Japanese Maple, or Acer palmatum, is a popular species of tree in bonsai culture due to its delicate leaves, vibrant colors, and unique branching patterns. With its graceful appearance and stunning fall foliage, Japanese Maple bonsai is highly prized among enthusiasts. Although it requires a bit more attention compared to some other bonsai species, the effort put into its care is well worth it for the end result.
In this blog, we'll delve into the fundamentals of Japanese Maple bonsai care, providing helpful tips and guidance for both beginners and experienced growers.
Acer Palmatum As Bonsai
Acer palmatum is a beloved and iconic species in the world of bonsai. Its smooth, unblemished trunk, plate-like nebari, and delicate, intricately ramified branches make it an ideal subject for bonsai enthusiasts.
The species is native to Japan, Korea, and China, and derives its name from the Latin term for palm, which references the five-pointed lobes of its leaves that resemble the fingers of a hand.
Acer palmatum is fairly straightforward to care for as a bonsai, but to really get the best out of them there are many nuances to their pruning, repotting, and fertilization that can take your tree to the next level. This is one of the reasons I love Japanese Maple as a bonsai subject - it is accessible to beginners but has plenty to motivate advanced practitioners to improve their trees.
Spring and summer
Japanese Maple bonsai should be cultivated outdoors, and thrive in sunny locations. They are vulnerable to leaf burn in higher temperatures, so when it gets hotter than around 30°C (85°F) consider placing them in partial shade.
Japanese Maple bonsai will develop longer internodes and less ramification if they are grown in the shade. This is the tree's natural response to being shaded out - it tries to elongate its growth to find better sunlight levels. This will diminish your ability to create a finely ramified bonsai with tight internodes, so avoid overly shading your maples if they don't need it.
Japanese maple trees are deciduous and require a period of winter dormancy each year. The species is frost tolerant and does not necessarily require any special winter protection. However, this all depends on your environment and the bonsai itself.
At temperatures above -10°C (15°F), you will not need to provide any winter protection for most Acer palmatum bonsai. However, if you have a tree that is very old, has been in a bonsai container for a very long time, or showed a health issue or weak growth through the year, you may want to consider some protection.
The winter protection you provide could range anywhere from heeling the tree in with leaves or mulch, placing it in a wind shelter or cold frame, or even moving it into a heated or unheated greenhouse.
Japanese Maple bonsai indoors
It is fairly common to see sources on the internet claiming Japanese Maple bonsai trees can be grown indoors. Sadly, I have not found this to be true (it may be possible, but it is very difficult and honestly not worth the effort).
The environment indoors tends to be far warmer and drier than outdoors, which increases the water demands on your tree. Also, the levels of UV light in your house will be less than they are outside, as the light is filtered by your windows. This can make it harder for the tree to photosynthesize, and will also contribute to the elongated growth we mentioned above.
Finally, the cold and dark conditions required for winter dormancy can't be replicated indoors. Without a period of dormancy every year, a Japanese Maple tree will slowly lose strength and die over the course of a few years.
So while it can be tempting to grow a Japanese Maple bonsai indoors, it isn't fair on the tree or yourself and it will be much happier outside where it belongs.
You can control growth on a Japanese maple using various methods, but it all depends on what stage of development your tree is at. Maples in early development will need the structural branches selected, while a bonsai in secondary development or refinement will need more focussed pruning of secondary branches, as well as partial defoliation and pinching to encourage ramification.
We'll look at each of these techniques in turn so you have all the tools you need to advance your maple bonsais.
When to prune Japanese Maple bonsai
There are two things to consider when deciding when to prune a maple bonsai: ensuring you prune at a time when the tree can repair the damage, and avoiding sap bleeding from the tree when you prune.
Japanese Maple trees move water from their roots to their branches very quickly. This rapid movement of resources helps the tree to grow strongly, but pruning at the wrong time of year can cause the tree to bleed sap.
If you prune a Japanese Maple bonsai tree in early Spring when buds are starting to elongate, you will likely see the tree start to 'bleed' within minutes. A clear sap will start running out of the cut site. This sap is very rich in sugars stored by the tree last season, which is moving to the buds to start this year's vegetative growth.
By pruning in Spring and causing bleeding in your Japanese Maple, you are weakening your tree, and obviously, this should be avoided.
A bit of confusion can arise here because early Spring is usually the best time to prune most other species of bonsai. We still want to prune at a time when the tree is not dormant, so that leaves us with a couple of ideal times to prune:
Autumn leaf drop
When leaves start dropping in autumn, it is the ideal time for pruning a Japanese Maple bonsai. At this point in the year the tree is busy reabsorbing elements such as chlorophyll from its leaves and redistributing them around the tree to boost its Winter hardiness. This movement of resources helps the tree to compartmentalize any wounds from pruning.
We also have the advantage of having a leafless tree when pruning during leaf drop, which means you can make educated design decisions.
Post-flush harden pruning
If you didn't manage to prune your Maple in autumn, the next best time is after the first flush of growth has hardened off in Spring. Once the initial set of leaves has grown out in early Spring, wait until the first two pairs have developed the deep green color and waxy coating that indicates they are fully formed.
At this point, you can safely prune your Japanese Maple without the risk of sap bleeding. This is a good time to be doing structural pruning as well as tidying up secondary branches. You can also partially defoliate at this time of the year which will help with ramification (see below for more details).
Structural pruning and selection for primary structure
If you have a piece of raw material or a tree in early development, your pruning should begin by setting the primary structure. The goals of this pruning should be:
- Create the lines of the bonsai
- Remove structural defects, such as 3 or more branches growing in one place
- Removing weak growth that will fail to contribute to the final design
Removal of big branches is best done with a fine-toothed bonsai saw. If you use concave cutters or a similar tool, the cut surface is a lot less clean due to the amount of crush caused by this kind of tool. This is absolutely fine for most bonsai trees, however since Japanese Maples have such thin branches and an elegant aesthetic, it is better to use a saw to give it a better chance of healing without a scar.
When removing larger branches, be sure to think about the potential for die-back along the trunk. Ideally, you should have other branches below the cut that will draw water and prevent a large amount of die-back. It is common to have a bud appear at the cut site of a large branch and I would encourage you to let this grow. It will help bring resources to the cut site to help with healing and can be pruned away after a year.
Pruning secondary branches
After handling the primary structure, you will need to prune secondary branches (the branches growing off the biggest, main branches). Secondary branches defined the shape of the branch and contribute strongly to the shape of the canopy of the tree.
When pruning secondary branches you should keep the desired shape of the canopy in mind. You will want to prune away any elongated growth to allow finer ramification to grow in its place, and similarly to structural pruning, you will need to prune out any branches that could grow into structural flaws.
If you have an established Japanese Maple bonsai it's unlikely you'll be doing much structural or secondary pruning. Some may be required every few years, such as cutting back a strong secondary branch to a finer, better-ramified one on the interior, but in general, you will just be managing tertiary growth.
The idea with refinement pruning on a maple bonsai is to cut back any elongated growth that is not finely ramified. If in doubt, it is best to cut back to the two buds closest to the base of the branch. These two buds will ideally grow into two branches, giving you two branches where you once had one.
Do this across the tree and over time you will massively increase the number of branches on your tree. As this happens, the strength in the tree will be distributed across more buds, which leads to finer and more balanced growth.
If there are areas of the tree you feel need refinement pruning and others that need to be grown out and developed, then simpler prune the refined section as required. Pruning in this area of the tree but not the area you are developing will shift the hormonal balance to focus growth on the developmental zone.
It is possible to achieve smaller leaf sizes and more refined growth in an Acer palmatum bonsai through the technique of partial defoliation. This method involves removing around 70-80% of the tree's foliage mass, which stimulates the tree to grow a second time. The second growth will have tighter internodes, smaller leaves, and a more delicate appearance due to the tree having already used up its stored energy on the first growth.
While fully defoliating the bonsai, or removing all of its leaves, can also achieve good results, there is a risk of the tree dying if it is unable to grow back its leaves. Partial defoliation is a safer option as it ensures the tree still has some leaves left to generate energy.
To perform partial defoliation on a Japanese maple bonsai, start by removing half of the leaves, alternating left/right to promote new branches to grow on alternate sides. Then, cut the lobes off the remaining leaves with sharp pruning shears to reduce the overall foliage mass of the tree. Be cautious with watering as the tree will use less water and may need protection from the sun for a short period.
Partial defoliation can be done multiple times a year depending on the bonsai's growth rate. It is important to note that this technique may not be necessary for other bonsai species with smaller leaves.
Pinching a Maple bonsai
Once you have an established bonsai with its primary and secondary structure set, you can start to pinch growth to distribute energy across the tree. The idea with pinching is to hold back strong individual leaders and allow interior shoots to gather more strength.
Pinching a Japanese Maple bonsai is a fairly easy thing to do, as there are clear signs the tree gives on when to pinch and which branches to pinch.
When growth first starts in Spring, the strongest buds in the tree will start to open first. These buds contain the highest concentrations of a hormone called auxin, which suppresses the growth of buds behind it and focuses energy and growth into the leading bud.
Once these first buds have opened enough for you to distinguish two leaves, you can go in with tweezers and pinch out the central growth from the bud. This will do a couple of things:
- Stop the tip elongating, encouraging the two new shoots to take over as the new leaders and increasing ramification
- Removes the concentrated auxin from the tip of the branch, causing the buds behind it to be more stimulated to grow
You can come back once a week or so over the course of Spring while buds open up to continue to pinch out the strongest growth. If there are areas of the tree that need to gain strength or develop a bit more then you can leave these unpinched, redirecting the tree's strength and thereby pinching everywhere else.
Over time, pinching your maple will dramatically increase the number of branches of the tree, making for a wonderful leafless Winter silhouette.
You can wire a Maple at pretty much any time of year from the beginning of Spring to around 2 weeks after leaf drop in fall, although it is definitely a good idea to wire at the same time as pruning.
Japanese Maple is a very thin-barked species that can scar easily, so care must be taken when wiring your bonsai.
Using aluminum wire, and wrapping it in a paper towel before applying it, can help reduce the chance of damaging branches when applying the wire. It's also a good idea to check the wire regularly once it is applied, since Japanese Maple scar easily due to wire biting in.
To get branches to hold bends you will likely need to unwire and then rewire them a couple of times until enough growth has occurred to hold the branch.
How to apply wire to a Japanese Maple bonsai
When applying wire to a Japanese Maple bonsai, it's important to select the appropriate gauge of wire. A good rule of thumb is to choose a wire that is roughly one-third the thickness of the branch you are wiring. This will ensure that the wire is strong enough to hold the branch in place, but not so thick that it causes damage.
Before applying the wire, it's a good idea to prune the branch to the desired length and remove any unwanted growth. This will make it easier to apply the wire and ensure that the branch is properly positioned.
When wrapping the wire around the branch, make sure to do so gently and avoid twisting the wire. It's also a good idea to start at the base of the branch and work your way up, rather than starting at the tip and working down. This will help prevent damage to the bark.
Once the wire is in place, use your fingers to gently adjust the branch into the desired position. Don't apply too much pressure, as this can cause damage to the bark and underlying tissue.
Check the wire regularly to make sure that it is not cutting into the bark. If you notice any signs of damage, remove the wire and reapply it using a different technique or gauge of wire.
There are some specifics to consider when watering an Acer palmatum bonsai. It's a tree that uses more water than other deciduous or coniferous species, but both overwatering and underwatering can be damaging to root health.
For most species, I would usually advise allowing the top half inch of soil to dry out completely before watering your bonsai, but for Japanese Maples I just let the very top layer dry out. They are more sensitive to drying out than a conifer would be, so I try not to take risks with underwatering, particularly when it's hot.
However, it is still very important not to overwater a Japanese Maple bonsai, as this will inhibit root growth and put the tree at risk of root rot, just the same as with other species.
The wet/dry cycle of thoroughly watering your tree, then letting it dry out to a degree that encourages strong root growth.
Repotting can be a broad topic in relation to bonsai, and I think with Japanese Maples there are even more factors to consider. This is because so much of the Maple bonsai aesthetic is linked to its flaring nebari, which requires special treatment of the roots to develop over the course of decades.
To achieve this iconic nebari on your tree, you will need to be meticulous with how you handle the roots during repotting. This topic alone could take up an entire blog.
Other considerations when repotting are the same as with other bonsai species, namely root health, soil compositions, and when to repot your tree.
The function of the bonsai container
Before digging into the specifics of repotting a Maple bonsai I think it's important to consider the purpose of the container it's planted in.
Bonsai containers are intentionally small. This is to restrict root growth, encouraging more ramification in the roots which will cause a more ramified canopy, resulting in a tree that looks miniaturized.
Japanese Maple bonsai trees are usually planted in containers that are very shallow. This is to encourage roots to grow laterally rather than downwards, helping to create the flared-out base that is so desirable in maples.
Understanding the horticultural impact of the bonsai container helps us to think about how often we should be repotting our bonsai. The aim of the container is to restrict root growth, but each time we repot we make space and add soil that is unoccupied by roots, which reduces the effect of restricting growth.
So basically, if you want your bonsai to have the desired horticultural effect please resist the desire to repot your tree too frequently. There are signs it will show you when it is ready to be repotted.
When to repot a Japanese Maple bonsai
In terms of the calendar, Japanese Maple bonsai trees should be repotted in Spring, just as their buds are starting to open. This is the best time for root reduction because the tree will only grow a quantity of foliage over the Spring season that it is able to support with its roots. If you repot in summer the tree will have banked on having lots of roots to hydrate its foliage mass, and root reduction at that time can be very dangerous for your bonsai.
When it comes to deciding whether your tree needs to be repotted, there are two signs to look out for:
- Decomposition of soil - Over time organic matter will break down into smaller and smaller particles. This leads to a soil that holds too much moisture, causing a wet root environment that is detrimental to the bonsai's health.
- Loss of percolation - As more roots grow in the pot, more organic matter accumulates on the surface from fertilization, and you may notice that water no longer penetrates the root ball. Instead, you will see it running off the sides of your pot. This can make it very difficult to get water to the core of the root ball, and it's a clear sign your Maple needs to be repotted.
How to repot a Japanese Maple bonsai
Before you begin the repotting process, you'll need a few tools. These include:
- A bonsai soil mix: Choose a soil mix that drains well, and will not break down easily. Avoid using garden soil or regular potting soil, as these can retain too much water and suffocate the roots. Akadama is an ideal soil substrate for Japanese Maples.
- Pruning shears: These are used to trim the roots and remove any dead or diseased roots.
- A chopstick: This is used to loosen the soil and untangle the roots.
The goals of repotting a Japanese Maple should include:
- Remove any organic or decomposed soil
- Remove downward-growing roots under the nebari of the tree
- Comb the roots to arrange them in a lateral orientation
By doing these three things, you can boost the health of the tree, allow it to continue to be cultivated in a shallow container, and help to develop the spreading nebari.
Steps for repotting
- Remove the bonsai from its current container. If it's in a nursery can, it will usually slide out easily. If it's an established bonsai you'll likely need to free the roots from the sides of the container using a root sickle. After that, cut any wires that are holding the tree in place and it should then be able to be pulled out of the container.
- Redefine the nebari using a chopstick to clear away soil and moss from the top of the root ball. The nebari can often be buried by moss and fertilizer and will grow under the soil as roots develop. Gently scrape away the covering soil to find where the nebari ends.
- The next step is to tackle the bottom of the rootball. Start by removing the congested matte of roots using a chopstick or paint scraper. Then systematically work away the soil to make the rootball as shallow as possible. Ideally, you want to work right to the underside of the trunk and remove all roots growing downwards. The ongoing theme here is to prune away all roots that are not growing laterally.
- The final step in tackling the roots involves removing the mattes around the edges of the rootball, then combing through the roots with a chopstick to ensure they are orientated laterally from the trunk. When you find a root that crosses over or under the others, prune this away. By being meticulous with this you can really have success developing a spreading nebari on your maple.
- Next, you need to prepare your pot for the tree. Add mesh to drainage holes and work out your strategy for placing tie-down wires. Ideally, the tree will be secured in two places, but it may need may if the rootball is less developed.
- Now you can plant the maple in its bonsai pot. Add a thin layer of soil over the bottom of the pot, then a small or medium-sized mound in the middle, depending on the depth of the pot. Place your bonsai on top of the soil mound then firmly twist and shift it down into the pot. This helps to introduce new soil into any small gaps in the rootball and helps to prevent large air pockets.
- Tie your tree into position and fill the rest of the pot with soil. Use a chopstick to push soil into any areas that are void of roots or substrate. This stabilization step is very important. If your bonsai is moving or wobbling in the pot, it will inhibit the growth of new roots. This can severely impact the health of your tree and is one of the major causes of death for bonsais following a repot.
- Create some topdressing to develop into a healthy sheet of moss on your bonsai. Grind up sphagnum moss and locally sourced green moss in a 1:1 ratio. If green moss doesn't grow in your environment, just using sphagnum moss is fine. Sprinkle the topdressing on your tree in a light.
- Finally, give your tree a thorough watering. Soil gets crushed during the repot so it's essential to wash away all these fine particles to stop them from holding too much water. Water your tree until you see it running clear from the drainage holes.
Repotting aftercare for Japanese Maple bonsai
Good quality aftercare is really important for the success of your repotting. Roots have been cut and reduced, so we need to create the ideal environment for them to regenerate.
Roots need air to promote growth. So the best thing to do for your tree after its thorough watering following the repot is to let it dry out.
To form a callus and heal, you must allow your bonsai to dry out after its first watering. Wait until the topdressing is dry before you water the tree for a second time. This gives roots a very oxygen-rich environment following the repot, which will stimulate callus formation and will promote wound healing. If you overwater or leave the bonsai out in the rain, there is a chance the roots will rot rather than heal. If this occurs across the whole rootball you may be in trouble.
The other thing you need to consider after repotting a Maple bonsai is the environment you place it in. Strong wind and hot sun should be avoided, as these will both increase the evapotranspiration rate and put stress on the tree.
You should also avoid fertilizer after repotting a bonsai. The salt concentration in fertilizer is very high (particularly in synthetic fertilizers), which damages new root tips as they grow and can prevent the rootball from regenerating itself. In late spring or early summer after the first flush of growth, you can consider fertilizing your tree again.
Fertilizing Japanese Maple bonsai trees should always be done with a goal in mind. Trees in early development will need heavy fertilization to encourage strong growth and thickening of the trunk and branches.
However, after this stage of development, you should hold back on fertilizer with a Japanese Maple more than you would with other species. This is because Maples will develop long internodes and coarse growth than can spoil the tree if they are fertilized too heavily.
Check out our full guide on how to fertilize a bonsai tree for more details.