Arborvitae are popular evergreens often used for dense privacy hedges. Because they retain their green foliage, they are frequently mistaken for actively growing plants year-round. However, like most temperate woody species, arborvitae do not grow during the winter months. Instead, they enter a state of deep physiological rest, known as dormancy. This protective response ensures the plant conserves energy and survives freezing temperatures and resource scarcity.
The State of Winter Dormancy
Plant dormancy is a dynamic resting state triggered primarily by shorter day lengths and cooling temperatures in the autumn. This initiates the slowdown of the plant’s metabolic rate, halting vegetative growth, including the production of new leaves, stems, and roots. The purpose of this resting period is to ensure the plant does not resume tender growth until the threat of severe frost has passed in the spring.
The arborvitae enters endodormancy in late fall, becoming temporarily unresponsive to environmental cues that might otherwise stimulate growth. Energy reserves built up during warmer months are conserved rather than spent on expansion. This state reduces the plant’s need for water and nutrient uptake, which is necessary since the root zone often becomes frozen. The plant remains in this non-growing mode until it has accumulated enough chilling hours to release the dormancy mechanism.
Physiological Changes for Winter Survival
To withstand extreme cold, arborvitae undergo “cold hardening,” which changes the chemical composition within their cells. A significant adaptation involves moving water out of the cells and into the spaces between them. This decreases the amount of water available to freeze inside the cell, preventing ice crystal formation that would rupture cell membranes.
The plant also increases the concentration of dissolved substances, such as sugars and proteins, inside the cells. This increase in solute concentration lowers the freezing point of the remaining cellular water, functioning as a biological antifreeze. These modifications allow the plant’s tissues to tolerate temperatures as low as -30°F to -40°F, depending on the variety. This cold hardiness allows the plant to appear green throughout the winter, even though its biological processes are largely suspended.
Understanding Winter Browning and Damage
While dormant, arborvitae are susceptible to environmental damage, often manifesting as “winter burn” or browning. This common injury is a form of desiccation, or extreme drying, occurring when foliage loses water through transpiration on sunny or windy winter days. Since the ground is frozen, the roots cannot absorb replacement water, leading to a moisture deficit in the foliage.
Browning is most often observed on the side of the plant facing the sun or prevailing winds, typically the south and west sides. This damage is a consequence of environmental stress, not a natural part of the dormancy cycle. Another common winter problem is mechanical damage, where heavy snow or accumulated ice causes branches to bend or split. This damage is particularly common in upright, columnar varieties and often only becomes apparent when the plant resumes growth in the spring.