Many gardeners have the idea that high temperatures are needed to grow. This is not altogether true as by choosing varieties carefully it is perfectly feasible to grow a vine in a cold greenhouse (there are, in fact, several varieties that can be grown successfully in the open). The themselves are perfectly hardy but some need a longer season than others to ripen their fruit.
One of the best grapes for a cold greenhouse is Black Hamburgh, and Buckland Sweetwater, a white grape, is another. One of the finest dessert grapes is Muscat of Alexandria, but it is not so easy to grow and must have heat to ripen the fruit.
Vines can be grown in a border but where space is limited a large pot or tub can be used if only a few bunches of grapes are needed each year.
The Vine Border
To grow a vine successfully themust be prepared carefully. The border can be inside or outside the greenhouse. In an outside border the main stem of the vine is passed through a hole in the side of the greenhouse. Good is important and if there is any doubt about this it pays to excavate the soil and lay rubble at the base of the border with tile to take surface water away to a lower level. In heavy, sticky soils it is also wise to have an enclosed border with brick or concrete sides so that the roots of the vine cannot penetrate into unsuitable soils. In poorly drained soils vine roots cannot function properly and troubles such as will develop. Unless the existing soil is good and well drained the border is best filled with a prepared mixture composed of 8 parts decayed turves to 1 part grit or coarse sand. Add a 5-in. pot of coarse bonemeal to each barrow load of the mixture.
The developing grapes must be thinned to allow room for them to expand. Long, pointed scissors are used for this operation
Dormant vines grown in pots can be planted just before new root growth starts in late winter. The roots should be disentangled so that they can be spread out when planting and covered with about 2 in. of soil. Vines must never be planted with their roots in a tight ball. Plants raised from ‘eyes’ in February in a warm greenhouse can be planted direct in borders inside the greenhouse in the summer, but as the roots are active at this time of year they need not be disentangled.
Vines flower and fruit on the new growths made each year and the usual method ofin winter is called spur pruning. All the sideshoots made in the previous season are cut back in November or December to within one or two buds of their base. Afterwards, it is wise to remove all the loose bark on the main stems of the vine because this forms an ideal hiding place for pests such as mealy bugs or the overwintering eggs of aphids and red spider mites. It can be done by scraping carefully with a knife or by twisting with the hands. Care must be taken not to expose the green rind beneath the bark.
The growths removed when pruning can be used to produce young vines. The best and sturdiest shoots can be bundled together and put outside, partly burying them in the soil. In February the shoots are cut up into ‘eyes’. These consist of pieces of stem each containing a single bud or ‘eye’. Trim the cut surfaces with a sharp knife and place the eyes horizontally in sandy soil in 3-in. pots. Add a little soil to keep them in position but do not cover completely, and place in a propagating frame with a temperature of 18°C. (65°F.).
New growth soon develops from each bud and good roots appear in about six weeks. The young plants can be moved to a border in the summer or to 5- or 6-in. pots. In the following winter they are repotted in 10-in. pots if they are to be grown permanently in pots and not in a border.
Pot-grown vines should be repotted after pruning. Scrape away some of the old soil, and return the plant to a clean pot of the same size. John Innes No. 3 Potting Compost can be used or the mixture recommended for vine borders. Firm the new compost with a rammer.
After potting, make a framework with stout stakes or bamboo canes on which the new growths which appear in the spring can be supported. One arrangement is to have a stout central stake with a circular wooden hoop fixed at the top with cross pieces. The new growths are trained and tied to the hoop.
The new growths made in the spring must be curbed if they are not to become a tangled mass. They should have their tips pinched out two leaves beyond the flower or embryo bunch of fruit. Secondary growths will develop and these should have their tips taken out at one leaf. Tendrils must also be pinched out.
Watering and Feeding
Vines in pots soon dry out in summer and watering must be attended to carefully. A vine border dries out less rapidly but also needs examining regularly. When the border is watered, sufficient must be given to penetrate deeply to all the roots. A mulch of decayed manure over the border greatly conserves soil moisture. A topdressing can be given to vines in pots using fresh potting soil and to make space for it a zinc collar can be placed round the rim of the pot. Feeds of a well balanced fertiliser must also be given to keep the plants growing well.
Pest and Disease Control
Pests which attack vines include aphids, mealy bugs, red spider mites and scale insects. These can be controlled by spraying each year with malathion when the new shoots first appear. Mildew may affect the fruits and can be controlled with a dinocap spray when thehave set.
Developing grapes must be thinned so that each berry has space to enlarge. Surplus berries are removed with a pair of long, pointed scissors. Start at the bottom of each bunch and work upwards, removing the smaller seedless ones first. To avoid touching the berries, and thereby spoiling their bloom, a small forked stick is useful.
After the fruit has been picked, vines in pots can be stood outside to ripen the wood. Watering must continue until the leaves fall and in December they should be brought into the greenhouse for pruning and repotting.
When the vines are dormant it is a good idea to untie the main stems (rods) and let them hang down on long strings. This will prevent the sap rising too quickly in spring and will encourage buds at the base of the rods to develop. When the new shoots are 2 to 3 in. long, the rods should be tied back in position.