Bonsai Repotting Guide


Bonsai (bone-sigh) literally means "plantain a tray." There's evidence that the art of bonsai is 1500 years old in China and 700-800 years old in Japan. Initially trees that had been stunted by nature were collected and potted. As those trees became harder to find, suitable plants were propagated for bonsai. Then to create the image of age the trees were pruned, wired, roots were exposed, limbs and wood were made to look dead, trunks were hollowed, all to make it look old. The beauty of bonsai does not have to be the actual age but its apparent age. The pleasure of bonsai is to be able to view in your own back yard what appears to be an ancient gnarled tree, or a huge forest that you could walk through.


Repotting is necessary when placing a newly styled bonsai in a new container or when an established bonsai becomes root bound and uses all the existing soil, nourishment, and space in the container. By root pruning you encourage new feeder root growth and provide room for new soil to keep your bonsai healthy. An indication that repotting is necessary is if watering becomes more difficult; for instance, water will not be absorbed and runs off the tree. It also becomes necessary to water more frequently. If your bonsai is root bound you will be able to grasp the trunk and gently lift the tree from it's container and inspect the root system. Some bonsai are wired into the container. If your tree does not easily come out of the pot look underneath for any wires that may need to be cut.


Spring is when most plants prefer to have their roots pruned just before awakening from their dormancy. This is the time that buds on deciduous trees are just starting to swell but before they open. Trees that flower before leaves appear are generally done after flowering but before the leaves open. Late spring and summer flowering trees are repotted before flowering. In some climates, fall and winter root pruning may be accomplished. If you live in an area where roots put on new growth, and there is no danger of this new growth freezing, fall repotting is acceptable. The time for tropical bonsai may be late spring to early summer. If you are in doubt about when to repot be sure to ask your nursery bonsai professional.


Frequency depends on the vigor of a particular plant. Repotting may be necessary every year or once every five years. Deciduous trees that are young and in training may require repotting every year. More mature deciduous trees may only need it every one to two years. Young conifers usually require it about once every two to three years and established ones only once every three to five years.


Container preparation consists of washing it with soapy water and then rinsing it well. For the health of your bonsai all containers should have adequate drainage holes in the bottom. Cover these holes with a rigid screen material that will allow water to drain through but prevent the soil from washing out. Too small a mesh, like window screen, will tend to get clogged up so use material with a larger hole. To hold the screen in place use a piece of wire bent in a hairpin shape and insert it through the screen and pot drainage hole from the inside of the container and bend the ends outward on the outside. If your pot is extremely shallow or your tree top heavy, it is a good idea to wire your bonsai into the container until new root growth holds it firmly in place. Use one to two pieces of wire long enough to insert from the outside through the drain holes of the container and secure the wire around the root system.

Root pruning

Root pruning needs to be done when repotting. There are several effective methods to root prune a bonsai. Most people use a chopstick or root rake to disentangle the roots around the outside and bottom of the root ball and then prune the roots with sharp scissors. The idea is to prune larger roots back to promote smaller feeder root growth. Approximately 1/4 to 1/3 of the root mass may be removed. Don't let your exposed roots dry out while performing this procedure, keep a spray bottle handy to moisten them.


Soil for bonsai is usually a mixture of pumice, lava, bark, and compost, all sifted to eliminate small dust and large particles. Check with your local bonsai professionals to find out what mix works best in your area. Soil is layered in the bottom of your prepared container and place your newly root pruned bonsai on top. If you are fastening the bonsai into the container with wires or string secure them now. Use padding material to protect heavy roots from damage. Add more soil and work it around the existing root system filling any air pockets. Sometimes a chopstick or small spatula helps to work soil into the smaller spaces between the root system and the side of the pot. Use your fingers or a trowel to tamp the soil down around the bonsai.


Water initially can be accomplished a couple different ways. The best way is to use a soft-spray bonsai nozzle on the end of your hose and saturate the soil several times until water is running out the bottom drain holes. Stand far enough away or use a low enough pressure so that you don't wash soil out of the pot. Another method is to set your newly potted bonsai in a tray of water that comes up to the rim of the pot. Leave it for a few minutes to saturate the soil than lift it out and let it drain. Most bonsai do not want to be left sitting in a tray of water.


Maintenance for about 1-2 weeks requires you to keep your bonsai in a shady location where it gets 2-3 hours of morning sun but is out of the drying wind. Then gradually work it into the spot it will be permanently. Proper watering is critical. Wait until the soil surface looks dry but there is still a slight dampness below. Then thoroughly water from above with a fine spray that will not dislodge the soil, gravel or moss. Be sure to water thoroughly when it needs water and don't water if it is still wet.