While the visible halt in above-ground growth suggests that all biological activity ceases during colder months, the subterranean world of a plant’s root system operates differently. Roots do not stop functioning; instead, they maintain a reduced, yet continuous, level of activity throughout the winter, provided the soil conditions remain tolerable. This slow, steady growth and maintenance underground is a sophisticated survival strategy that ensures the plant is ready to burst forth with vigor once spring arrives.
How Soil Temperature Governs Root Activity
The primary factor governing whether roots remain active is the temperature of the soil surrounding them, not the freezing air temperature above. Soil acts as a natural insulator, which means the root zone often stays significantly warmer and more stable than the atmosphere. For most plant species, the physical process of fine root growth can continue as long as the soil temperature remains above a specific threshold, generally between 32°F and 41°F (0°C to 5°C).
As air temperatures plummet, the ground releases stored geothermal heat upward, buffering the root zone from extreme cold. This buffering effect is greatly enhanced by natural covers, such as a blanket of snow. Fresh, uncompacted snow is an excellent thermal insulator, capable of creating a dramatic temperature difference between the air and the soil surface. Studies show that sufficient snow depth can increase the soil temperature by dozens of degrees compared to bare ground, limiting the depth of frozen soil and maintaining conditions conducive to root function.
The soil’s thermal properties allow roots to continue growing even when the visible parts of the plant are in deep dormancy. This biological decoupling means that while the trunk and woody branches of a tree cease growth in the autumn, the roots below ground can continue to expand slowly throughout the winter months. This subterranean activity is a distinct growth phase regulated independently of external signals, such as short day length, that trigger above-ground dormancy.
The Biological Process of Slow Winter Root Growth
When soil temperatures hover just above freezing, a plant’s internal metabolism shifts to support survival and long-term preparation, rather than rapid expansion. The slow growth that occurs in winter is fundamentally different from the fast, cell-elongation-driven growth of summer. This period is dedicated to maintenance, repair, and the formation of new, fine feeder roots that are essential for nutrient and water absorption.
This activity is fueled by energy reserves that the plant stored in its roots and woody tissues during the previous growing season. Carbohydrates, primarily in the form of starch, are converted into soluble sugars and translocated downward from storage sites to the active root tips. These sugars serve as the plant’s energy for immediate expenditure and metabolic functions, allowing cell division to continue at a reduced pace.
The accumulation of soluble sugars also serves a secondary function by acting as a form of cryoprotectant. By increasing the sugar concentration within the root cells, the plant effectively lowers the freezing point of the cellular fluid, a process similar to applying antifreeze. This mechanism is an integral part of cold acclimation, preventing the formation of damaging ice crystals inside the cells.
Specific genes are activated in response to cold to boost tolerance, allowing the plant to manage low temperatures and repair any damage to the delicate root tips. By maintaining this low-level biological operation, the roots can quickly ramp up their water and nutrient uptake capacity the moment conditions become optimal in the spring.
Why Different Plants Maintain Root Activity in Winter
For woody perennials, like deciduous trees and shrubs, the cold season is used to establish new fine roots that will be responsible for nutrient scouting and uptake once the canopy demands spike. This pre-emptive root growth gives the plant a significant advantage, allowing it to support the sudden flush of leaf production in spring.
Evergreen plants, which retain their foliage throughout the winter, have a more immediate need for continuous root function. Although their metabolic rates slow dramatically, evergreens lose water through their needles via a process called transpiration, even in freezing conditions. Therefore, they must maintain a minimal level of water absorption to replace this loss and avoid desiccation, which is only possible if their roots remain active and the soil is not completely frozen.
Winter annuals and some perennials also rely on this underground activity to position themselves for the next season. The slow expansion of their root systems deepens their anchor and widens their search area for scarce winter nutrients.