When a tree is described as dormant, it is entering a state of suspended growth and minimized metabolism. This biological adaptation allows deciduous trees in temperate climates to survive the harsh conditions of winter. The dormant phase protects the tree’s living tissues from damage caused by freezing temperatures and lack of available water. By conserving energy, the tree preserves vital resources until favorable conditions return in the spring.
Understanding the Concept of Dormancy
Dormancy is a temporary suspension of visible growth and active metabolism. This state is primarily triggered by two environmental cues: a reduction in daylight hours (photoperiod) and sustained cold temperatures. As days shorten in late summer and early autumn, photoreceptors within the tree sense the change, initiating a hormonal response that prepares the tree for winter.
Endodormancy
This process is divided into two parts: endodormancy and ecodormancy. Endodormancy, or “inner dormancy,” is controlled by internal physiological factors, primarily hormones like abscisic acid, which actively suppress growth regardless of external weather. During this phase, the tree must accumulate a specific number of “chilling units”—hours spent at temperatures just above freezing—to internally break the rest.
Ecodormancy
Once the tree meets its chilling requirement, it transitions into ecodormancy, or “outer dormancy.” In this phase, the tree is physiologically ready to grow, but unfavorable external conditions, such as freezing temperatures, prevent emergence. The tree will not resume active growth until the environment warms sufficiently, preventing tender new growth from being damaged by late frost.
Physical and Physiological Changes Inside the Tree
The transition into dormancy involves adjustments that prepare the tree for cold hardiness. For deciduous trees, the most visible change is shedding leaves, which prevents excessive water loss through transpiration when frozen ground makes water absorption difficult.
Internally, the tree’s cells undergo controlled dehydration by moving water out of the protoplasm and into the spaces between the cells. This reduction in intracellular water defends against freezing, as the remaining water is less likely to form ice crystals that would puncture cell membranes. The tree increases concentrations of dissolved sugars and starches in its cells. These compounds act as a natural antifreeze, lowering the freezing point of the cellular liquid and protecting the tissues.
Stored starches are converted to sugars and relocated to the roots and core woody tissue, serving as energy reserves to fuel reduced metabolic processes throughout the winter. The tender growing points, known as meristems, are converted into hardened, protective buds covered in specialized scales. These adaptations allow the tree to withstand temperatures far below freezing and ensure sufficient energy for new growth in spring.
Caring for Dormant Trees
A dormant tree still benefits from care. Winter watering is necessary for newly planted trees or during prolonged drought when the ground is not frozen solid. Water mid-day when temperatures are above 40 to 45 degrees Fahrenheit, allowing the water to soak in before freezing overnight.
This season is the optimal time for structural pruning of most deciduous trees, as the absence of leaves allows clear identification and removal of dead, diseased, or crossing limbs. Pruning minimizes stress and reduces the risk of attracting pests or diseases.
It is also the most successful time for transplanting. Since metabolic processes are slowed and energy is concentrated in the roots, moving a tree while dormant allows the root system to be less disturbed. This gives the tree time to establish itself before the demanding growth season begins.