The physiology of Trees in the Winter

tree1 by Eric Putnam


Eric Putnam is a local arborist with a passion for taking care of trees. He is owner of Arbor-Tech Consulting and gives free tree diagnosis. Check out his FACEBOOK page!


Even though your trees may look bare and dead during the winter season, they are actually still living, breathing and growing the entire time. That is why buds form on the bare branches and then the leaves reappear after the wintertime. Leaves are not essential for the cellular growth of a tree. Tree respiration moves the minerals and other nutrients throughout the tree thus feeding the cells, allowing them to expand and grow, and store energy. Tree respiration along with their continual growth is essential for their survivability during the extreme temperature changes throughout the winter season.


While you don’t see it (because it is occurring underground), a tree’s roots are actually growing all year round along with different fungi and bacteria necessary for root growth. Temperature ranges affect the different types of bacteria and fungi that are  present to help keep the plants and trees functioning throughout these temperatures. During near freezing or scorching temperature points, not much happens in the growth of trees or plants. This is usually when taking care of your plants and trees is very crucial to their life span.


Ever wonder why the deciduous trees in our area still have most of their leaves during the wintertime? It is mostly due to the fact that we do not have very many days with temperatures below 50 degrees. Temperatures below 50 degrees cause a tree to go into full dormancy and can cause the leaves to turn from green to maybe yellow or even brown. However, it can make for a very beautiful and colorful fall if there is enough cool weather before the leaves drop. If this happens, a set of hormones are released, causing the leaves of most trees to turn from green to rich reds and deep purples during the fall season. Whether or not a tree reaches full dormancy, it is still actively growing in its root zone and as long as the temperatures stay above 50 degrees the trunks and branches of the tree will continue to grow.


When the temperatures stay above 50 degrees for a certain number of days the tree will bud break or spring to life. The total number of days it takes to bud break is known as Degree Days. Every plant and tree has a different Bud Break/Degree Day need. However, there are things that can alter the amount of Degree Days for a plant or tree. For example, when we get that last tricky cold snap it will actually stop Bud Break for a tree or plant and the Degree Day count is set back and sometimes it can cause the count to start over completely. This often happens to pecan trees. This is the reason some pecans do not bud break until mid-May. However, if in the month of January it does not get below 50 degrees we will have bud break in January.


When bud break happens and the leaves start growing, the tree starts rebuilding the stores of energy that it will take to survive the winter and the flowering. If a tree flowers before it starts growing leaves after the wintertime, it will take an incredible amount of energy. The amount of energy used is based on something called a carbon ratio. For example, on an average a tree will allocate 1 carbon for general tree growth and 7 carbons to produce flower and fruit growth. That means that while a tree is flowering it is growing seven times slower than without flowers. A tree’s root zone is responsible for the majority of the supply for this extra energy. So, even though your tree doesn’t look full and lively during the winter time, beneath the ground there are tons of things going on that are keeping your tree alive and preparing it for growth.