Plums and gages may be grown as standards, half-standards, bushes, pyramids and fans. They are unsatisfactory as cordons or espaliers. They do best on deep heavy loam or on a well-drained clay soil. Ex-vegetable plot soil (rich in nitrogen) is excellent. Light soil results in poor fruit quality, brittle branches and a short life.
Plums are extremely susceptible to a fungoid disease known as silver leaf which is most infectious during the winter months. Pruning should be carried out during the summer, in spring or in early autumn immediately after the crop has been picked.
- FIRST YEAR: If a maiden is planted, behead it at about 3 ft. from soil level in spring just before growth starts. Existing feathers may be selected for the fruit branches if well spaced. Plums are very liable to branch breakage and it is important to select as branches laterals with a wide angle to the main trunk, these being much stronger than those which are more nearly vertical and make an acute angle with the stem.
- SECOND YEAR: In early spring cut back the selected branch leaders by a half.
- THIRD YEAR: Again cut leaders by a half of the previous season’s growth.
- SUBSEQUENT YEARS: Between June and August remove crossing branches to open up the centre of the tree. Cut out any dead wood before mid-July. Some laterals may have to be cut out or shortened in summer to avoid overcrowding but otherwise plum pruning is best kept to a minimum. Should growth be poor, cut back some laterals drastically to encourage new wood. Plums fruit on second-year wood and on spurs which will develop without your assistance on older wood.
The pyramid is the best form for plums in most gardens because it only needs 10 ft. of lateral space and can be restricted to a height of little more than 9 ft. You cannot buy plum trees already trained as pyramids so you must start with a maiden which should be on St Julien A rootstock. Apart from size a great benefit with this type of tree is that branch breakage is substantially reduced and, with it, there is less possibility of silver leaf infection.
- FIRST YEAR: In late March behead the maiden 5 ft. above the ground. Cut off at source any feathers up to a height of 18 in, and reduce those above that point to half their length. When the growth of new shoots ceases (about the third week in July) cut back branch leaders to 8 in., pruning just beyond a bud pointing downwards or outwards. Shorten laterals to 6 in.
- SUBSEQUENT YEARS: In April cut off two-thirds of the central leader’s new growth. To keep the trees straight, cut to buds on opposite sides of the three each year. When new growth ceases about the third week of July cut branch leaders to 8 in. and laterals to 6 in. Once the tree has reached 9 ft. cut the central leader to an inch or less of new growth each May. Should a new vertical shoot grow up to replace the central leader, remove it at source.
Pruning Fans Form the framework of the fan in the manner described for peaches. When the tree reaches fruiting age, however, the treatment has to be slightly different as the plum fruits on old and new wood and one does not, therefore, have to cut back a lateral as soon as it has fruited. Any new laterals not required to extend the tree or to replace old laterals should be stopped when they have made six or seven leaves. It may be necessary to ‘go over’ the tree several times during the growing season for this pinching. Also rub out entirely any shoots pointing at the wall or directly away from it, doing this as soon as these unwanted growths are noticed.
As soon as the crop has been picked cut back by half all the laterals previously stopped and either tie down towards the horizontal any vigorous shoots growing vertically or cut them out entirely.
With glut crops on bush or pyramid trees, the thinning of plums not only increases fruit size but reduces the risk of branch breakage and the entry of silver leaf disease. Proper thinning also promotes regular cropping.
Thin gradually in two stages—early in June and, later, after the natural crop during stone formation. Break or cut the fruitlets off so that stalks remain on the tree, eventually reducing dessert plums to a minimum spacing of 2 in. Cooking plums may be slightly closer.