Bonsai are miniature trees cultivated in containers, but they are genetically identical to their full-sized counterparts and follow the same natural life cycles. For many species, this cycle includes a period of physiological rest known as dormancy, which is a survival mechanism triggered by the seasonal changes of cooler temperatures and shorter daylight hours. Dormancy allows the tree to conserve energy, protect its delicate tissues from freezing damage, and prepare for vigorous growth when warmer weather returns. This phase is not a sign of poor health but a necessary biological process that ensures the tree’s long-term health and vitality.
Species Dictate Dormancy Needs
Whether a bonsai requires winter rest depends entirely on its native climate. Owners must correctly identify their tree’s species to determine if it is temperate or tropical. Temperate species, such as Japanese Maples, Pines, and Junipers, have evolved in regions with cold winters and must experience a period of cold to survive and thrive. This necessary cold exposure, sometimes called vernalization, resets the tree’s internal clock and allows for proper bud break in the spring. Tropical and subtropical species, including Ficus, Jade, and Serissa, originate from climates with stable, warm temperatures and do not require cold exposure. These trees grow year-round and will suffer severe damage or death if exposed to freezing temperatures. Keeping a temperate species indoors during winter prevents dormancy, causing the tree to weaken and eventually fail to flush new growth.
Recognizing the Signs of Winter Rest
For temperate bonsai, the visual cues signaling the onset of dormancy are distinct. Deciduous trees, like Elms and Maples, undergo the most noticeable change by gradually yellowing and shedding all their leaves in the fall. This leaf drop reduces surface area and minimizes water loss during the cold months. Evergreen species, such as Pines and Junipers, show more subtle indicators, including a slight dulling of needle color or the hardening of new growth. Physiologically, all temperate trees reduce their metabolic activity and move sugars into their cells. This increased sugar concentration acts as a natural antifreeze. A simple scratch test on a young branch, revealing green tissue beneath the bark, confirms the tree is dormant and alive.
Essential Care During the Dormant Period
Maintaining a temperate bonsai during dormancy requires specific environmental control to protect the roots while ensuring the tree remains at rest. The goal is to keep the root mass between approximately 33°F and 40°F, cold enough to sustain dormancy but warm enough to prevent deep freezing. Unheated garages, sheds, or cold frames are ideal locations for overwintering, as they shield the trees from harsh winds and repeated freeze-thaw cycles. Watering frequency must be drastically reduced because the dormant tree uses very little moisture. Overwatering is a common mistake that leads to fatal root rot in a cold, inactive root system. The soil should be checked regularly and watered only when it feels slightly dry, remaining damp but never saturated. Furthermore, all fertilization must cease entirely, as the roots are not actively growing and cannot process nutrients. Feeding should only resume once the tree shows visible signs of new growth in the spring.
Managing Bonsai That Do Not Go Dormant
Tropical and subtropical bonsai species require a different winter strategy focused on maintaining a warm, stable environment indoors. These trees must be moved inside before outdoor temperatures consistently fall below 50°F, as they lack cold-hardiness. They continue a slow rate of growth throughout the winter, requiring year-round attention.
Light Requirements
Adequate light is a primary concern in winter. Placement in the brightest location, such as a south-facing window, is necessary. Alternatively, supplemental grow lights should be used for 15 to 18 hours daily.
Humidity Control
Heated indoor environments tend to have extremely low humidity, which can desiccate tropical foliage. To counteract this, owners should increase localized humidity. This can be done by placing the bonsai on a tray filled with gravel and water or by using a dedicated room humidifier. These species still require watering, though less frequently than in summer, as soon as the topsoil begins to dry out.