibility would need further research for its confirmation.

        The authors highlight the fact that this new discovery has emerged at a time at which moth populations are declining world-wide, and that there may be a risk of losing essential pollination services at a time when we are only just beginning to understand them. Conservation efforts involve the provision of plant species that can provide nectar for moths in an agricultural environment.So, what goes on in an orchard after sunset? The natural world – even in a small and restricted habitat such as our garden – is still a planet with many areas still awaiting discovery.