This section is from the book "The Gardener V3", 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.
At the end of the first season's growth - no matter whether the tree has been grafted or budded - the cultivator must determine what mode of training he intends to adopt. The fan is by far the best method, although the horizontal may, for variety's sake, be introduced occasionally. The principal objection to horizontal training being adopted for the Plum is, that the very best managed and the most healthy trees will at times lose a branch or two. Sometimes they wither and die without any apparent cause, and at other times gum or canker will be the destroying enemy. It will thus be evident that when a branch gives way through any of the above causes, the vacancy cannot be so readily made up, and, as a natural consequence, the tree becomes unsightly and irregular in its outlines and general appearance. Further, if the horizontal method be adopted, the tree is not nearly so easily managed, nor yet is the general effect ever so good. It is a well-known fact to all gardeners, that to obtain a good and well-balanced Plum-tree by any mode of training is a difficult task, as it is very apt to produce large strong branches on the one side, and short weak ones on the other.
It is therefore much easier to work against these inconsistencies where the fan system is adopted, as means can be used to greater advantage to obviate this than when horizontally trained. No doubt it can be managed either way - by bending and twisting down the strong shoots, and elevating and encouraging the weaker, at the same time root-pruning the tree on the strong side without touching the other. These and some other methods may be adopted with success in either case; but where the fan system is taken, the shoots can for a season or two be twisted and bent, yet so well disposed all over the wall that the existence of such contortions can scarcely be detected; whereas, by the other method, it cannot be done without presenting a very ungainly appearance. For these reasons we would recommend the fan as the best mode of training to be adopted for the Plum.
This having been decided upon after the wood is ripe and the foliage fallen, if three nice ripe young shoots are upon the young tree, let the centre one be selected and cut down to within 3 or 4 inches of where it started, and each of the side ones cut back to about 18 inches or so. Care should be taken to try, if possible, to get the top bud of the leader on the front of the shoot with two well-placed eyes below it, one looking towards the right and the other towards the left, the former to form a leader and the latter to make side branches. Should the two side shoots prove of very unequal proportions, we would recommend that both be removed, and the tree treated as if it only possessed one shoot. Where one shoot only is produced from the graft or bud, it ought to be cut back to within 9 inches of the base, remembering to have the buds placed as already directed for the leader. We recommend this height as the best for the Plum, so that if the shoots or under branches of the tree are wanted about 1 foot from the ground, they may not be taken away at right angles from the stem.
Mr Thompson, in the 'Gardener's Assistant,' says, that where this method is adopted, "the branches will not be so liable to die off as if they were taken at right angles, which, in training stone-fruits, should never be the case." Two very good reasons may be given for this - the first, that in a tree of so robust and vigorous a nature as the Plum, if the branches are taken away directly at right angles from the stem, those branches above which are trained oblique or perpendicularly will be sure to rob these under ones to a very great extent, and, as a natural consequence, if they do not die, they will, at all events, make but very slow progress. If, however, they are started 4 inches below, and brought up to the horizontal line at an angle of 45o or so, the results will be very different, as the juices of the tree will be introduced into their proper channels ere they reach the horizontal line, and they will therefore be the better able to fight the battle for existence. The second reason is, that, from the very nature of the Plum, it is to be expected that in bending down its branches to the right angle of its perpendicular there will be considerable damage done to the cellular tissue of the branch - the evil of which no gardener needs to be told.
Where the young tree has been cut as we have above recommended, it will the second year have three branches, and should be managed as already directed for a young tree having three growths.
The following season it will probably make from seven to twelve or fourteen shoots. We would advise all to be kept regularly pinched during the summer, save seven. Three of these ought to be on the leader, and two each on the two side branches, one being at the point of each shoot to form leaders, and the other four so placed that they will form well-positioned side branches, those upon the under shoots being upon the upper side thereof. The under branches ought at first to be trained at an angle of about 45°, and gradually brought down into their horizontal position, as we have recommended for the Pear. Where, however, the branches may be of unequal proportions, it may best suit the purpose to elevate the weak side to this angle, and depress the strong to the horizontal line; and where this does not of itself accomplish the end, let the roots upon the strong side be cut pretty well back, which will greatly assist the accomplishment of this object. If the tree is pretty vigorous all over, we would recommend that this season ought to be the first to introduce a regular course of root-pruning, which, while tending to make the tree healthy and robust, will also be the means of inducing early fertility.
Out of about four dozen young Plum-trees here, about four years of age, there were a considerable number which bore a few Plums this year; and one in particular - Coe's Golden Drop - produced and perfected twenty-eight handsome Plums, which, in a commercial point of view, were at least equivalent to the price of the tree. If thus a little trouble in root-pruning can, at the end of four years, produce such results, no one can doubt the practical utility and economy of so doing. The operation may be performed any time during autumn and winter, but the earlier it is done the better will be the results. In the case of a tree bearing fruit, we would not do so till November, in case of losing the crop; but where there is no crop, the end of August, September, or October will be the best time to do so, and the probability is, that next year there will be as much fruit as may counterbalance the cost of labour employed. In root-pruning the Plum, the same things are to be remembered as we have already mentioned when treating of the Pear and Apple - viz., digging right round and underneath, cutting the roots back pretty freely, according to the size and age of the tree, and providing the roots with some nice fresh materials, such as shall be hereafter recommended.