The Importance of Pollination

MANY OF US HAVE FRUIT TREES, NUTS, AND VEGETABLES THAT REQUIRE POLLINATION FOR THE PRODUCE TO DEVELOP FOR HARVESTING

Ioften get asked why someone didn’t get any apples, pears, plums, walnuts the previous year. There are many factors involved, including late frosts, but often it is because something has hindered the process of pollination.

First of all, it is useful to understand that (most) fruits cannot develop without the process of pollination having taken place. Literally it is the same as in humans; without fertilisation a baby will not develop, and biologically the process is very similar in humans as it is in plants. Plants have organs that are very similar to animals; there is an ovary where the seeds develop, these organs being contained within the flower. Many plant species are hermaphrodite, and also contain the male parts that produce the pollen. In these flowers self-pollination is possible, although many plants have developed mechanisms to avoid self-pollination, and promote cross-pollination which strengthens the gene pool. For example apples and pears have evolved a process called Protogyny, where the female parts of the flower reach maturity before the stamen (the male structures) ripen and release the pollen, and therefore selfpollination is avoided. Apples are also unreceptive to their own pollen and for this reason you need to have at least one different type of apple tree growing nearby to ensure that crosspollination occurs and that the fruits develop.

Apples are also unreceptive to their own pollen and for this reason you need to have at least one different type of apple tree growing nearby

Caroline has been a lecturer in horticulture for 20 years and now runs a nursery and ‘garden craft’ courses in the Haute-Vienne at Le jardin créatif

When pollen lands on the stigma (a sticky pad that sits above the ovary) if the pollen is receptive it begins to grow a tiny tube that pushes its way down into the style (a stalk that connects the stigma and the ovary), through tiny channels called ‘stylar canals’, and when it reaches the ovary it connects with an ovule (an undeveloped seed) and the two nuclei fuse to form the first cell that will develop into the seed - this is the process of fertilisation. For plants that produce more than one seed within their ovary, each ovule requires a separate pollen grain to fertilise it. Apples typically produce 5 seeds, and therefore need at least 5 grains of pollen for a complete process. If complete pollination does not take place then the apple will develop misshapen.

Some plants produce separate male and female flowers, i.e. some of the flowers will only have the male structures, and some will only possess the female organs. Courgettes are a good example of thisyou can tell which are the female flowers because they have a miniature courgette (the ovary) behind the flower. You need to plant a couple of courgette plants to ensure that there will be male flowers (without the ovary) open at the same time. Cucumbers and squash are the same.

Some plants go one step further and are gender specific, they produce some plants that are male, i.e. possessing flowers with only the male parts, and some that are female. You need to have at least one male and one female in order to get the fruits. Kiwi and holly are examples of this type of plant. So if you have no berries on your holly, it is either a male plant, or a lonely female!

There are two main agents of pollination: insects and wind. Insect pollinated flowers will be bright and showy and often scented, with tiny glands at the base of the petals that produce nectar which entices the insects to visit the flower. If the weather is bad at the time of flowering and insects have difficulty flying then poor pollination will lead to a lack of fruiting. As I write, it is very windy with heavy rain, and the peach tree is in full bloom, so this will inevitably have an effect on my peach crop this year.

Wind pollinated plants such as walnuts, hazelnuts, grasses, and sweetcorn produce masses of pollen, causing many people with hayfever days of misery, but it is necessary for the plant because the process is much more haphazard; it relies on a chance meeting of the pollen and the stigma as pollen grains drift on the wind. Again high winds and heavy rain can hinder this process.

Therefore it is important to choose a sheltered spot for planting your fruit trees, and for your potager, but avoid the bottom of a slope too because this can cause a frost pocket, where cold air naturally collects. If you already have an orchard which is in an exposed position, consider planting a hedge or shelter belt to protect from strong winds. You can also encourage pollinating insects by planting scented and flowering plants close by that will attract them. Keeping bees will also be a bonusthis is something that we have done for the first time this year.

There is also an interesting biological process called Parthenocarpy, where a plant will spontaneously produce fruit without fertilisation having taken place. In nature this does not ensure the success of the species because ultimately no seeds are produced, but in cultivation this can be a useful process and growers will propagate vegetatively to produce seedless vegetables such as seedless grapes, some citrus fruits, bananas, and some varieties of cucumber.

If you have any horticultural questions, I am happy to take suggestions for future articles, or if you need planting advice you can pop along to our nursery and garden on Saturdays (10am-4pm, March –end of October) and have a chat with us.

You can find details of our workshops, courses and browse our plant list: www.lejardincreatif.net