The above-ground portions of woody plants can tolerate far more cold than the roots can. Roots may start to be damaged if the soil temperature drops to 25 F. or below. Even with air temperatures well below zero, the cold only slowly penetrates into the soil. This is especially true if there is a layer of good insulating material on top of the soil, such as snow, mulch, leaf litter, etc. However, with a container plant, the cold can far more easily chill the soil to damaging temps by penetrating not only through the top but also through the container's sides. If left outside in Zone 5, containerized plants are likely to perish unless surrounded on all sides and on top by ample insulating material (e.g., straw).