Even though trees are inactive or in dormant state, its roots are self-reliant and can still grow during the winter season. Some roots remain dormant in this frigid weather, but if the soil reaches favorable temperatures root growth could occur. For example, temperatures that are suitable for roots in winter are between 32 through 41 degrees Fahrenheit.
Roots take up the responsibility to take in essential nutrients from the soil and water for the whole tree. Whatever food a tree obtains, goes towards the trunk, new buds, leaves, fruit and flowers that will grow into spring.
Sources: Ridgewood Tree Corp., “What are your trees doing in the winter”, http://www.ridgewoodtreecorp.com/what-are-your-trees-doing-in-the-winter/
Micheal Snyder, “What Do Tree Roots Do in Winter”, http://northernwoodlands.org/articles/article/what_do_tree_roots_do_in_winter, December 1, 2007