WINTER CARE: REPOTTING. (February, 2010)
Start repotting your bonsai now in the San Francisco Bay Area. Some of us have already repotted all type of bonsai in December. If you are not in the Bay Area, the best time to repot deciduous trees is after the buds start to swell, but before the leaves open; but do not wait too long. It is not safe to repot after the buds have opened.
Among deciduous trees, flowering apricots and Japanese maples are repotted first. Japanese beech is the last to be done because its buds open later than buds on other deciduous trees (do not wait until you see the buds swell; it may be too late). Repotting should be done by middle part to the end of February. Trident maples can be repotted as long as new leaves still have a reddish color.
You can bare root deciduous trees and cut the tap roots off completely. You need to have a strong healthy tree to go to this process. Otherwise do not cut too much roots off.
Do not bare root conifer. It is dangerous. You should make a plan to remove all the original soil after 2–3 repottings. If you repot collected trees, you need to learn how to secure them in the pot. You need to be creative in order to secure the tree into the pot, since they do not have a developed root ball to anchor with wire.
In the warmer climate zones, it is safe to repot earlier. In the San Francisco Bay Area, we can repot as early as December. Again, early is better than too late.
Azaleas, other broadleaf evergreens, and pines are repotted in February and March. If spring comes early in your area, repot early—many trees start growing as soon as the weather begins to warm up. Shimpaku and sonare (J. procumbens) are repotted in January-March, tosho (needle juniper) around the end of May to beginning of June.
After repotting, the trees should be left outside in the sun. Heat from the sun will stimulate root growth. The sun's warmth will also make shorter internodes.
If you live in a colder climate, you have to put the tree in the cold frame, and you may need to slowly introduce it into the sun. Place trees under shade cloth until they get used to the light before exposing it into the full sun. Sudden change in temperature and light can burn leaves and needle.
If it is still cool (day time temperature is below 55°F), you can spray lime sulphur as dormant spray.
Do not spray spruce and azalea and the tree that you have just wired or the tree that has just been repotted. with lime sulfur. It can burn them. Tropical trees should not be sprayed with lime sulphur.