Bonsai need to be regularly repoted their pots become root bound; soil runs out of nutrients and imbalances of salt and Ph levels occur. Repotting helps correct this, by introducing fresh growing medium and pruning overgrown and excessive roots, there by allowing the bonsai to grow at its full potential.
When to repot a Bonsai?
The best time to repot a Bonsai is dependent on the species and to some extent the area that you live in. For example, a Ficus retusa is best repotted in spring in its native lands, in the UK it is safest to repot when minimal temperatures are above 15C, which usually means early summer.
Deciduous Bonsai Trees are best repoted when they are comming out of their dormany which usually means early spring onwards. Look for signs of swelling leaf buds, this usualy the best indicator of the Bonsai coming out of its winter dormancy.
Conifers (and even evergreen broadleafs) are best repotted in late summer just before autumn begins to set. Newly repotted conifers will recover quickly as they weather is still quite warm. The new root growth activity is also reflected in a stronger flush of buds in evergreen trees which are by this time also coming out of a short period of summer (hot weather) dormancy.
If you expect to prune the roots quite vigorously, best to leave it till just after spring begins so the Bonsai has a better chance of recovering from its trauma, as the weather warms and the sap flows more freely and the new leafs are producing much needed nourishment which will aid with new root development and regeneration.
How frequently to repot your Bonsai?
In general younger Bonsai, i.e. bonsai under 10 years old which usually are vigorous growers, will need to be repotted more frequently than older bonsai of the same species.
Young deciduous bonsai can usually be re-potted every year, while young evergreen coniferous species every other year. Young evergreen broadleaf species can also be repoted every other year.
Mature Bonsai need only be repotted every 3-5 years, earlier if you notice the Bonsia is obviously root bound. More details on when to repot individual species can be found in Ma-Ke Bonsai - Species Guide
Preparing to Repot
Having decided that you are going to repot, there are a few task that need to be completed before you actually start uprooting your tree from its pot. You will need to collect and prepare all the materials that you will need for the re-potting, including suitable pot if required, repotting medium and appropriate tools to complete the job. We shall explore each of these subject areas below:
Usually will start with selecting a suitable Bonsai Pot, for younger trees this often means a larger pot, giving the tree the additional root growing area that is required, especially if it will support a larger leaf crown. For more information on choosing a suitable sized and aesthetically pleasing pot lookup Which Bonsai Pot?
Suitable growing medium for your Bonsai - Bonsai Trees Species do best if they given a suitable growing medium, some species like well drained soils, some species like growing mediums with loamy mediums, so it is important to collect the right mix of growing medium for your Bonsai Tree. You may even be able to acquire a ready mix from your get your favourite Bonsai retailer.
Tools will be needed to help you with the task - so have the right tools handy. Here are some you may need.
Root Shears & Root Cutters - to prune the heavy roots
Aluminium Wire (2mm and 1.5mm) - to help hold the tree firmly in the pot.
Wire Cutters - help with removing existing wire ties and to prepare new ones
Jin Pliers - To help with wiring and other odd jobs.
Repotting Sickle - to help with removing the Tree from the Pot.
Steel or Bamboo Spatula - to help with prising the Tree out of the Pot
Soil Sieves - to help strain the right sizes of growing medium
Potting Mesh - ensure soil stays in pot and bugs and pests out.
Root Hooks Single and Multiple Pronged - to untangle the root ball.
Chop Stick - to help with ensuring the growing medium gets in to spaces between the roots.
Tamping Tools - to help flatten and firmly compact growing medium in the pot
Cleaning brushes/Tooth brush - to help with cleaning the pot, the bark etc.
Camellia Oil - to give your pot the added clean and shine - for the perfect finish
Turntable - Nice to have a potting turntable (even a solid Cake Decoration Turntable will do)
A large bowl into which you will place your repotted Bonsai tree to water it.
Grated dried sphagnum moss or fine fresh moss. The moss layer will protect against excessive evaporation of water
Suitable place to work - a sturdy flat surface large enough to hold you tree and tools
It is important to prepare yourself as you could cause damage to your Bonsai if you go around looking for items once you have uprooted your tree. You must aim to complete your re-potting as quickly as possible and without letting tree roots dry out too much.
Before we start with the removal of the tree from the pot - a small diversion into understanding root pruning practice is necessary. Let us presume that you have removed the tree out of the pot and have combed out all the roots so the present a root mass that look like long beard.
One of the critical questions to answer is how much of the root mass to remove.
The actual amount you can safely remove is dependent on a few simple factors; the species of the tree, the age of the tree, the time of year, the location and the environment in which the tree will then be allowed to recuperate.
In a young deciduous tree, you may be able to safely bare root the tree in between late winter early spring, in its native climates. However, if your try and bare root a tropical tree in a temperate climate in early spring you are most like to kill it. Removing too much of the root mass may mean your tree will go into shock and may take years to recover, or worse - never recover.
The idea of root pruning is to provide a fresh and suitable growing environment without stress the plant too much.
If you unsure as to how much root ball to remove for your Bonsai then:
Do NOT totally "bare root" the Bonsai - keep the root ball intact.
Ensure that only the fringes of the root ball are combed out and pruned back - do not prune away more than 15-20% or the root ball.
If you need to replace all the soil of the bonsai, do so over a few years - or gradual repotting. With older and more mature trees this is the preferred method - rather than using the bare-rooting, washing and pruning method.
As a general rule I do not advocate bare-rooting bonsai - it can stress the tree quite significanlty
Repotting your Bonsai - Step-by-Step
Prepare the Bonsai Potting Medium
Make sure that your have sieved your Bonsai Soils so that you have removed all the fine dust from the growing medium, ensure that your particles are between 3-6mm for small pots and between 6mm-10mm for larger pots. Fine dust particles will clog your your soil and make it prone to water logging and poor air circulation. Some Bonsai Enthusiast use use larger (10-16mm) granules at the base of the pot for better drainage. Use a blend of growing medium suitable for your Bonsai Species. A good general purpose mix for Conifers is 1 Part Akadama to 1 part Pumice, for Decedious Broadleaf Species is 3 parts Akadama, 1 part Pumice, 1 part Bark Chips. (Look up the Species Guide for a growing medium mix suitable for your species of Bosnai)
Prepare your Bonsai pot
If you are using your original pot - use this opportunity to remove all the salt scale from the rim of the pot, clean the outside and inside of the pot thoroughly, an old tooth brush or plastic toothed brush will help in this task. Wash and wipe clean the pot. Polish the outside of the pot - Camellia Oil gives the pot a new shine and lease of life. Remember Pots age naturally and gain a natural patina and have a charm all their own, which can add to the value of the pot - so it is well worth looking after your pots.
Cover the drainage holes with small squares of drainage mesh, and secure down each mesh with 2mm Aluminium wire that has been bent into a U Shape and then inserted on to the mesh and through the drainage holes and the legs of the aluminium wire are bent over on to the outside of the pot to hold the mesh securely in place.
Wire the Bonsai Pot with Aluminium wire so as to be able to hold it in place when you repot the bonsai into it.
Removing the Bonsai from the pot
Cut any retaining wires from the bottom of the pot.
Thump around the edges of the pot with your clenched fist - to see it the root ball dislodges from the pot.
If the root ball does not dislodge - use a repotting sickle and cut the edges of the of the root ball - thump to dislodge or use a Bamboo or Steel Spatula to lever out the root-ball from the pot. If using a steel spatula - use a piece of cloth under the spatula so as not to damage the pot.
Untangling the root ball and combing out the roots
Place the root ball on the turn table and using a single pronged root hook start untangling the root ball turn the root ball on the turntable as you work. Move deeper into the root mass gradually - exposing roots only to a maximum 10% of the diameter of the root ball along it width and lenght.
Lift the root ball off the turntable and gently untangle any roots at the bottom of the ball.
Use a multi-pronged root rake to comb out the root all around the root ball.
Follow - any large root to its source - it may mean gently making your way deeper into the root ball - cut away the large roots. Cut back all the other roots till the root ball surface - the root ball should now be marginally smaller.
Reseating the bonsai tree
Place the root-ball back in the pot and determine a suitable position for the tree in the pot, you may want to pace the tree to once side of the pot to accentuate or emphasise the flow of the tree.
Make sure that there is enough room around the root ball, in its new for new soil in it. Reduce the root ball slightly on this side if you need too.
Once you have determined a suitable position - fill the bottom of the pot with growing medium so can sit on it and the top of the root ball is about 1.5cm below the surface of the pot.
Tie the tree down with the guide wires that are already in place. Make sure that you have the tree leaning in the right direction and that the trunk stands in just the position you want it.
Use a Jin Pliers and tighten the wires by pulling and twisting around simultaneously. The pulling will ensure that you have even twists and the wire does not break and twisting will ensure that the root ball and tree is held firmly in place.
Fill more growing medium round the pot and gently tap around the pot for it to settle in.
Use chop stick to gently work the soil between the roots if required.
Use a tamping tool to press down the soil gently and firmly into the pots to level out the growing medium. The level should be highest near the tree trunk and gently slope down to the side of the pot with the soil levelling off between .5-1 cm below the rim of the pot.
Watering the repotted tree
Hold the pot partially submerged in a bowl of water, the water rises to the surface gently as we lower the pot. You may just be able to distinguish low hissing noises as the soil soaks in the water and expels air. Let the water rise above the level of the soil and remove.
Make sure the pot drains properly of excess water by leaning it at a slant on a wall or a block of wood.
After a couple of hours when the water has drained you may want to cover the surface of the soil with sieved dry or live moss. This moss will prevent excessive water evaporation from the soil.
Clean up and wipe down the outside surfaces of the pot a little bit of Camellia oil will give you pot an added lustre.