If there is a prolonged spell of low temperatures and freezing weather, it is possible for Water butts to freeze and this expansion will cause the plastic to fracture as in the image left. If severe weather is forecast consider draining the water butt, otherwise it can be an expensive mistake.
Winter protection is needed for non hardy plants, follow this link for an explanation of Frosty Hardy. Popular examples of shrubs which are not fully hardy are Olive trees a Mediterranean shrub sold in various retail outlets and whilst Olives, especially once established will tolerate a significant level of cold, but not a prolonged spell of low temperatures which we often experience. Olives are ** hardy as is Laurus nobilis, common name Bay, both of which are increasingly grown in containers. Some varieties of Agapanthus, Escallonia, Herbs are not fully hardy, and need winter protection under glass.
To ensure winter survival, it is best to bring the plants in under glass and raise the container so it is off the ground by inserting feet or raising on bricks. If overwintering outside, it is important to wrap the container with something insulating, bubble wrap, Hessian, or old jumpers and if it's going to be freezing, cover the plant with a fleece. It is equally important to remove the fleece when the cold spell finishes to ensure the plant gets plenty of air circulation. Plants covered all winter, or covered up for too much often get damp and rot which means it is best to limit cover to the cold spell.