Winter Management Methods and Care Points for Stump Bonsai

  As the weather turns colder, many flowering plants and bonsai are gradually being moved indoors and entering the winter management stage. We have seen quite a few articles on the spring, summer and autumn management of stump bonsai, but the winter management of stump bonsai is often overlooked, and often by the following spring, when all things revive, some bonsai still show no sign of budding. Bend a small branch lightly and it has long since dried out; only then do you realize that these stump bonsai quietly died during the winter, and by the time you want to save them, it is too late, which is deeply distressing. Therefore, the winter management of stump bonsai must never be treated lightly. Below are several issues to note in the winter management of bonsai.

  1. Timing of Moving Indoors and Outdoors

  After “Frost's Descent”, the climate turns cold, plants begin to shed leaves, nutrients begin to transfer and accumulate within the plant body, and the plants enter a dormant state to meet the arrival of harsh winter. As the prelude to winter management of stump bonsai, the work of moving them indoors should first distinguish which ones need to overwinter indoors and which ones overwinter outdoors in the open ground. For southern tree species that overwinter indoors, they should also be moved indoors as late as possible, so that the trees go through a relatively long period of low-temperature adaptation, which is the so-called “no moving indoors in autumn”. This not only allows the trees to gradually adapt to the low-temperature environment of the long harsh winter, avoiding excessively high indoor temperatures that cause new shoots to sprout and consume nutrients and reduce cold resistance, but also allows parasite eggs on the trees to be killed by freezing in the low-temperature environment. There is no uniform, standardized timing for moving indoors across different latitude regions. During the period before moving indoors, pay close attention to local weather forecasts and focus on preventing frost. When the temperature drops to 0°C, there is no need to move them indoors immediately either; you only need to put up plastic film or other coverings at night to prevent severe frost from directly hitting the branches and leaves. Lower temperatures are not a big problem; it is very necessary to ensure about 20 days of hardening at around 0-5°C. Influenced by the trend of global warming, in the Huang-Huai region, moving indoors around “Minor Snow” is not considered too late, and after moving indoors, the doors and windows should not be closed tightly right away; gradually shorten the window-opening time so the plants gradually adapt to the climate environment.

  After the harsh winter has passed, when to move them outdoors is also extremely important. It must be said that we often find a strange phenomenon: many bonsai are fine throughout the winter, but as soon as they are moved outdoors in spring, they die immediately. This is what people often call “easy to get through winter, hard to get through spring”. As is well known, plant physiology has much in common with humans. If a person stays in a warm environment for a long time, going out suddenly will make them very uncomfortable; plants are the same. Having been in a relatively enclosed environment for the whole winter, plus sprouting early, internal energy is severely consumed and they are quite weak, and their ability to adapt to the external environment is greatly reduced. Suddenly moved outdoors, upon encountering a late spring cold snap, they are almost certain to die. Therefore, the correct approach is to gradually increase the frequency of opening windows for ventilation, extend the ventilation time, and delay moving them outdoors as much as possible. The purpose is also to let the plants go through a process of gradually adapting to the environment, which is the rationale of the so-called “no moving outdoors in spring”. Therefore, after the harsh winter, when it is warm but still cold, the weather is very unstable, and the bonsai must not be moved outdoors early. The bonsai should gradually adapt to the climate changes of early spring, and only after the weather is relatively stable should they be moved outdoors. The timing of moving outdoors also differs by region. As long as one principle is grasped, namely taking the end of frost occurrence as the limit, in the Huang-Huai region it is advisable to move them outdoors around “Grain Rain”.

  2. Indoor Temperature

  For bonsai overwintering indoors, what indoor temperature to maintain is also a key issue in winter management of bonsai. Many people have a misconception, thinking that the higher the indoor temperature the better, not realizing that the level of indoor temperature varies greatly depending on tree species and cultivation purpose. Broadleaf evergreen types are mostly medium- to high-temperature tree species; the winter work is temperature and humidity management, and room temperature can be appropriately higher to ensure normal growth in winter. What is most taboo is temperature fluctuating up and down, especially at night; species such as banyan, Fujian tea, and serissa shed old leaves and replace them with new leaves in late winter and early spring, at which time they should not be exposed to cold or lack water, so as to facilitate renewal; for bonsai such as orange, azalea, and cyclamen that need to be enjoyed in flower during the Spring Festival, heating can be increased to above 20°C around the twentieth day of the twelfth lunar month to ensure flowering for the Spring Festival; deciduous tree species are cold-tolerant trees, shed leaves in winter, and need full dormancy. Placed outdoors in a wind-sheltered, sunny spot, they can overwinter safely, and if conditions permit, burying the whole pot and tree in the ground is even better. For such stumps, many people lack sufficient understanding of the harm caused by excessively high temperatures. Most northern homes have heating or other warming facilities, and it often happens that excessively high temperatures break the plant's normal dormancy, causing early sprouting in an environment with insufficient light and poor ventilation. Not to mention that these long, thin, weak shoots cannot develop normally the following spring and will wither and die as soon as they are moved outdoors; more importantly, early sprouting causes the nutrients stored in the plant body to be exhausted during winter, leaving no reserve strength, so the following spring it cannot produce robust branches. Even if it does not die, it is very difficult to restore vitality. This is an important reason why many bonsai remain weak year after year. Even when placed indoors, low temperatures must be maintained, with the limit being that early sprouting does not occur, generally not exceeding 5°C, so as to ensure sufficient internal energy stored in the plant body, ready to burst forth and produce robust branches the following year.

  3. Keeping Warm and Preventing Cold

  For northern deciduous tree species and pine and cypress bonsai suitable for overwintering outdoors, a one-size-fits-all approach should not be used either; they must be treated differently according to specific conditions such as the tree species' degree of cold tolerance, placement location, and pot depth. For example, tree species such as honeysuckle and ginkgo can withstand low temperatures of -40°C and can overwinter safely without any cold protection measures. For deep-pot stump bonsai placed on the ground, especially some stump bonsai that have developed adventitious roots, these adventitious roots can greatly improve their cold resistance. Never cut off the adventitious roots, do not move the position of the pot, and they can overwinter safely in place without any special protection. For shallow-pot stumps and stump bonsai placed on elevated stands, especially stump bonsai placed on rooftop platforms, regardless of pot size or depth, cold protection measures must be taken. The lessons in this regard are profound. Taking Xuchang as an example only, in the winter of 2004 the weather was unusually cold, and several dozen top-grade large pomegranate stump bonsai were killed by freezing, with economic losses reaching several hundred thousand yuan, and regret came too late. These frozen-to-death stumps were, without exception, bonsai grown on rooftop platforms. The main reasons were that on platforms the wind is strong and the air dry, the temperature extremely low, and most bonsai pots are shallow and small, making them very easy to freeze through. Especially for fruit tree bonsai, which bear large amounts of fruit, the nutrients produced by photosynthesis are all transported to the fruit, the tree body's nutrient consumption is severe, the concentration of cell sap in the tree body decreases, the freezing point of the tree sap rises, and the tree's cold resistance is greatly reduced. Therefore, regarding the overwintering of these stump bonsai, one must not harbor a mentality or treat it lightly; one must take preventive measures. In fact, the solutions are very simple; methods such as mounding soil, digging trenches, sheltering from wind, and simple warm sheds can be chosen according to different situations. If conditions permit, remove the pots or bury them together with the pots in relatively concentrated groups in the ground, with the covering soil slightly higher than the original soil surface, water thoroughly once, and cover with straw or similar materials; then no special care is needed for the entire winter and they can overwinter safely. For those without such conditions, even more effort should be made to create conditions for safe overwintering to ensure the bonsai overwinter safely. They can be relatively concentrated and then covered with sawdust, furnace slag, and similar materials, or a simple large shed can be built. For large pots that are not easy to move, they can also be wrapped on the spot with old quilt covers, cotton wadding, and other warm materials, with a large plastic bag over the outside.

  4. Water and Fertilizer Management

  Watering is a very important issue in winter management of bonsai and should not be underestimated. Winter watering is not like summer watering, which must be done every day; it is relatively easier, and precisely because of this, it is easy to forget to water. For bonsai in a greenhouse, they are relatively concentrated and can be observed regularly, so generally no omissions or mistakes occur. But for bonsai placed separately indoors, especially in places not easily noticed such as room corners and under beds, they are more likely to be neglected, and often dry up because watering is forgotten. Experienced people say that when flowering plants and bonsai die in winter, eight or nine times out of ten they die of drought because watering was forgotten. This should be taken very seriously; therefore, check frequently and replenish water in time. In winter, bonsai are in dormancy, plant metabolism is slow, and water needs are relatively low, so the potting soil should preferably be appropriately dry, to prevent waterlogging and root rot or early sprouting, and also to maintain a relatively high concentration of cell sap in the tree body to enhance cold resistance. Generally, no fertilizer is applied in winter, but for those that will not be repotted the following year, solid organic fertilizer can be applied in holes around the sides of the pot in winter, taking care to avoid damaging the roots as much as possible.

  5. Pruning to Preserve Shape

  Winter pruning is an important measure for maintaining tree shape and renewing and rejuvenating the tree. This issue has been discussed in many articles, so there is no need to elaborate further here. What needs to be pointed out is that it should differ according to different pruning purposes and the degree of shaping of the stump. For finished trees, the purpose is to preserve shape; prune away the weak and keep the strong, completing it in one go, but winter pruning directly affects the trend of branch growth the following year. Prune close to the bud eye, distinguish inner and outer buds, and generally keep outer buds. Also pay attention to heading back and renewal pruning for rejuvenation. For stumps in the training stage, the pruning principle is to use surplus to fill gaps and move solid growth into empty spaces, mainly with medium pruning and supplemented by long shoots left unpruned. Practice has proved that pruning to the plump buds in the upper-middle part of the branch allows these plump buds to produce thick, strong branches, which is more conducive than leaving long shoots to thickening the supporting branches, completing the gradual transition from trunk to branch supports as early as possible and shortening the time to finished form. At the same time, wire and bend the pruned branches so that they cannot be bent after thickening. Winter-flowering types are mostly used for festive decoration and seasonal effect, such as plum, peach, winter jasmine, wintersweet, and crabapple. For such bonsai, flower-forcing measures can be taken according to the tree's sensitivity to temperature. One month before the Spring Festival, gradually raise the temperature for bonsai that have already formed flower buds, breaking dormancy early so they flower and sprout. After flowering, such bonsai are extremely frail and weak, so after the flowers fade, remove the faded flowers promptly and carry out shaping pruning to promote the sprouting of new shoots and the formation of more flowering branches for the following year. By no means should heavy pruning be done during the dormant period. After sprouting, apply thin fertilizer frequently to help them recover slowly. There is one more point: for tree species such as tamarisk, Japanese bush cherry, and firethorn that easily develop lower budding points and withdraw branches, leave them appropriately longer when pruning in winter. For tree species that flower in early spring, the flower buds on the branches were differentiated last year, so they must not be short-pruned in spring; the correct pruning time should be after the flowers fade. For species such as pomegranate and crape myrtle that flower on current-year branches, heavy pruning should be done during the dormant period, which can promote the sprouting of new branches and increase flowering.

  (Note: This article has been published in the first issue of 2009 of the magazine “China Flowers & Bonsai”)