Tree Facts: The sleep cycle of trees

Farm Forum

People go to sleep every night and wake up in the morning. Trees go to sleep every autumn and wake up every spring. This sleep, technically called dormancy, enables trees to survive harsh winter periods. In tree species, dormancy and cold hardiness are closely linked. In general, trees are most cold hardy when they are in deep dormancy.

Growth above ground essentially stops during dormancy and ends only after a period of sustained chilling. Besides cold temperatures, the shorter day lengths help to trigger dormancy. Growth resumes when warmer temperatures and longer days return in the spring. However, roots are never truly dormant but are in a resting state called quiescence. When only a portion of the soil is warmed, roots in that area will grow. This can occur even when air temperatures are well below freezing.

Dormancy happens in phases as follows: pre-dormancy, true dormancy and post-dormancy. In pre-dormancy the tree starts napping and wakes up during periods of favorable growing conditions. Next is true-dormancy, the tree has entered into a deep sleep and will not wake up easily. Deciduous trees have lost their leaves and a period of prolonged chilling is required before growth resumes. The final stage of dormancy is post-dormancy, occurring in late winter and early spring. The tree starts to wake up, is drowsy, buds start growing, but can temporarily stop if cold temperatures return.

A tree is full of vascular cells that move water and sap from roots to leaves. As the amount of sunlight decreases in autumn, the veins that move sap into and out of a leaf slowly close off. A separation layer develops at the base of the leaf’s stem and it falls off. It happens with all deciduous trees except oaks. In oaks, the separation layer does not fully develop. This is why most dead oak leaves remain on the tree through winter and even into early spring.

Day length and temperature are important factors that trigger dormancy. Long days promote vegetative growth and short days trigger dormancy. Dormant buds form in preparation for next year’s growth as days begin to get shorter in later summer. Cool temperatures are needed for the plant to enter true-dormancy. Dormancy happens more quickly when short days occur in combination with cool temperatures.

Besides water and wood, trees contain various chemical compounds. When day lengths get shorter, higher amounts of a growth regulator called abscisic acid (ABA) occur, causing growth to stop. The ABA breaks down and decreases during the winter. Simultaneously in the spring, soil begins to warm and growth promoters, gibberellins and cytokines, build up causing bud growth to resume.

Evergreen trees include pines, spruces, cedars and firs go dormant less conspicuously than deciduous trees. They do not lose their leaves, or needles, in winter, because their dormancy is different. The needles of evergreens are covered with a heavy wax coating to help prevent moisture loss, and the fluids inside the cells contain substances resistant to freezing, essentially evergreen antifreeze. Evergreens shut down for winter dormancy but continue most basic metabolic functions; the plant’s super-cool. Water in the cells is chemically maintained in a liquid state below 32° F. Evergreen leaves can live for several years, through all four seasons, before they are dropped and replaced by new growth.

My sources for this news release were the Virginia Polytechnic Institute and State University and Community Channel 101. If you would like more information about “The Sleep Cycle of Trees,” call Bob Drown at the Conservation Office at 605-244-5222, Extension 109 or by e-mail at