As the weather gets warmer it is time to consider what changes need to take place for the bonsai plant. As spring is generally a time of accelerated growth, it is often a good time for some of the more stressful activities of caring for the bonsai. As stated earlier, late winter or early spring is the time for repotting and pruning, as the spring growing season will help the plant heal. If fertilizer is to be used, springtime is often the time to apply it. Spring may also be the time to consider wiring of the plant if the shape needs to be altered.
When summer comes, the focus will quite possibly turn from making ambitious changes in the bonsai plant to ensuring it survives what may be hot, stressful weather. While watering will be covered more in depth in a later section, it stands to reason that the soil mix should be checked more often in the Summer months to ensure the plant has enough moisture. It is also helpful to situate the pots above a yard or gravel rather than cement. Many trees will benefit from reduced fertilization during the summer. Most plants should not be in direct sunlight for the entire day, four or five hours should be more than adequate and afternoon sunlight should be avoided. During the slower growing period of summer, it is still important to keep up with pinching or nipping the plant so that it retains its desired shape. It is also necessary to regularly rotate the plant so that the growth throughout summer is even.
As the days shorten, most plants will require less fertilization and watering. Deciduous trees may loose their leaves, and the dead leaves should be removed from the pot. Less care will need to be taken to ensure that the plant does not get too much sun. Stress on the plants should be avoided during this time, as the plant’s natural defense systems against the cold of winter will be taking effect. For many species it is important that the tree remain outside during the fall, as being kept indoors will hamper the hardiness of the plant in the coming winter.
Most trees, with the obvious exception of tropical and sub-tropical varieties, will grow dormant in the winter and survive the cold without mishap. In fact, deciduous trees that are brought indoors will fail to grow dormant, and in the long term this will be detrimental to the tree and possibly even hamper its healthy growth. Nearly all species of hardy trees will withstand temperatures of -10 C without a problem. If the temperature falls much below that, however, the roots may be in danger. The top-side portion of the plant will be much more resilient to the cold than the roots.
To protect bonsai from temperatures that fall below -10 C in the winter, most enthusiasts will place the plant in unheated shelter. A garage or shed is commonly used. Cold-frames will be discussed in a later section. The plant must be prepared for winter shelter, all debris, leaves, and moss should be removed from the top of the pot to prevent storing insects with the plant over the winter that can lead to infestations when the weather gets warmer.
While the plant is dormant it will not consumer much water, but the soil should not be allowed to dry completely. Deciduous trees need no sunlight once the leaves have fallen, and evergreen trees need a limited amount. Dormant trees should also be sheltered from heavy winds during the winter months.