Is the root inside or outside the greenhouse?. If inside the vine will be hard pressed to plump up the grapes. We have a huge vine completely outside on a verandah. I never water it or fuss over it. It reaches over 30 feet. It has masses of grapes formed and getting quite large. A grape does not need to be under cover really. After a few years the greenhouse will be full of grapevine and need drastic pruning......which grapes don't mind.