Growing Vines Indoors and Outdoors
While the vines are lying on the border during the winter, they should be sprayed to protect them against. Scrape off any loose bark, and then spray the vines with a 5% tar oil wash if trouble was experienced in the summer with scale insects, mealy bug or other pests. At the same time, tidy up the greenhouse, removing old leaves and other debris.
Although grapes grown in Mediterranean climates often flourish on thin, infertile, in cool temperate climates they appreciate feeding. In late winter, rake off any old mulch around the plant and dress the surface of the soil with proprietary vine fertilizer at the rate of 0.5 kg per sq m. If you cannot obtain specialized vine fertilizer, use 0.25 kg of good, general fertilizer. Fork the fertilizer in lightly and then mulch the soil surface with half-rotted manure.
Feed the plants as soon as the fruit have been thinned in the early summer, using a proprietary vine fertilizer at the rate of 120 g per sq m. Once set, grapes swell fast until the stones or ‘pips’ begin to form. Then they cease to swell until stoning has finished, when they begin to enlarge again. Give another application of fertilizer at the end of the stoning period.
If you are growing for exhibition, however, give an early summer dressing of fertilizer at 120-240 g per sq m and then from the second swelling stage, at two-week intervals, water on a liquid fertilizer, at the rate of 12 L per vine, until the berries begin to change colour.
In late winter or very early spring, when the top-dressing has been applied, begin watering. Soak the borders with water at the rate of 10 L per 100 sq cm (2 gal per sq ft) and water generously in this way two or three times more: once when the shoots are growing strongly, but before the flowers open, once when the berries have begun to swell and at other times if necessary, the object being to prevent the border becoming dry, especially deep down. Maintain a moist atmosphere by damping the floor and staging frequently.
Greenhouse Heating and ventilation
Vines are extremely hardy once they have become dormant in the winter and grow better in summer if they have been subjected to winter cold. During the winter, fully open the ventilators of the house to allow the cold to enter.
Close the ventilators when you wish the vines to begin shooting in early spring, and allow the greenhouse to warm up. In a heated house try to maintain a minimum temperature of 5°C (40 °F) and ventilate if the temperature rises above 10°C (50°F). Nevertheless, close the house down before it becomes dark in the evening to retain some of the sun’s warmth.
Maintain the same temperatures as above while the fruit is setting; but keep the atmosphere a little more bouyant by keeping a crack of ventilation open at night.
Once the fruit has set allow the temperature to rise to 19°C (65°F) before ventilating and maintain, if possible, a minimum night temperature of 10°C (50°F). As the summer progresses you will need to gradually admit more air to keep the temperature down. Generally speaking, grapes prefer those temperatures which are comfortable to the gardener himself. If you find the greenhouse too hot for comfort then the grapes, too, will be too warm. Too high a temperature is liable to cause scalding. If necessary reduce the temperature by painting the glass with a proprietary shading paint.
In late spring, the vines will begin to flower. Good fruit setters, such as Black Hamburgh, will set pollen if the vines are merely tapped smartly by hand at the base of the permanent rod. This will dislodge the flower caps, allowing the pollen to disperse.
Setting of the fruit in other varieties, particularly the Muscat kinds, however, should be aided by cupping the embryo fruit bunch in your hand—which should be dry—and then running your hand caressingly down the bunch. If you do this to different bunches in turn, you will transfer pollen from one bunch to another. Better still, if you are growing more than one variety, the best set of all is obtained by cross-pollinating two varieties by alternately running your hand down a bunch belonging to one variety and then the other. Particularly difficult setters, such as Muscat of Alexandria, can be cross-pollinated using a camel hair brush or a rabbit’s tail.
After setting, remove all the dead flowers by syringing.
About a month after fruit set, the grape bunches should be thinned. It is important that this should be done to all young vines—they are severely weakened if they are allowed to bear a heavy crop before their sixth year. But you should also do it to mature dessert grape vines to improve size and quality.
Harvesting and storing
Approaching ripeness is indicated by a dark tinge on black grapes and by a slightly transparent appearance to white grapes. When you see this, reduce the humidity of the greenhouse by ceasing damping and by opening the ventilators. Black grapes colour best if kept shaded, but white grapes ripen best in direct sunlight; remove foliage shading these.
Birds may be a problem if the ventilators are open. Fix wire netting across them to let in air but to exclude the birds.
Grapes are rarely as ripe as they look; so taste one before deciding whether to pick a bunch. Harvest the bunch by cutting the lateral 5 cm (2”) on either side of the stalk holding the bunch.
If you are not ready to eat all the bunches when they are ripe, they can be left on the vine, removing any berries that begin to go rotten. Alternatively, you can cut the bunches off with a long piece of stalk. Place the stalk in a bottle of soft water placed at an angle, with the grapes hanging over the side. Then store in a cool but dry and frostproof place.
Growing in pots
If you have a very small greenhouse, most of which must be devoted to other plants, but you would still like to grow greenhouse grapes, try growing one in a pot. Obviously, the size of the vine must be reduced to balance the restricted size of the roots, and pot-grown vines cannot be expected to produce such heavy crops as those in deep well-prepared borders. Nevertheless, a small crop of reasonable quality can be obtained from a pot-grown vine.
Use the biggest container you have space for. An un-creosoted wooden cask or plastic tub 60 cm (2’) in diameter and 60 cm (2’) deep is ideal. The container should be filled with a rich potting. Once the buds of the vine have 1. 2 burst, feed with liquid fertilizer every week and water daily in hot weather. Otherwise treat the vine in the same way as those grown in borders.
Train a pot-grown vine as a single rod as far as the greenhouse roof. Repot in fresh compost every other year, and topdress with rottedin alternate years in late winter.