Plastic wrap holds moisture. When overwintering bananas, you don't want moisture to be held near the stalks during the winter. A better idea would be to wrap the stalks with layers of cotton towels. Then get some old tires and place them over the stumps. Fill up the tires with mulch. Then place an old towel over the stacked tires and then finally top with plastic on the OUTSIDE of the tires. Plastic against the stems will only encourage the water inside the cut trunks to stagnate and rot will occur. Try to cut off the mushy part now, re-wrap without plastic on the trunks and take everything off when heavy frosts are done for the season.