How to Grow Grapes
Yet again, the section on growing vines out of doors should be read first, since the principles apply equally. While the varieties and the environment are different, much more skill is required. The rewards are, however, very great, since there is nothing in horticulture more thrilling than producing a well-grown crop of glasshouse grapes.
Varieties and propagation
The choice of variety is crucial. The superb high quality muscat grapes, typified by Muscat of Alexandria and Canon Hall Muscat, are difficult to grow and require facilities beyond those at the disposal of most amateur gardeners. Reluctantly, there-fore, one is obliged to suggest that even the keenest of amateurs should lower his sights and restrict himself to the easier, but still demanding, varieties of which Black Hamburgh is the most popular. Foster’s Seedling, a good natured white grape of fair quality, is an alternative.
The vine is usually grown on its own roots. Propagate by taking I .5 cm (2 in) hard-wood, each bearing a single bud, in the dormant period. Place in 7.5 cm (3 in) pots in a warm place.
Grow glasshouse vines in borders rather than containers. You can either plant the vine immediately outside the glasshouse and train the stem inside, or grow the whole of the plant, roots and all, inside the glasshouse. The latter method has the advantage of giving more control of the total environment, but circumstances may make it necessary to use an outside border. Some excellent grapes have been produced from vines thus planted.
Whichever approach is used it is important that the root-run should not be less than 60 cm (2 ft) deep and 1 m (3 ft) wide. Good drainage is essential, and the bordershould be thoroughly dug and enriched before planting takes place.
This will depend on how many main stems, or rods, are to be grown on the one vine. The best results are obtained from single rod or cordon vines, which are spaced 2 m (6 ft 6 ins) apart, to allow the fruit-bearing laterals to develop on either side to 1 m (3 ft) without tangling with the neighbour. One rod of Black Hamburgh, well grown, at the end of the mixed plant glasshouse will meet most purposes.
Allow the vine to enter into a dormant state after fruiting and keep the house cool during the winter, the needs of other plant inhabitants permitting. Some of the old school of gardeners used to hose the rod down during frosty periods until encased in ice.
In spring, when growth begins, the minimum temperature may be raised to 7°C (45°F). Bud break can be assisted by syringing with clear water in the mid-morning of fine days.
As growth extends, the night temperature should be raised to a minimum around 10°C (50°F). Much higher temperatures will occur during the summer and a light shading may be required. Regular, as distinct from sporadic, watering is necessary and particularly so with vines grown in indoor borders.
Syringing should continue until the fruits begin to colour, but every effort should be made to ensure that the rods are dry by nightfall. A sharp friendly rap on the rod when the vine is in flower will help to distribute the pollen. Choose a warm period of the day for the rapping session. Varieties other than Black Hamburgh call for rather more complicated procedures.
Basically, pruning is very simple, but the timing calls for skill and experience. The fruits are carried on lateral shoots produced annually from spurs on the main rod. The art of growing consists in controlling these laterals. Beginning at the beginning, and assuming that the vine is established, pruning is done immediately after leaf fall, usually in December. It is important not to make major cuts when the rods are active because the sap pressure is such that the vine ‘bleeds’ from the cut surface.
All the lateral shoots are cut back to two buds. Shorten the extension shoot at the same time, back to two buds if the rod has reached its allotted length, or about halfway back if extension is still permissible.
The result is a rather stunned-looking rod, deprived of all growth save for the knobbly spurs.
When growth starts in spring the young shoots should be reduced to leave two to each spur. This reduction process continues as the need dictates throughout the growing season, so that all energy is concentrated on the two selected shoots. Some of these will tend to grow vertically towards the glass instead of laterally and it is necessary to pull these down gradually, not suddenly, by an adjustable tie attached to the middle of the shoot at one end and a wire, or the main rod, at the other.
Generally speaking, and assuming that large berries on large bunches is the aim, the bunches should be reduced progressively so that only one well-placed bunch is left on each shoot by say mid June. Even this may be too much for a vine not in the peak of health and vigour; a dozen large bunches per rod is as much as should be asked of a willing vine.
Left to its own devices a lateral will grow on and on; don’t let it. Pinch out the growing point two leaves beyond the selected bunch. Sub-laterals should be pinched back to one leaf, to give active leaves without undue competition for light.
Fruit thinning and harvesting
Thinning of the fruits is an unnatural process, but a necessary one if large berries are wanted. The task is best accomplished by the use of a pair of scissors with long narrow blades, known in the trade as ‘grape’ or ‘vine’ scissors.
The point of these scissors can be inserted into the heart of the bunch to remove single unwanted grapes. Most attention should be paid to the insides of the bunch, the shoulders needing little thinning. Indeed it helps to lift the shoulder away from the main part of the bunch by means of a tie, to give the swelling berries more room and to admit air to the centre of the bunch. Thinning should be gradual, and carried out in stages. The small berries should be removed first, together with all those in the centre and any damaged or diseased ones.
It is a drastic process, requiring the removal of some two-thirds of the berries. Thinning will be completed by mid-July, and colouring of Black Hamburgh will start a month or so later.
Harvesting can be staged according to use; the berries will keep well on the vines in a buoyant atmosphere, but you must trim out any over-ripe or rotting berries.
After harvest the vines are kept as cool as conditions permit, but the border must not be allowed to become really dry even in the dormant period.
Pests, diseases and disorders naturally abound on such a choice subject, but all are within control. The most likely disease is powdery mildew, controlled by spraying with dinocap in spring as a preventive measure. A further precaution is not to allow the foliage to become overcrowded.
A summer fillip is a great help in June when the vine is under strain. This can take the form of a quick-acting nitrogenous fertilizer, a favourite being dried blood.