Evergreen trees retain their foliage—typically needles or scale-like leaves—throughout the year, maintaining biological activities different from deciduous species. This continuous leaf cover raises the question of whether they continue to grow when temperatures drop. Understanding the physiology of these hardy species requires examining how they manage water, energy, and cellular function during the coldest months. While they appear static, the internal processes governing their survival are complex and regulated by environmental cues.
The State of Winter Growth
The straightforward answer to whether evergreens grow in winter is that visible, primary growth largely stops. Growth involves cell division and expansion, processes that require substantial water pressure and metabolic activity, which are severely limited by low temperatures. When the soil freezes, the tree cannot readily draw up water, leading to a state often referred to as “physiological drought.”
The tree enters a period of rest, which can be distinguished between true dormancy and quiescence. True dormancy, or endodormancy, is an internal, genetically programmed state that prevents growth even if a brief warm spell occurs. This state is triggered by decreasing day length and cooling temperatures in the autumn, ensuring the plant is prepared for winter before the onset of freezing.
Quiescence, or ecodormancy, is the environmental phase where the tree’s growth is suppressed solely by unfavorable external conditions, such as freezing temperatures or lack of water. Although the tree is not actively elongating its stems or producing new needles, it is not biologically inactive. Maintenance metabolism continues at a significantly reduced rate, ensuring that the existing cellular machinery remains intact until conditions improve.
Cold Hardening and Survival Mechanisms
Evergreens survive freezing temperatures through a two-stage process known as cold hardening. This preparation begins in late summer and early autumn, initiated by shortening photoperiods. The first stage involves biochemical changes within the cells that alter the cytoplasm’s composition to resist ice damage.
Trees significantly increase the concentration of solutes, such as soluble sugars, proline, and specific antifreeze proteins, within their cells. These compounds act as cryoprotectants, lowering the freezing point of the cytoplasm, much like antifreeze in a car radiator. This prevents the formation of damaging ice crystals inside the cell, which would rupture the membranes and lead to cell death.
The second stage of hardening involves structural changes, primarily focused on managing water content. As temperatures continue to fall, water is actively transported out of the cell and into the extracellular spaces between the cell walls. When freezing occurs, the ice crystals form safely in these non-living, extracellular locations.
By reducing the amount of free water inside the cell, the tree ensures that the cell’s contents become highly concentrated and less susceptible to freezing. On warmer, sunny winter days, evergreens can also engage in minimal photosynthesis, utilizing the retained needles to capture light energy. This low-level energy production is used for maintenance and repair, rather than generating new growth.
Resumption of Growth
The transition from the hardened winter state back to active growth is governed by a precise sequence of environmental signals. To break true dormancy, the evergreen must first satisfy a specific chilling requirement, accumulating a certain number of hours below a threshold temperature, often around 41 degrees Fahrenheit (5 degrees Celsius). This chilling period ensures that the tree will not prematurely start growing during a false warm spell in mid-winter.
Once the chilling requirement is met, the tree moves into the quiescent phase, where it is ready to grow but waits for external signals. The primary cues that trigger the eventual spring “growth flush” are the increasing photoperiod and rising soil temperatures. Longer days signal to the tree that the risk of a deep freeze is diminishing and that sufficient light energy will be available for sustained growth.
Rising soil temperatures are particularly important because they signal the availability of liquid water and the reactivation of root function. As the ground thaws, water uptake resumes, providing the turgor pressure necessary for cell expansion and the transport of stored nutrients. The combination of these signals releases the tree from its winter hold, leading to a rapid burst of new shoot and needle production, often called budbreak. This rapid growth phase utilizes energy reserves that were stored during the previous summer and fall, allowing the tree to quickly maximize its photosynthetic surface area.