The Plum Family
There are at least 14 species of cultivated plums, of varying quality. The species grown in the U.K., Primus domestica, is derived from a natural hybrid between the cherry plum and the sloe. To this mixed genesis we owe the remarkable range of colour, form and flavour.
Plums are divided into the general run of varieties such as Yellow Egg, used in the main for preserving; the dual purpose varieties, such as Victoria, and those varieties which develop a high sugar content and are delicious to eat raw. For convenience we group the latter type under the heading of gages, a term more strictly applied to the greengages and similar varieties. Greengages make a splendid jam, than which there is no better—but the buds of gages are high on the sweet-toothed bullfinch’s list of winter delectables. Damsons stem from a distinct but closely related species. The cultivation details are very similar, so they can be discussed under the same heading as plums.
These delightful fruits, once so easy to grow, are now under a cloud. The problems include increased bird damage, mostly by bullfinches and sparrows; the ancient disease which causes silver leaf; erratic cropping due largely to cold springs, and a new menace caused by a horrid virus disease with the dramatic name of plum pox, or sharka disease, which seriously affects the fruits. Silver leaf continues to elude satisfactory control, although there are some rather slight hopes that one of the new systemic fungicides might succeed where other measures have failed. There is also the intriguing possibility, now being researched, that a biological control may be achieved by the introduction of an antagonistic fungus.
Frosty springs we can do little about, short of providing protection by cover, heating or water sprinkling, none of which is easy to provide in gardens.
Plums should be afforded the same considerations as those given to pears and the same preplanting operations used for apples. The only difference is that most of the plums will tolerate heavier, wetter soil than either apples or pears. So if there is a spot in the garden which lies a bit on the wet side without being waterlogged, use it for one or two plum trees.
The form of tree to grow is to some extent governed by the nature of the plant. Many of the popular varieties, including Victoria, possess weak pendulous branches. To keep these branches off the ground it is usual to grow the trees as half-standards, so that the first branch starts at about 1.5m (5ft) from ground level. There are a few strong-branched varieties, such as Czar and Marjorie’s Seedling, which can be grown as open-centre bush trees with a short (70cm) leg, but tradition calls for a half-standard tree which, grown on St Julien A rootstock, will give an average branch spread of 4 to 5m (13 to 16ft) across, and which can be planted accordingly.
A half-standard tree must be securely staked to prevent rocking or blowing over.
The best result is achieved by a double-stake and cross-bar, using two posts each 2m (6ft 6in) long. The tree is tied to the crossbar and padded to prevent chafing.
Plums and gages can also be grown as fan trees against walls, but these rather special sites should be reserved for the choice gages.
Pruning is based on rather different principles to those which govern other fruits. For the first four years or so prune the bush or half-standard tree in the winter as for an apple of similar form, with the intention of making sure that the primary branch formation is strong and permanent, and able to carry heavy crops without breaking. Thereafter keep pruning to the very minimum: remove only broken or diseased branches. Do this work in the summer, not the winter, because there is much less likelihood of silver leaf spores invading the wounded tissues during June, July and August. Take care to remove all sucker growths from the roots as they appear.
Branches carrying heavy crops should be temporarily supported to prevent breakage. Use struts, carefully positioned to avoid chafing, or a tall central stake with ties suspended from it as from a maypole. It is seldom necessary to thin plum fruits for garden use, save in exceptional years. In any case the natural drop of young fruits is often ‘severe and alarming, but it is nature’s way of shedding the superfluous. Feeding
The recommendations made for the manuring of pears apply equally to plums, which, conveniently, have similar needs. Plums do, however, differ in their reactions to, being particularly allergic to .
Plums, ready for picking in September, cannot be stored. Use them as soon as you can: eat them, make jams or bottle the fruits. If in difficulty in a glut period, Victoria can be kept in the refrigerator for a week or two. All good quality fruits can be frozen.
Pests and diseases
Much damage is caused by bullfinches and sparrows, which eat the blossom buds in winter. Reduce damage with black cotton, or spray the trees with one of the temporary deterrents.
Aphids and caterpillars can be readily controlled by spraying with malathion or rogor in accordance with instructions just before blossoming begins. Take great care not to spray open blossoms because of the risk to bees and other welcome visitors. If you miss the right time and there appears the characteristic leaf curling caused by aphids, or the small—later large—holes left by hungry caterpillars, then spray as soon as blossoming has finished.
Silver leaf can be reduced by avoiding the large wounds through which most infections occur. Hence the emphasis on building a strong early branch frame-work and pruning fruiting trees in the summer months. The silvering of the foliage is due to air in the leaf tissues caused by toxins produced by a fungus which may be lower down the branch. The spores of the fungus are not produced from the silvered leaves, so do not be alarmed if you see silver appearing in your or your neighbour’s plums—or apples, laurels or even poplars, all of which can be attacked.
Occasionally trees which are short of nitrogen are thought to be suffering from silver-leaf because of the pale colour of the leaves. Sometimes the tree or bush manages to control the fungus, the silvering disappears and the plant grows normally. More often, unfortunately, the silvering becomes more pronounced and the branch begins to die. It is sensible to cut out and burn affected branches as soon as they are seen, cutting well back and dressing the wound with one of the proprietary wound dressings, or at least with a thick layer of white lead paint. But if the whole or the major part of the tree is affected wait to see if natural recovery-occurs, taking care to act immediately the branches begin to die. This is crucial, because the fungus enters into its fruiting stage once the tissues die.
At this point purplish bracts appear on the surface issuing vast numbers of dustlike spores to be carried away on the breeze. The original diseased tree, if left alone, will continue to produce spores and menace the neighbourhood for years. Always cut out and burn dead wood from trees affected with silver leaf, and try to persuade your neighbours to do likewise.
Plum pox, or sharka disease, is incurable and can only be prevented by the eradication of all diseased trees. What makes this virus disease particularly difficult to control is that, unusually for fruit tree afflictions, it is carried by aphids. By sucking the sap of diseased trees the aphids may convey the virus particles to healthy trees on which they subsequently feed. The Ministry of Agriculture are working very hard to control this disease. Let us hope they succeed, because the fruits from sharka-affected trees are useless.
There have in recent years been developments in plum growing of great benefit to gardeners. New rootstocks producing small compact trees are now available, and their propagation has been made easier by work at East Mailing. Additionally, new systems of training in pyramid form, referred to later, offer hopes of small but fruitful trees. Nurserymen’s catalogues will offer trees on Myrobalan B and Brompton root stocks. Both are excellent, but make big trees needing some 6m (20ft) of space. Choose instead either Pershore or St Julien A. Before long a new series will be available, carrying such intriguing names as Pixy, which is said to be very promising for small trees, although the prospect of uniting the majestic Victoria to Pixy seems a trifle undignified.
Relatively few of the innumerable varieties available are really suitable for gardens. Any short list, in order of ripening, would include:
River’s Early Prolific
To reduce the list to four, consider the following varieties:
Czar is the most reliable of plums, bearing attractive purple fruits in early August. Its only fault is that, like Victoria, it is susceptible to silver-leaf.
Victoria is still much the best of the dual-purpose plums and if there is room for only one tree then this is a clear choice.
Severn Cross is a reliable heavy-cropping jam-making plum of dessert quality.
Marjorie’s Seedling, a good-natured, late-ripening purple plum completes the quartet. The four chosen will cross-pollinate and fertilize each other admirably.
An added qualification for the choice of Victoria as sole representative is that it will set fruit with its own pollen, a quality shared by the other three, but it is thought that in cold springs all plums set better with the aid of cross-pollination.
Damsons are very easy to grow, treated as plums, choosing the variety known variously as the Prune damson, the Shropshire damson or the Westmoreland damson. They all appear to be identical, unless you are fortunate enough to live in Shropshire or Westmoreland, when you use the local name for this hardy fruit. The damson must be tough to grow, as it does, in the Lake District.