Bonsai plants need to be repotted from time to time in order to accommodate their growing roots, as well as to replace the soil they are living in, which can harden and make it difficult for water and air to permeate. If the bonsai is not repotted at the appropriate time it can become unhealthy and possibly die. The repotting process is also a good time to reshape the plant and prune the roots, both of which can help your bonsai grow and remain healthier.
It is time to repot your bonsai when the roots outgrow the size of the pot it is currently in, or when you notice water remaining on the surface of the soil, which indicates the soil has hardened and is no longer allowing water to permeate it properly. As a general guideline, you should repot a young evergreen conifer every two to three years and an adult tree every five years. Young deciduous trees should be repotted once a year and adult trees every two to three years.
The environment and climate conditions will also serve as guidelines for the necessary frequency of repotting. Trees grow slower when living in harsher climates so they do not need to be repotted as often. Repotting should be done at a time when damage to the plant can be kept as minimal as possible. The best times are in early spring, before the buds emerge, or in early autumn, after the buds mature. These time periods will also promote rapid growth in the newly pruned roots. If you choose to repot your plant at a time other than these two specified, you should be careful to prune the roots as little as possible.
Before you begin the repotting process, you should reduce the amount you water your bonsai because the repotting process is easier if the soil is dry and loose. It also helps minimize possible damage to the roots. Repotting needs to be done fairly quickly or the roots can become too dry.
Before you remove your tree from the pot, it is important to see if it has been wired to the bottom of the pot. You should cut off any exposed wires you find before proceeding with removal. Be extra careful when removing your bonsai from the pot to ensure you do not damage any of its delicate roots.
Repotting a bonsai plant is accomplished in stages. First, you need to remove and replace some of the old soil. For younger evergreen conifers, about half of the soil will need to be replaced; for adult trees, replace about one third of the soil. Deciduous trees require slightly more soil to be replaced – about two thirds for younger trees and half for adult trees.
You will also need to clean the old soil from the roots. The easiest way to do this is by gently kneading them. Sometimes bamboo chopsticks or a root rake can be helpful with this process. If the soil has lumped the roots together, an ice pick, tweezers, bodkin or awl can be handy to help break up and remove the clumps. Begin cleaning the roots at the bottom, paying close attention not to damage the thin, fine roots. Move up the roots while simultaneously working vertically, as they will be growing in all different directions.
Once your soil has been replaced and your roots have been cleaned of the old soil, you can begin the next stage of repotting – pruning the roots, which will stimulate growth of new roots. Begin by spreading out the thinner roots and then start the pruning process with the long, thick, entwined, upward-growing roots. Next prune the thick tap roots that grow vertically downward. Since there is a limited amount of space in the pot and roots absorb water from their tips, it is best to keep an abundance of slender roots and prune the thicker ones.
You need to determine the height of the exposed surface roots and prune anything above that level. Then evenly spread the roots out in multiple directions beginning where they meet the surface of the soil. Finally, prune any roots that are unnecessary for the plant.
Once you have completed the soil replacement and root pruning, you are ready to repot your bonsai. You will need to cover the drainage hole at the base of the bonsai pot with a screen, which will keep insects out of your pot. Next, you need to place the wires that will eventually be used to fix your tree.
The bottom layer of soil will be course, with a grain size of 1/4” or larger. The next layer will be planting soil, with a grain size of 1/32”-1/4” in diameter. Next, the bonsai tree is placed firmly on top of the planting soil. Once you have made sure there is no space between the tree and the soil, you can then fix the tree with the wires you placed earlier.
When placing your bonsai tree in the pot, you should determine how to place your tree so the best view is from the front. You will need to use your imagination and try to picture what the tree will look like as the trunk and branches grow as well, keeping this in mind while determining which side of the tree will be the frontal view. Bonsais are very visually appealing plants and should be treated as such.
Once your tree is placed, continue adding planting soil until all the gaps are filled in and your soil is level. (It helps to gently tap the side of the pot while adding the soil, so it can properly settle and fill in the gaps). You can add gravel or mosses on the surface soil to help prevent it from turning over after watering. Finally, you need to flush the plant with water until the water coming out of the draining hole is clear.
For the next two to three weeks, keep your newly repotted bonsai in a bright, yet shaded area, being careful to avoid direct sunlight or wind. Do not fertilize it until the roots grow and make sure to keep it from drying out.