Most gardeners are well aware of the benefits of thinning when applied to seedlings. Few of us allow our sowings to remain as congested a stand as a good germination can give. We soon learn to thin them out to give each sufficient space to develop, and if in their final site to mature. If we have ever left any too crowded the detriment is only too obvious with the plants becoming spindly, weak and often bolting into early flowering rather than growing on strongly.
Mind you perhaps we may not realize how soon such thinning needs to b done for maximum effectiveness. Experiments on tomato seedlings showed that separating them within a few days of emergence rather than leaving them clustered for over a week gave plants and crops both considerably bigger. Thus the great benefit obtained from sowing but one seed apiece in individual cells or pots, or if several seeds are sown together of removing the surplus as soon as possible.
Likewise most of us can hardly have failed to notice how when there is a bad season for setting fruit those that do form are almost invariably much bigger than normal. Whereas conversely, when many fruits set they inevitably are smaller and ripen later, sometimes failing to mature at all in the case of some such as grapes. These are notorious for over-cropping themselves to the point of exhaustion and if all potential bunches are left on a vine then often these do not ripen at all well.
The thinning of fruits is thus a simple and effective way to improve their quality. Especially as sensible and careful thinning will remove not only the surplus congested fruitlets but will select out those showing any trace of pests or diseases. This not only means that affected fruitlets do not waste the plants energies on their lost cause but it removes the threat of infections or infestations spreading to healthy fruits this year and in future. Of course that assumes that the thinned fruitlets are disposed of by burning, burying or composting and not just discarded on the ground.
Nature is wise and many plants will thin their crop themselves if they ‘perceive’ it to be greater than their capacity. Apples are the prime example and the June Drop as it is called occurs in late June or early July when large numbers of small applets are allowed to fall. This can be aided by a vigorous shaking of the trees at that time. However further thinning by hand is even more useful. This is because the plants assess their capacity to produce viable seeds and thus retain a large number of fruits accordingly. Whereas it is the fruity pulp that we are after not the seeds. Now it is the production of seeds that is a drain on the plant as they require large amounts of nutrients especially minerals and fats. The fruity bit we want is easier for the plant to produce as it contains more sugar and water and less of those other nutrients. So if we remove many fruitlets, before the seeds swell, then the plant is spared scarce resources resulting in it being better able to ripen those remaining, and to go on to crop even better the following year. And of course this also helps the plants general health, well being and longevity.
Indeed some plants, such as grapes and blueberries do seem almost suicidal in their attempts to over-crop and although death only seldom occurs it does sometimes follow an immense crop. However far more often a tree crops so heavily one year that it exhausts itself and has to take a year off the next which is known as biennial bearing. This is almost always cured by heavy thinning when a large crop has set thus ensuring more regular harvests.
Many gardeners are worried about thinning- or rather are reluctant to be ruthless enough. An interesting tale dispels this doubt. Some decades ago some American farms in a co-operative specializing in peaches were going out of business. There was a glut on the market year after year and the bulk of their crops were either sold for a pittance for processing or even just left unpicked. The cost of picking and grading just not covering the price received. They debated their position and decided to thin their trees heavily in order to try and get larger finer fruits which would receive a premium. The idea was that the extra work of thinning would be recouped by less work picking the fewer better fruits at harvest. This thinning worked, but not well enough and they still faced disaster. Then one farmer hit on the solution- instead of thinning their own trees they would thin each others! It is easier to be ruthless with anothers’ crop, and they were, almost viciously so. But although each farmer felt robbed when he surveyed his thinned trees when it came to harvest the result was spectacular with them all producing peaches of a size and quality that commanded the highest prices they had ever dreamed of. They may have had far fewer peaches to sell but the higher value more than compensated and their businesses were saved.
So be brave and when you have an apparently heavy crop setting be prepared to be ruthless and thin, thin, thin. You will not over-do it. Of course do so systematically. Once a crop has set thin, not once but make several passes a few weeks apart. Each time remove the surplus taking out the most congested and obviously infected, infested or damaged. Then take out those in poor positions such as those in heavy shade or where they will be pressing on stems or branches as they swell. Indeed for apples, pears, peaches, quinces and other large fruits it is best if no fruits are close enough to touch when fully swollen. With the smaller fruits such as plums and apricots it is simpler to reduce the size of the bunches. In fact it is possible to simply shear off some of the surplus with a hedgetrimmer without doing too much harm and is certainly better than leaving huge crops to go on to swell and break the branches. Cherries are not normally thinned but would benefit like any other fruit if you wanted bigger examples say for show.
The soft fruits are likewise less often thinned than the tree fruits but will similarly respond with bigger better fruits though in most of their cases the same result may be achieved by judicious pruning. This is especially true for blackcurrants and blueberries where removing the older wood is far simpler than thinning out individual fruits or bunches. Red and white currants are likewise easier thinned by pruning out surplus wood than by reducing the fruitlets. Gooseberries are best both hard pruned and then thinned if it is large berries you are after- the champions grown for show becoming many times larger than the small sized ones we are all used to. Even strawberries benefit if odd and imperfectly formed fruitlets are removed before they swell much.
Although in theory most fruits will benefit from thinning a few, particularly the blackberry raspberry tribe are seldom bothered with, Even so if you do want bigger better fruits they could be thinned if you wished though as with the other currants and berries you may find it simpler to just retain fewer canes when pruning. Kiwis tend to over-crop and are worth thinning to get bigger fruits as small ones tend to be all skin. As they are such rampant growers it may be physically difficult to reach them unless they are kept as trained forms such as espaliers.
Citrus will set vast numbers, far more than is good for them and if left these will make tiny fruits. Obviously the bigger species such as oranges and grapefruits need to be thinned much harder than limes, lemons and tangerines, Kumquats can usually be left unthinned. The small fruitlets of citrus you thin out need not be wasted as they can be used in pot pourris.
As mentioned above grapes are notorious for over-cropping and if you want large ripe grapes with a good flavour then prune them hard, winter and summer and be more ruthless removing bunches than for almost any other fruit. The old boys went further and using long thin scissors would carefully remove grapes from within each remaining bunch. This is too much work for most of us but again worth it if you are after prize winning bunches, and by reducing the congestion much reduces the incidence of rot. I cannot over-emphasize the need for ruthlessness with grapes- in anything other than perfect years grapes will always tend to over-crop and not ripen well at all.
Melons are another crop that MUST be thinned. The old books make much of a fuss about getting four fruits to set at one time as if one sets ahead of the others it robs them. However this was in order to get four marketable fruits- but each will have it’s own skin and seeds. If you want huge melons then forget all their advice and remove all other fruitlets once one has reached golf ball size. This will then give you far more flesh with less waste on skins and seeds. Sweet peppers rarely need thinning and hot peppers even less so but large fruited aubergines may be worth thinning.
One other crop that is rarely thinned but can benefit is the tomatoes- we often expect them to ripen more fruits than they can manage and simply removing any new trusses forming after mid to late summer will help the others to ripen better. And for those growing beefsteak varieties who wonder why they are not so big- try reducing each truss to the best fruit when still smaller than marbles and no more than three fruits per plant at any time.