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 …