Gardening Tips: Guide to pollination
he subject of pollination is one which closely affects everybody who grows any kind of fruit or any vegetable for the sake of its fruit, such as, tomatoes and ridge .
With the one notable exception of, it is essential for a pollen grain, bearing the male germ cell, to reach the egg in the female part of a blossom before that flower can ‘set’, as gardeners term it, and so develop its fruit or seed-pods. It is possible to induce fruit formation artificially by chemical means, but this is normally unnecessary in home growing.
To understand what happens and what may possibly go wrong, knowledge of a little basic botany is most helpful. If you look within the petals of an apple flower, you will see several tiny stalks (filaments) terminating in bulbous swellings (the anthers) encircling a central tube, as if guarding it. The anthers and the filaments on which they are borne are together known as the stamens. The central tube is the style, or female organ, and the portion at its tip is called the stigma. The base of the style is enlarged to form the ovule, which contains the egg.
When the flower is ready to be fertilized, the stigma becomes sticky, and the dust-like grains of pollen which form on the anthers are easily dislodged by wind, animals or some other means, so that some of them find their way to the stigma, and thence to the egg in the ovule.
In some cases the egg can be fertilized by pollen from the very same flower. Such plants are said to be self-fertile, and in this case the pollen may be knocked off the anther by an insect, by rain or by wind causing the anthers to vibrate, or even by an accidental or purposeful nudge by the gardener.
In other cases, such as most varieties of apple, pear and many plums, the pollen must come from another compatible variety of the same species. Such varieties are said to be self-sterile and as the pollen has to be transferred over a distance, cross-pollination is usually carried out by insects (often bees). These visit one blossom, collect some of the sticky ripe pollen on their body, wings or legs and then fly on to the flowers of another tree of the same kind but a different variety, where the pollen now rubs off onto the sticky stigma.
Stormy weather, prolonged rain and cold temperatures can all impede or prevent pollination. What is not always appreciated by gardeners is that when this successful transference of pollen to the stigma has occurred, it is only the first step towards fertilization of the egg.
After this, a grain of pollen must ‘germinate’, producing a tube which grows down the inside of the style until it reaches the ovule. Only then can the male germ cells fuse with the female and fertilization be complete.
Frost damage to flowers
Frost immediately before the blossom opens can so damage the ovule that sterility results. And so, of course, can frost when the blossom is wide open. Most gardeners who grow strawberries are only too familiar with the condition known as black-eye when the centre of the flower turns black as a result of frost.
Frost and possibly near-freezing low temperatures can also cause damage which may not immediately be noticeable to the gardener but which can cause some degree of sterility.
Low temperatures, too, can adversely affect the growth of the pollen tube, perhaps slowing it down so that when it reaches the ovule, fertilization can no longer take place.
After considering all these hazards, you may think it is a marvel that any fruit ever does set. However, nature makes allowances for these potential losses of pollen. Only one pollen grain has to grow successfully: a countless multitude of grains is produced in each ripe anther. If only one blossom in twenty on an apple tree sets, you may still have quite a reasonable crop.
There are cases, however, where it is helpful, sometimes essential, for gardeners to pollinate their own crops.
Artificial pollination by hand is not a difficult process, but it can be finicky and tedious work.
Instances where hand pollination is essential are when strawberries are being forced and when early peaches and nectarines are grown in the greenhouse. In these cases, the flowers open at a time when few insects are on the wing and, owing to cold weather, ventilators may be closed or opened only slightly so that the insects have little chance of entering the greenhouse.
All the varieties of strawberries and those of peaches and nectarines normally grown in Britain are self-fertile, so it is only necessary to distribute the pollen, not to transfer it from one variety to another. An exception is the peach Hale’s Early, which is self-sterile.
To dislodge the pollen, ‘tickle’ the centre of the flowers with the hairs of a brush. Old gardening books used to recommend a rabbit’s tail tied on the end of a bamboo cane so that one could easily reach strawberry pots on shelves close to the greenhouse roof or the higher branches on trees. These days rabbits’ tails are relatively uncommon, and a child’s camelhair painting brush will serve just as well. Many gardeners, in fact, make do with a wisp of cotton wool tied to a cane.
All you have to do is simply dab the top of the dry brush or piece of cotton wool gently into the centre of each flower. Do this around midday if you possibly can, because this is the time when the greenhouse is likely to be at its warmest, and the pollen most easily detached.
Hand pollination will also be necessary for marrows grown in the greenhouse, and for pumpkins, both in and outdoors. In this case there are female flowers, easily identified by the embryoto be seen just behind the petals of the flower, and male flowers which bear the pollen and have no swelling behind the petals.
Pollinate the female marrow flowers by carefully removing the petals from a male flower, trying not to dislodge the pollen in doing so, and then plunging the pollen-covered centre of the male flower into the centre of a female bloom and leaving it there.
The main difficulty with the pollination of marrows is that they tend to form male flowers in advance of the female, so that when the latter eventually open, male flowers are few and far between. In extreme cases you can adopt the camelhair brush technique, and transfer pollen from one open male flower to several females.
With outdoor marrows, there is less likely to be any failure in the natural pollination by insects but when flowers open during inclement weather you can again assist nature by hand pollination.
With melons, whether they are grown in the greenhouse, in a frame, beneath cloches or, in very warm favoured districts, in the open garden, another problem arises. As with marrows there are male and female flowers, the latter being recognized by the round, pea-like swelling immediately behind the petals. If one female flower is pollinated ahead of the others, however, it then swells at their expense and all you get is one melon per plant.
Greenhouse melon plants are each capable of developing four good melons, provided that all the flowers are pollinated on the same day. Therefore you have to pick off female flowers which appear on their own, before other female flowers. After pollination, pick off any subsequent female blooms.
With melons grown in frames or cloches, you must decide between two possible planting plans to ensure polli-nation. With the first method, you plant two melons per light, at opposite corners, and wait until two female flowers on each can be pollinated on the same day. The second method involves setting a single plant towards the back, training out four side growths and waiting until four female flowers can be pollinated at once, which may mean a delay of a week or more.
Greenhouse grapes usually produce plenty of pollen. Its distribution is aided by a warm, dry atmosphere and by tapping the vine rods around midday to dislodge the pollen from the anthers.
The condition in tomatoes known to growers as ‘dry set’, when the flowers fall but the fruits refuse to swell and remain no bigger than a pin head, is the result of indifferent pollination.
Tomatoes in the greenhouse normally require a rather dry atmosphere, but to assist pollination damper air is beneficial. The lowest trusses on the earliest plants are most likely to be reluctant to set, and with these it is always helpful if you can syringe them overhead with clear water, doing this between mid-morning and midday. Where syringing at these times is impossible to arrange, spray in the evening as soon as you get home. It also helps to distribute the pollen if you gently shake the plants by tapping their supporting strings or canes.
It is also possible to buy a hormone setting preparation and you can spray the tomatoes with this. Chemically induced setting will result in seedless fruits. In practice it is not often necessary to fall back on this expedient, except possibly with very early plants.
Tomatoes in the open generally set quite naturally without assistance. High day-time temperatures and warm, dewy nights provide the ideal conditions, but when these are absent you can help the first trusses to set by spraying the plants overhead with clear water on bright days.
Runner beans in the open are rather like early greenhouse tomatoes in that the first trusses of flowers often show a marked reluctance to set. Failure to set in hot, dry weather is due to the closing up of the flowers so that pollinating insects cannot enter. You can aid pollination in the early stages of this crop by spraying the blossom trusses with a fine spray of clean water every morning and evening, but when the plants set into the full swing of flowering, no amount of overhead spraying will help pollination, if they are short of water at the roots, as the flowers will be closed.
There is one vegetable which is odd man out in this pollination routine and that is the greenhouse cucumber (which may also be grown in a cold frame or under cloches). The fruit of these cucumbers will swell naturally without pollination and, indeed, if the female flowers are fertilized, the resultant fruit develops hard, inedible seeds within it and may swell up in a bulbous manner. They are often bitter, as well.
Ventilating apertures with curtain netting, coarse hessian or very fine nylon net. Then, with a small crop, it is not too tedious to pick off all the male flowers (those without any embryo cucumber behind the petals) as soon as they show. Practically all such trouble can be avoided, however, by growing one of the new, virtually all-female varieties which produce very few male flowers. These include Femdan, Femspot, Fertila, Pepinex, and Topsy.
Outdoor ridge cucumbers, like vegetable marrows, will not swell their fruits unless they have been pollinated, so in these cucumbers the male flowers must be left.
Sweetcorn is one more vegetable crop where some thought needs to be given to pollination. The pollen from the male flowers is blown by the wind on to the silky tassels of the female flowers which produce the cobs of corn. The wind is more likely to aid pollination if theplants are grown together in a block rather than in long single rows across the plot.
All the crops we have considered so far have been self-fertile. There has been no need to think of cross-pollination by another variety. Soft fruits too, are mostly self-fertile, although blueberries are an exception and the yield of blackcurrants may possibly be improved by cross-pollination. But the tree fruits present a different picture, because most apples and pears, many plums and all sweet cherries, except the very new variety Stella, must be cross-pollinated.