Wednesday 31 January 2024

The Essentials of Fruit and Nut Tree Pollination

 During this post we’ll cover the essentials of fruit and nut tree pollination to ensure that you make the correct species and cultivar selection for your regenerative landscapes that will result in healthy, productive trees for you and future generations.

An often overlooked reality for novice growers is that the blossoms of fruit/nut trees or shrubs serve as the plants' sexual organs. The remarkable journey from flower to fruit hinges on the transfer of gametes – a process encompassing pollination and fertilization. In simpler terms, the successful union of these tiny elements within the flower's intricate makeup is what transforms these blooms into the delicious fruits we enjoy.

Pollination is a fascinating process involving the transfer of pollen from the male reproductive organs (anthers) of one flower to the female reproductive organs (stigma) of another flower of the same species. This transfer of pollen is crucial for fertilization, which eventually leads to the development of fruits and nuts. For the majority of fruit and nut species the transfer of pollen is carried out by insects, particularly flying insects. Mammals and birds will also do the job for certain species (mostly in the tropics) and in other cases the essential task is left to the wind.

Pear - Pyrus communis

Self fertile, self infertile and cross pollination  

Some plants can achieve sexual reproduction i.e. pollination and fertilisation using their own pollen and eggs. These are referred to as ‘self fertile’. Some plants must receive pollen from another compatible plant (a pollinating partner) in order to produce fruit, and these are referred to as ‘self infertile’. The majority of fruit and nut trees - even those that are self fertile - will produce higher quality and more bountiful fruit when receiving pollen from another compatible tree. This is known as cross pollination. For example, apple cultivars ‘Granny Smith’ and ‘Golden Delicious’ are both self fertile and will produce fruit if planted all alone. However, when planted close by each other (within 30m) the pollen transfer (which is facilitated by insects in this case) from one cultivar to the other, will result in more abundant yields on both trees.

NB - you will often find the self fertile /self infertile categories referred to as self fruitful/self unfruitful and/or self pollinating/self sterile. 

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How do you know which plants can cross pollinate each other?

Cross pollination compatible plants are generally of the same genus. For example, there are some 30 - 40 species of apple - Malus sp. The orchard apple - Malus pumilla (that represents mosts of the temperate apple cultivars) can be pollinated by any of the other apple species, such as the European crab apple - Malus sylvestris, so long as the blossom time coincides and insects are around to move the pollen from one plant to anotherIn fact the crab apple makes an excellent pollination partner for many apple cultivars as the flowering period is often prolonged over many weeks and the plants produce masses of flowers. 

While on the topic of apples it’s worth mentioning that some apple cultivars are triploid which basically means they are unable to produce viable pollen. They are receptive to pollen from other apple species that bloom at the same time, but they cannot pollinate other cultivars. Triploid cultivars will be listed as such in all good nurseries and databases. 

Plums, cherries, peaches, nectarines, apricots and almonds are all in the Prunus genus. Most of these plants are self fertile but all will benefit from cross pollination. All species in this genus will cross pollinate with each other as long as they flower at the same time and there are invertebrates around to move the pollen from one plant to another. There are however a few exceptions to this rule. European plum cultivars will not cross-pollinate with Japanese plum cultivars and sweet and sour cherries can cross-pollinate each other but are not compatible with ornamental flowering cherries.

The basic and important thing to remember is that for cross-pollination to be successful, the pollen must transfer between compatible plants that flower at the same time.

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Other factors that influence pollination

Pollinators - pollinators are the biotic agent that move the pollen from one flower to another. In temperate climates these are generally insects and include: bees, butterflies, wasps, ants, flies, mosquitoes, moths and beetles.

Some pollinators in our gardens 

Temperature - the temperature at blossom time is very significant for effective pollination. Late frosts can damage blossom and pollen, and even when pollination and fertilization has occurred cold weather can destroy the young embryo.

Wet and windy weather - if your plants rely on insects for pollination be aware that cold, wet and windy weather will discourage insect flight. The good news is you only need 1-2 fine warm days during the bloom period for flowers to be successfully pollinated. 

Plant spacing and location - in order to gain the highest benefits of cross pollination, your pollination partners should not be too far from each other so insects can easily move from one plant to another. For wind pollinated plants, it’s important to plan your plant positions to optimize pollen dispersal. As a general rule, plant at least two compatible-pollen varieties within 15m of one another. Pollination will still occur if trees are planted closer together, and may even occur between trees planted farther apart than this, but when in doubt a 15m distance between trees is good to aim for. 

Competition for pollinators - plants compete with each other for the attention of pollinators by offering nectar and pollen incentives. During the blossoming period, you want the pollinators to focus primarily on your crop plants. During these periods I will often cut the surrounding wild flower patches, herbs and flowering biomass plants in order to drive the pollinators to the crop blossoms.

Flower/Fruit buds

Pollination and fertilization cannot occur without flowers and it’s worth knowing that the flowers emerge from buds that are grown in the previous year. Next time you pick cherries from a tree, take a look at the tip of the branch you are picking from and you will see a cluster of buds. Some of the buds will be leaf/shoot buds and some will be flower/fruit buds that will burst into blossom the next spring and if pollinated and fertilized, will be fully ripe cherries in 12 months time. It’s useful to know where the flower buds on your fruit trees are so that you can avoid pruning them. The location of flower/fruit buds vary from species to species, and from cultivar to cultivar. Generally they are either located on the tips of branches or on the spurs. With some careful observation and a little practice you can identify the flower/fruit buds on your plants.   

An example of flower and shoot buds on pear, apple and apricot

Biennial bearing - some fruit trees will flower profusely one year and produce a very heavy crop. The following year (sometimes two) the plant may only bear a few flowers and little fruit. This is thought to be due to the energy used to grow the bumper crop reduces the plants ability to invest in flowering buds for the following seasons. You may be okay with a boom and bust cycle, but if you prefer consistent yields you can achieve this by removing a portion of the blossom by hand during the boom years.  

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Key points to guide selection 

With so many factors to consider, planning your fruit and nut tree selection and placement can make your head spin but there are some simple measures you can take to make the whole process easier.

  • Grow a range of wild native fruit trees. They will provide a good source for cross pollination to occur. You can grow some of these trees in hedging and windbreaks and they do not need much attention. 

  • Select species and cultivars that are commonly grown in your region. Get in touch with local fruit growers and orchards and see what they are growing. 

  • Every good fruit and nut tree nursery will have the flowering times listed for each cultivar and often you will find a list of recommended pollination partners.

  • If you are in an isolated area and only have room to plant one tree, choose a self-fertile cultivar. If your site is surrounded by many different fruit trees it’s likely there will be compatible trees in neighbouring gardens and hedgerows.

  • Choose cultivars that have different harvest times so that you have a spread of fruit throughout the season.

  • Consider that some fruit and nut trees such as kiwi - Actinidia spp. and persimmon - Diospyros spp. are dioecious and need male and female plants in order to bear fruit. 

The below table indicates the pollination requirements for common fruit and nut trees and shrubs, including the recommended distance from a pollination partner and the temperature at which damage occurs in flowers and blossoms.

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