Rootstocks can affect the development of cold hardiness in apple trees, but this is a complex trait, and it may depend on what time of the winter with which one is concerned. Some rootstocks, such as Robusta 5, can cause a tree to harden early in the winter, and these trees are very cold tolerant during the early winter. Some rootstocks, such as Robusta 5, that induce early cold hardiness also lose their hardiness early in the winter in response to warm periods in late winter. Other rootstocks can enhance the maximum cold hardiness of a tree during the middle of the winter. Therefore, a rootstock may survive winters in Ontario, where temperatures are consistently cold, but the same rootstock may be winter injured in Arkansas where winter temperatures fluctuate. Find more information in this article on Winter Hardiness of Apple Rootstocks.
Answer provided by Dr. Richard Marini, Penn State University.