If its rooted you should take the plastic off. Probably a good idea to overwinter it in a cold greenhouse. Don't overwater it - just enough to prevent it drying out completely - the amount really depends on the weather - it will need less when its very cold. You can usually tell by the look of the plant if it is very short of water.If there's a likelihood that the soil in the greenhouse may freeze you should protect it as when soil freezes the plant can dry of dehydration.