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How to Repot a Bonsai Tree (and the Reasons for Doing So)

Updated on December 28, 2016
A little slice of heaven in the backyard.
A little slice of heaven in the backyard. | Source

Unearthed and Potted

Bonsai having originated from China has been introduced to Japan as a way to create living works of art for the social elites. Miniaturization is what the hobby is based upon. The goal is to recreate a scenic view of a mountainous landscape or down to a single tree perfectly trained to look like nature had sculpted it with wind and water. Having a bonsai, Japanese for a potted plant, or penjing, Chinese for a potted scene, means you've taken the organism from the nurturing earth and placed it into a confined space. The plant is no longer receiving nutrients from the earth and no longer receiving life-giving rain, either. You are the caregiver, the guardian, the all-powerful God to your potted bonsai trees. So it's important to understand and to assume that responsibility, the burden to all of your potted bonsai plants, as they are now at your mercy. So be a merciful omniscient being and attend to the needs to your living art creatures.

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Repotting Bonsai: Why Do We Need to Do It?

Aside from watering, feeding, pruning, and training, repotting is one of the biggest responsibilities any bonsai-tender needs to get familiar with and perform on a fairly regular basis. But why do we repot a tree? What purpose does it serve?

The answer is directly relative to your #1 responsibility: Because you have taken the tree/plant out of the nurturing earth. In a natural environment, a plant will have ample amount of space to spread roots and absorb untold nutrients from the earth. In a confined pot, based on what kind of soil mix you have, the soil will decompose slowly and roots will fill the pot. Not introducing fresh material and not pruning roots is a recipe for disaster.

Chinese woman in traditional dress in front of mirror.
Chinese woman in traditional dress in front of mirror. | Source

Root Bound

In ancient China, during the 10th to 11th century Song Dynastic period, foot binding was very popular for the upper crust of society. The philosophy behind the tradition is largely due to not having the woman do any strenuous labor. But the aesthetic reason behind foot binding was because it makes the foot look daintier, and that was quite sought after.

There is a phrase in Chinese, "要靚不要命," which loosely translates to "desire beauty rather than desire a good healthy life." How does this correlate to bonsai? Like foot binding, binding the roots is as harmful to the tree as it is to the woman. It inhibits growth and doesn't allow for much mobility which can hinder the health of both tree and woman. But unlike foot binding, root pruning can solve the issue. Although a plant contained in a pot is already considered unnatural, and the mention of pruning its roots back to stop it from being root-bound is yet another unnatural practice, still it is the only natural means of allowing the plant to survive in its confined home.

Note how the roots have started to encircle the pot. This is the beginning stages of being root bound. In another year or two, the roots will start to thicken and the space between roots will shrink and soil mass will deplete.
Note how the roots have started to encircle the pot. This is the beginning stages of being root bound. In another year or two, the roots will start to thicken and the space between roots will shrink and soil mass will deplete. | Source

What Are the Signs That a Bonsai Needs Repotting?

More often than not, there are a set amount of rules that will dictate when to repot a tree. It is important to know that for bonsai, the rules are simply suggestions, because every bonsai is different, from place to place, tree to tree, and person to person. Some rules will dictate that a bonsai get repotted every 2 years or so and that is dependent on what kind of soil medium is used.

The 2 year rule came about because akadama, a type of bonsai soil mix, will break down after a few years because it is a type of clay. When this soil material breaks down, it will compact and lose its great permeability. Also, breaking down into smaller pieces, this medium will lose its capability to efficiently absorb nutrients. So one can only surmise how a 2 year rule can come about.

However, time is not the only factor to use when deciding to repot a tree. A plethora of signs can hint as to when a tree should get replanted.

  • One very good sign is water. How does water tell you when to repot a tree? More often than not, when you water a tree, water seeps pretty instantaneously into the soil and drains out just as quick. However, when you see signs of that it's taking longer for water to penetrate the soil surface, it's a clear sign that the roots and soil has compacted to a degree where the tree's health will be impacted.
  • Another good indicator is the root ball. After 2 years, if it isn't already cut, you can cut the wires that hold the rootball to the pot. Take the tree out to inspect the root mass. Any roots that encircle the pot will continue to circle and thicken. Over time, this will continue and begin to hurt the tree because important feeder roots can't be sent out by the tree to take in much needed water. If the tree's roots don't fill and encircle the pot, it's a good idea to leave it and let it grow a bit more.

If you've ever repotted your bonsai trees, how long do you typically wait (in years) before your next repot?

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What Time of Year Is Best for Repotting?

Remember following rules to bonsai? Well, repotting too has a set time frame rule and honestly, it should be loosely followed.

So when do you do the deed? That will depend on the species of tree but typically, it's good practice to repot during spring, but tropical trees can be repotted at any time of year. Again, it differs from place to place and tree to tree. It's important to pay attention to the signs the tree gives you it's ready to be repotted.

So, yes, spring is the broad time to aim for. It can be further separated to early spring, mid spring, and late spring. Assuming that your tree is indeed ready to be repotted because it is root bound and or having issues with soil impermeability, then it's a good idea to observe the tree during spring to best gauge when is the most optimal time to do it.

What's the Deal? Why Are We Waiting?

The energy cycle of the tree is why we wait. It is also the reason why we observe the tree to accurately pinpoint the best time for repotting. So to read the signs, one needs to understand how seasonal changes affect the tree.

To be very brief, a tree loses its leaves to safeguard itself from the harsh winter. Trees who want to keep their fleshy leaves will have to toughen up to safeguard their foliage during the winter (like pine trees that are evergreen). Having fleshy, broad leaf foliage means the tree is constantly using energy to keep those leaves lush so they can absorb sunlight to produce glucose. When fall comes, the tree must shed its lush green leaves so it can store energy for the harsh winter since winter will freeze any fleshy foliage. This transference of energy can be seen as winter comes: Energy is transferred to the roots to store and use through the winter season. Once the bitter cold leaves and the spring comes, the transference of energy can be seen as it moves from the roots above ground and into the body of the tree to once again produce leafy green foliage.

So by following a very simple equation, LEAVES = ENERGY, we can gauge when is the best time to repot.

Notice how the buds have swelled and turned a green color.
Notice how the buds have swelled and turned a green color. | Source

The Best Time Is NOW!

The best time is in spring when it's starting to warm up. You observe your tree and notice that the buds that were left on the tree after all the leaves fell off are starting to perk up and swell. This, ladies and gentleman, is the golden moment to execute repotting.

It is at this moment that the transference of energy is shifting and at this moment, it is possible to repot the tree without causing catastrophic damage. One can imagine that if a tree is repotted too early, one will prune away the energy stored in the roots which will lead to a tree dying or leave a tree severely weakened. Even advanced hobbiers will confess to having repotted at the wrong time, which caused the end of one if not several of their trees.

Summer and Fall Repotting

What about repotting in the summer and fall? All mandates state that spring is the best time, and winter is the worst time; however, what about summer and fall?

Repotting in the summer or fall is not typically done, but it can be done if performed with skill and experience. Summer and fall repotting can be done on any species, but in truth, a summer or fall repot is not exactly comparable to a spring repot. How are the two different?

A summer or spring repot is less intrusive as the tree is repotted to provide emergency action to address poor soil drainage, soil compaction leading to oxygen deprivation, and possible fungal infestation. Most of the time, a summer repot is where a tree is removed from the pot and "slip potted" into a larger pot where the angle of the tree is adjusted. New soil is then added to the pot to cover the existing root mass.

Note the tree on the left is slanting downward. The emergency repot is to correct the slant and restyle the tree. It's placed into a larger pot where the roots aren't disrupted. The tree's slant is corrected and new soil is added around.
Note the tree on the left is slanting downward. The emergency repot is to correct the slant and restyle the tree. It's placed into a larger pot where the roots aren't disrupted. The tree's slant is corrected and new soil is added around. | Source

How to Repot a Bonsai Tree

Typically these steps can be applied to any tree of any style, the only difference is time when it's done and how much root pruning a tree can endure. Assuming that this is mid spring and the buds have swelled, the bonsai tree has been in its pot for about 4 years and is ready for a new pot, it's time to repot.

Bonsai Tree Characteristic Gender Features

Masculine
Feminine
Heavily tapered trunk
Delicate trunk
Mature Bark
Smooth Bark
Angular Branching
Sleek and smooth Branching
Deadwood
Some deadwood
Features of masculine vs feminine features of a bonsai tree.

Step 1: Pot Selection

The very first thing one should do even before taking the tree out of the existing pot is to prepare its next home. The new home should be a pot that is similar in size. Too large a pot could cause the tree to grow exponentially. If that is the goal, your tree is most likely in need of training; therefore, should be put into a very large training pot to allow the tree to grow without much restriction. However, if the tree is not in need of growth training, the next home should be similar if not slightly smaller. How do you pick out your tree's new home? There are a few guidelines to follow.

Quite often, this is the most difficult part of repotting. Because this is what will tie the tree together.

Choosing a pot for your bonsai is not exactly an easy task. But a pot is a pot is a pot right? Not exactly. In bonsai, a pot will finalize the tree creating harmony between the tree and the pot. Bonsai pots are used purely for aesthetic reasons; therefore, it is important to understand the rules of pot selection.

Dimensions:

A general rule typically requires that the depth of the pot is equal to the diameter of the trunk after it's planted into the soil, so the surface of the soil.

Masculine vs Feminine:

As trees can be categorized into having gender features; pots can also be categorized into these two categories. Because pot shape and depth determines the gender of the pot, it is important to know how to pair it up with your bonsai.

- Rectangular Pots:

These pots are typically associated with masculine trees (featuring all the traits listed in the table). The one tree that fits the rectangular pot are conifers, almost exclusively, as they typically have a more robust trunk and mature looking bark.

- Round Pots:

These pots are associated with more delicate and more feminine trees. This pot shape can suit both conifers and deciduous trees.

- Oval Pots:

This pot shape can be associated with delicate bonsai as well. Typically used in group or to display a scene, due to the elongated pot to offer more depth.

Color and Texture:

Bonsai pots come in an array of colors and textures. Combinations of colors and textures are used to; yet again, compliment the tree. Though, not as important as the shape of the pot, colors are a final touch to adding that final harmony to a tree.

As many pots come glazed or not, it is good to know how the glazing of the pot can further help define the prospective tree.

- Glazed/Colored

Typically used for flowering trees to further accentuate the special feature of that tree.

- Glazed/Earth Toned

Can be used to suit any tree that is of the feminine quality.

- Non-Glazed

Typically used for conifers. Masculine quality.


Step 2: Preparing the New Pot

Once your pot has been selected, it is ready to make the move to its new home. The very first thing to make are drainage screens. The screens will allow water to drain; however, will still hold soil medium back. What holds the screen in place is a copper wire formed into a shape of a staple that will hold the screen in place. Note how the thin wires are protruding out of the pot's bottom. It's threaded through the smaller holes to ensure when you place the root ball in, you can fasten it to ensure the pot and tree are bound together so the tree doesn't move as it establishes its new root system. The bottom of the pot is also covered with the largest coarse component of the bonsai mix. This will allow for better drainage.

Note how the copper staple top is larger than the hole; but, smaller than the screen, this will hold the screen in place and once threaded through the drainage hole, the excess wire can be folded outwards to securely hold the screen into place.
Note how the copper staple top is larger than the hole; but, smaller than the screen, this will hold the screen in place and once threaded through the drainage hole, the excess wire can be folded outwards to securely hold the screen into place. | Source
The completed preparation of the pot. Not, other than the drainage holes are smaller holes for anchor wires. These can be used to hold the root ball in place when repotting.
The completed preparation of the pot. Not, other than the drainage holes are smaller holes for anchor wires. These can be used to hold the root ball in place when repotting. | Source
Note the wires that stick out to hold the pot in place. Also note the mound placed where the tree will go. This will allow the tree to be wedged into place.
Note the wires that stick out to hold the pot in place. Also note the mound placed where the tree will go. This will allow the tree to be wedged into place. | Source

Step 3: Removing the Tree

Once the pot is prepared, it is now ready to receive the new tree.

Firstly, it's important to take a kama or a bonsai sickle and go around the perimeter of the pot to detach the root ball. After removing the tree from the pot, a portion of the tree's root ball can be removed. This is called root pruning. Take as much out as needed to fit the tree into the new pot. Take off about 20% of the roots, but no more than 40%. Generally speaking, this works for most tree species. Pines and junipers can take a harsher beating. Soil can be loosened further to reveal more roots to be pruned further. It's a good idea to insert a long rod-like instrument (screw driver) to jiggle the soil to dislodge the material off the roots.

Note how the tree is in its existing training pot.
Note how the tree is in its existing training pot. | Source
Once out of the pot, a portion of the rootball can be removed. Remove as much that's needed to fit the tree into the pot.
Once out of the pot, a portion of the rootball can be removed. Remove as much that's needed to fit the tree into the pot. | Source
Note how the roots amass along the bottom and how dense the feeder roots have become.
Note how the roots amass along the bottom and how dense the feeder roots have become. | Source
Note how much soil material has been dislodged off the root ball. This approach will allow you to introduce new fresh material into the root system.
Note how much soil material has been dislodged off the root ball. This approach will allow you to introduce new fresh material into the root system. | Source

Step 4: Check the Angle and Find the Front

Once ready to place into the new pot, the small mound of soil you have placed in preparation of the bonsai pot comes into play. Assuming you have the front of the tree already picked (because this is an already styled tree), if not, a front will need to be found.

The front of the tree is just a term to denote what the face of the tree will be. The front is typically the side of the tree that looks the best. Once found, place the tree onto the mound and wiggle it into place. The goal is to make sure the proper angle is achieved. If the desired angle is not right, add more soil and rework the tree back into the soil mound to get the desired angle.

Note how the tree is placed directly over the mound and the angle is found.
Note how the tree is placed directly over the mound and the angle is found. | Source

Step 5: Fastening and Introducing Fresh Soil

The next step is refilling the soil. But before this can happen, the root mass should be fastened tight by the protruding wires. Once the tree is secure, the soil can then be added.

Introducing new fresh soil is not as simple as just shoveling new material onto the root ball. The soil has to be "worked" into the root mass. The best way to do this is to take a chop stick or a piece of bamboo and swivel/rotate the stick to work the soil into the empty voids within the root mass.

Note how the bamboo stick works the new soil into the voids of the rootball.
Note how the bamboo stick works the new soil into the voids of the rootball. | Source

Step 6: Finishing

Once the root ball is full of fresh soil, the repotting process is nearing its end. At this point, it's important to make sure all the soil has been worked into the pot. To check, you can make a fist and gently pound the side of the pot with the fleshy side of the palm to settle the soil.

Finishing touches to the tree is to make sure all soil is incorporated into the pot and root mass.
Finishing touches to the tree is to make sure all soil is incorporated into the pot and root mass. | Source

Final Step: Water

The last step is to water the tree. This step is crucial as the material that's been introduced is dry and can pull out all the remaining water in the roots. It should get a thorough watering to finish off the repotting process. Watering should be instantaneous as the water seeps into the medium, it should quickly leave the pot. The water that drains out of the pot will be murky at first but keep watering until the water runs clear.

The last step is to water the newly introduced soil.
The last step is to water the newly introduced soil. | Source

A Comprehensive Video to Repotting

Conclusion

Repotting is something that any bonsai hobbier should look forward to. It's a great time to check on the health of the tree and correct any problems. It's at this point where the tree's nebari can be created and this can take decades to create. Repotting a bonsai is in part therapeutic and calming and can serve as a great teaching tool for anyone who wants to understand the tree's cycle.

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      Mary 4 weeks ago

      Thank you for your insight; it is most helpful.

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