This section is from the book "The Gardener V1", by William Thomson. Also available from Amazon: The New Organic Grower: A Master's Manual of Tools and Techniques for the Home and Market Gardener.
It will now be generally known what the crops of fruits are to be. Except in the case of those which are not right at their roots, and liable to throw off the greater portion of what have set, one may fairly judge of the amount of thinning which will be required. This operation should not be left in the hands of a novice, who might remove the most promising of the crop. The finest and best-placed fruit now, are generally the best in the end; but better to go over the trees several times with the view of doing the safe thing. These remarks apply more to Peaches, Pears, Plums, and Apricots on walls, than to standards; but with them it is better in every way when they are not overcrowded. The distance between fruits on walls is a subject which cultivators are undecided on. Peaches, to average all over, about a foot apart is a safe distance; and if such can be had year after year, we would consider it successful growing, and wish that we could always be content to act on this principle; but circumstances often compel one to crop too heavily all kinds of fruits. Nectarines may average 8 or 9 inches apart; but the strength and health of the trees are a guide to practice, and when the roots are a mass of fibre, quantities of manure-water can be given with great advantage.
The thinning of the shoots is a matter of much importance. When a tree is so well under command that all the wood can be removed except that which is to bear next season, the work is made very simple. The new shoot starting from the base of the predecessor, and one leader left to each shoot with fruit on it, may meet all wants, except when trees have to be extended or vacant spaces filled up. Whatever form the tree is in, the same rule applies as to absence of crowding. With amateurs, smaller sizes of trees are preferred - many have cordons for sake of variety; but on the whole we prefer moderate-sized trees - and one objection to large ones is, when accidents occur a large space is denuded, and the loss is greater than when trees of moderate size are grown. The form of training is more a matter of taste than any advantage to fruiting or to quality. We find our few horizontally trained trees more easily and quickly managed than other forms. Pears are as common in this form as any other; Plums, Peaches, Apricots, and Cherries are not so common. One thing we would consider of much more importance is to be able to save the trees from the mischief caused by late frosts.
We have often alluded to the destruction, more or less every year; and although every precaution is used as a preventive, in no case is there absolute success. Protecting material is too often useless. For example, during the last week in April, when we had a splendid set of Apricots and Peaches about casting their flowers, frost from 8° to 10° continued the most of the week. The Apricots are severely handled, and Peaches are shrivelled up in great numbers - probably enough left, but such thinning we would like to dispense with. A friend called on us lately from Inverness-shire, and he describes his Peaches and Nectarines as most promising, his Apricot-trees in perfect health, and fruit in clusters all over. This, from former information, we were prepared to hear. What can experienced men in the south do to help us to similar results? Former directions as to watering young trees, and old ones too if they want it. In training the newly planted trees, cover as much of the wall near the base of them as possible - a few extra shoots will do no harm where there is space to place them; stop any shoots which are robbing their fellows; get all growth even.
Let trees which are spurred, such as Plums, Pears, and Cherries (not Morellos), have the thumb and finger applied in due time; but not so close as to cause the buds at base to start. Examine grafts and remove clay from them if they are advanced enough. War against insects and grubs, - Gishurst's Compound, 2 oz. to the gallon, is a remedy now much used - also fir-tree oil. Net Cherries, etc, from birds. Place litter between Strawberries to keep the fruit clean. Figs may be stopped at fourth or fifth leaf; these are too often crowded : mulching may be used with much advantage to roots.