While bananas, pineapples, and the various members of the genus Citrus require so much heat that they are mostly outside the range of the ordinary amateur gardener, a number of hardier fruits can be obtained earlier with the aid of glass protection.
Some heat is necessary. The blossom and newly-set fruitlets of all our so-called hardy fruits are vulnerable to frost. In an unheated greenhouse, sun heat is stored up during the daytime so that the average temperature is well above that prevailing outside. Further, the occupants of such a house are not exposed to chilling, growth-inhibiting winds, and these two factors result in earlier blossoming This, of course, increases the risk of frost coinciding with flowering. Just sufficient heating, therefore, must be available to keep the temperature above 32°F (0°C) during the coldest weather.
Greater heat than this is not really necessary but if you aspire to eat ripe strawberries in April and early May some regular heating will be required.
Any hardy fruits can be grown under glass but those generally considered the most rewarding are peaches and nectarines, figs, and grapes. Strawberries are accommodating because they occupy the house for three or four months only, leaving the space available for other purposes during the remainder of the year. Further, you can force strawberries one year, if you wish, and not the next, but once a tree fruit, or a vine is planted, it is ‘there for good’. Melons are another possibility for the greenhouse owner and these are in a category on their own.
Where space is at a premium, fruit trees may also be grown under glass in pots. This restricts root action so that the trees remain small. Pot-grown specimens of apples, cherries, figs, peaches and nectarines, pears, plums, and gages may be obtained for this purpose from specialist nurseries. Peaches and nectarines Both these fruits are treated in the same way, the latter simply being a smooth-skinned form of peach. They are usually grown as fans and are most easily accommodated against the back wall of a lean-to-house, but they may also be trained on wires fixed 0.6m (2ft) away from the glass or on a trellis arranged across the house.
Although plenty of water will be required in the growing season, drainage must be impeccable. If necessary, therefore, the border should be excavated to a depth of about 1m (3ft) and either field-drain pipes laid or a foot depth of rubble put in. A layer of turves, laid grass-side down, will prevent the soil above from clogging the drainage. The peach tree will need a depth of 46-60cm (18-24in) for its roots but this soil should not be too rich or excessive growth will be encouraged in the early years at the expense of fruiting.
For a path, wooden duck-boarding is preferable to concrete so that moisture can penetrate and the roots be free to extend.
Trees should be planted in the usual way and the young growths tied to canes which in their turn, are tied to wires. Training and pruning follow similar lines as for outdoor trees but remember that in the greenhouse the trees will receive only the water you give them. The border will need to be flooded in January to start growth and frequent watering will be necessary when growth becomes active. Syringing is also necessary, once or twice daily, from bud break until the fruit begins to color. This syringing may be quite forceful, to discourage pests. While the flowers are open, the syringing should be reduced to once a day, around midday, to encourage pollination.
In January the house ventilators should be closed early in the afternoon to trap the sun’s warmth, the temperature is maintained as near to 40°F (4°C) as manipulation of the ventilators can secure. In February the temperature may be allowed to rise to around 50°F (10°C) but, should very cold weather prevail during the blossom period, and immediately after, some heating will be necessary to keep the temperature at least above freezing point.
Pollination is the difficulty with fruits under glass because the insects which usually perform the service for us are not present. A hive of bees may be kept in the greenhouse, but for most gardeners this is impossible, and resort should be made to hand-fertilization of the blossom, transferring the pollen about midday with a rabbit’s tail (if you can get one), a camel’s hairbrush, or a small piece of cotton wool tied to the end of a stick. Do this before syringing as an additional aid.
In March the temperature may rise to 55-60°F (13-16°C), but when outside temperatures begin to exceed this try to keep the atmosphere in the house buoyant by adequate damping down and ample ventilation. From May on some light shading should be provided—with blinds or strips of plastic netting shading material or by spraying or painting the roof glass with a proprietary shading preparation.
Thinning of the fruit should be carried out, as with the outdoor crop, in easy stages. If large fruits are wanted, the final allowance of space should be a 30cm (1ft) square per peach and 23cm (9in) square for each nectarine, but for an average size, thinning may be less drastic, allowing from 16-23cm (6-9in) square per fruit.
As ripening begins, all syringing must stop and more air should be given. Foliage which shades the fruit should be tied back temporarily.
Once the peach crop has been gathered, spray as forcefully as you can and ventilate as freely as possible to assist the ripening of the new wood. Continue to water the border regularly. Ventilators should be left open, night and day, until January when growth is to be restarted. Untying and pruning may be done early in the autumn.
If spraying with oil is considered necessary to control aphids and scale insects, this may be done as soon as it is certain that the tree concerned is quite dormant.
Feeding is seldom necessary in the first year or two as it may result in growth rather than fruit. Some food, however, will be needed once heavy crops are being carried. A mixture of 2 parts of sulfate of ammonia, 2 parts of superphosphate and 1 part of sulphate of potash, all parts by weight, should be scattered over the border at the rate of 140g (5oz) per 2 sq m (2 sq yd), and lightly raked in and then watered. In March put down a mulch of rotted stable manure or garden compost. If the burden of fruit-bearing appears to be too great and fresh growth is being made only slowly, an extra fillip can be given after stoning, in the form of liquid manure or dried blood applied at the rate of 112g (4oz) per sq m (sq yd) of border.
Growing Peaches in a greenhouse are liable to the same pests and diseases as those in the open, although peach leaf curl is usually less troublesome and red spider mite very much more so. The following varieties, given in order of ripening, are suitable for greenhouse culture, those marked with an asterisk is the most reliable:
Peaches : ‘Duke of York*’, ‘Waterloo’, ‘Peregrine’, ‘Royal George’, ‘Dymond*’, ‘Bellegarde’.
Nectarines : ‘Early Rivers’, ‘Lord Napier*’, ‘Pineapple’.