Grape vines are lovely plants to train along the inside of a greenhouse or conservatory, but they do require a lot of room. One grape vine is plenty for a small greenhouse, but for larger ones allow 1m (3¼ft) between each vine.
Greenhouse grapes grow best when the roots are planted outside the greenhouse, and the vine is trained into the greenhouse through gaps near ground level. However, where this is not possible, the vines can be planted directly into the greenhouse border, but more irrigation will be required.
Double dig the ground and then incorporate a light dressing of well–rotted manure or compost, plus John Innes base fertiliser. If the soil is waterlogged, dig a hole 75-90cm (30in-3ft) deep and create a 15cm (6in) drainage layer of brick rubble, gravel or similar in the base. Greenhouse vines should be planted at the opposite end to the door, with the stems trained along the side of the greenhouse parallel to the ridge of the roof and running towards the door. Outdoor vines should be planted 4-5ft (1.2—1.5m) apart, 6in (15cm) away from wall/fence. November and December are a good time to plant, as the vine can be pruned back without bleeding at this time of year. Vines should be planted at the same depth that they were in the pot, teasing the roots out so they are well spread out in the planting hole. Grape vines can be grown in containers of loam-based John Innes No 3 compost. Use a pot about 30-38cm (12-15in) in diameter and depth.
In the first spring after planting, choose one shoot to be the leader and train. Any laterals produced from the leading shoot should be stopped at 5 or 6 leaves, and any flowers or tendrils should be removed.
Just before growth starts in the spring, mulch the rooting area with well-rotted manure and sprinkle the with John Innes base fertiliser and dried blood. During the growing season, vines benefit from an occasional extra sprinkling of dried blood. When growth starts in the spring, feed every three weeks with a high potassium feed, such as tomato fertiliser. Once the vine is in full leaf, increase this feeding to weekly intervals. When the grapes start to ripen and colour up, stop feeding with tomato feed, as extra feeding at this time may spoil the flavour of the fruit.
Water the vine thoroughly every seven to ten days in the growing season. Vines with the roots inside the greenhouse will need more frequent watering than vines with their roots outside the greenhouse. Where the roots are outside the greenhouse, be guided by the weather and concentrate your watering in dry spells. During the summer, it is a good idea to mulch the greenhouse borders with straw to keep the atmosphere dry. This will aid pollination of the vine flowers and subsequent fruit set.
Every three to four years, it may be necessary to ‘re-soil’ by digging a trench about 2m (6½ft) from the main stem, 60cm (2ft) wide and deep enough to reach any rubble put in to improve drainage. The soil removed form the trench should not be used on vines again, but may be used elsewhere in the garden. The trench should be backfilled with fresh topsoil.