Potting and Repotting
I am frequently asked how often an established tree or shrub should be repotted. It depends, first, on how fast it grows. A willow may need this attention twice a year, a conifer once in five years. A general rule is to repot flowering and fruiting plants about once a year, most deciduous varieties every other year, evergreens every three to five years. But don’t follow that schedule too literally. If a tree begins to wither or look weak, if its color turns sickly and it shows no sign of growth, if its roots are so packed they hump up the soil, root-pruning and repotting are often the “shot in the arm” that can save it.
The most favorable repotting seasons vary with types of plants and the climate. Again, it’s a good idea to consult some authority or reference book. In general, and in most areas, it is safe to repot evergreens, deciduous foliage varieties, and summer-or fall-flowering types in early spring before new growth begins. As for spring-flowering plants, repot immediately after flowering; for fruiting types, in early autumn before cold weather sets in.
As a side light, it is possible to use watertight containers minus drainage holes. That is rather desirable when a plant is to be displayed on finely finished furniture in the house. The container should be a shade larger than usual so that, in potting, a corner area or space along the side can be left empty and the soil sloped down toward it. If there is any standing water it will show up in this section. Just up-end the container and drain off the excess water. In the process make sure that the plant is not also drained off or disturbed. But that you will know from plain common sense.
If a pot has drainage holes, I always make sure they are partially covered with crockery or aluminum window screen so the soil will not sift down and clog the holes. To insure even better drainage, I screen the first layer so that it is coarse and granular, put a little soil on top, and then set the plant in place. In oblong or oval containers, the trunk is usually one-third of the distance from the end and a little behind the center line. Of course, that depends on the shape of the containers. In round or square containers, the plant is usually centered.
When planting, fill in the soil gradually. Use a pencil, or if you have the true Oriental feeling, a chopstick, to tuck the soil in tightly around the roots. Whatever you use, be sure to eliminate any pockets of air. Roots don’t like it. As to how tightly you pack it, that depends on the type of plant and the texture of the soil. If your soil is built up around the edges and sloped toward the center, you will have a depression that will hold water until it can seep down to the roots below. Unless you have some particular plan or design, finish off the surface with a ground cover of moss, or perhaps a layer of gravel. Moss is not only an added attraction but functions as a mulch and delays the evaporation of moisture from the soil underneath.
After repotting, soak the soil thoroughly, and soak the container, which is probably dried out. Mist the foliage and remove any debris such as fallen leaves and petals. Then keep the plant sheltered for several weeks until it is re-established. Remember, it has had what a human being would regard as a major operation.
Some of the most appealing bonsai plants are grown with their roots around a rock of interesting and harmonious size, shape, color, and texture. Soft, porous tufa is especially good for this purpose because you can cut and shape it easily. You can hollow out cavities and crevices, just the place for roots to grow. Any rock should have a stable base. Please don’t let it tip or wobble.
For this sort of planting, select a healthy plant in vigorous growth with roots long enough to reach down the side of the stone. Prune away any roots that are too short or dead, and remove the soil from the others. Set the base of the trunk in place on top of the rock and arrange the roots so that the tips reach down and can be buried in the soil in the container. Both roots and plant may need to be anchored in place for a while; that is, until they can stand on their own.
Such plants naturally need special watering and protection for the first few months, perhaps even for a year or so. You can help them along by rubbing soil into the roots after they are fastened in place, or covering them temporarily with a thin layer of moist moss. Should you do any repotting, be careful not to loosen the roots on the rock.