A greenhouse is not strictly necessary for growing grapes as a few varieties can do quite well outdoors. But by far the tastiest, more luscious and gorgeous varieties do have to be grown under cover and indeed there are few more rewarding crops.
Now a large greenhouse used to be needed as grapevines are very vigorous plants. However these can be grown in a small greenhouse if you adopt a completely different method to the traditional. Vines always used to be planted in a border inside, or outside trained in through a hole. The problem with this is the vines then make far too much growth and become huge requiring almost continuous pruning yet still filling all the space available. And then they ripened a massive crop maturing all at the same time. In more modest greenhouses vines are much more suited to growing in tubs. This cramps their roots and thus their growth so they can be restricted to growing up a pole to a height of six feet or so. This means that in a small greenhouse you can fit in several different vines each of which will yield up to a half dozen bunches and so much the same in total as from one vine growing in the ground.
And there is another huge advantage to growing in tubs. In the late summer or early autumn once the crop has been gathered the vines can be moved outside and left there as they’re quite hardy. This together with a ruthless autumn prune makes it more difficult for pests and diseases to over-winter than when a vine is permanently housed. Then in late winter or early spring the tubs are brought under cover again where the sudden warmth kicks the vines into an earlier start than if they were housed there all the time. By having several tubs of the same favourite variety you can bring them in sequentially a fortnight apart so spreading your harvest over a much longer period. Or have several different varieties both extending the harvest season and giving a range of rose, white, and black grapes, personally I find the blacks the most desirable. I’d strongly recommend Boskoop Glory for reliability, Siegerrebe (rose) for earliness, Black Hamburg and Madresfield Court for lusciousness, Muscat Hamburg for outstanding flavour, Flame (rose) and Polo (white) are modern seedless ones, and there are many dozens of others.
Their compost should be top dressed in spring and they need regular watering with the occasional liquid feed. In autumn all growth is pruned away leaving a stump with say five stubs of this year’s young canes each carrying two or three buds. When these burst in spring reduce the shoots to the best half dozen each carrying a good bunch and remove all others. Wind these canes about a central pole and remove the tips when they reach the top. Other than keeping an eye out for the usual greenhouse suspects there is seldom any problem as their winter sojourn outside helps keep them clean.