The production of containerized nursery stock started in southern California because of its mild climate and long growing season (Whitcomb, 1987). As production of containerized stock moved into areas of the country with harsher climates, methods of overwintering were developed to protect plants from winter damage. Proper winter protection in the production of container-grown woody plants is crucial because a plant is no hardier than its root system (Patterson, 1936; Mathers, 2003). Unlike stems, roots exhibit little dormancy and can grow anytime soil temperatures permit (Romberger, 1963). Young roots tend to grow on the outside edge of the root ball in contact with the container wall. Because young roots are less hardy than mature roots they are often the first to suffer winter injury (Mathers, 2003). When overwintering practices do not adequately protect young roots, new root regeneration in the spring is retarded resulting in plants that flush later and grow slower. To ensure that plants remain viable and marketable, overwintering protection must be adequate to protect plant roots from both extremely low and drastic fluctuations in temperature (Iles and others, 1993; Mathers, 2003).
Kirk, Steven D.; Starbuck, Chris J.; Van Sambeek, J.W. 2004. Missouri gravel bed and a pot-in-pot system superior to white polyethylene and foam for overwintering Syringa pubescens subsp patula Liners. International Plant Propagators Society, Combined Proceedings (2003) 53: 132-137.