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.
Site and soil preparation
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 at 90g per sq m (3oz per sq yd). 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.
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. For more detail on planting technique, see planting trees and shrubs.
Just before growth starts in the spring, sprinkle the rooting area with John Innes base fertiliser and dried blood at a rate of 120g per sq m (4oz per sq yd) each.
During the growing season, vines benefit from an occasional extra sprinkling of dried blood at 30g per sq m (1oz per sq yd).
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 10 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.
Just before growth starts in the spring, mulch the rooting area with well-rotted manure.
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’ the rooting area during winter 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 from the trench should not be used on vines again, but may be used elsewhere in the garden.
With a fork, gradually draw the soil away from the roots into the trench, working towards the stem. Empty the trench as necessary. Take care not to damage the roots during this process, and cover any exposed roots with damp sacking or straw to prevent them from drying out. Stop removing soil when you get 60cm (2ft) from the stem. Ensure that the roots are well spread out, and then backfill the trench with fresh topsoil. Firm well so that the roots make good contact with the new soil.
Growing grapes in containers
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.