If you want new potatoes in December, then they can be grown in bags in a sheltered place. In early August you can buy second earlies from a garden centre that have been stored at low temperatures to prevent them from sprouting. They will be ready to harvest in November and the tops will need to be trimmed to about a couple of inches as they begin to die down. They then can be moved to a frost-free environment and allowed to stay in the bags and harvested as required in December. Regular watering and feeding is essential during the growing season.