Adding growth in bonsai pots?

Hey Mirai Family,

This may seem like a stupid question (when it comes to bonsai i more often than not feel stupid!!!)…

I have lots of little conifer trees which i’ve made out of nursery stock. The basic structures are set but i want to add a lot more vascular tissue, thicken trunks etc They are in nursery pots and soil at the moment and i’m going to repot them all into an Akadam/Pumice/lava mix in spring.

My question is what sort of pots are best to achieve this type of growth? It seems that Ryan’s method is to repot into the smallest container that the root mass will comfortably fit in, looking to develop the peripheral root system and work on the central roots following the second repot. Can you achieve good growth and significant thickening in a small-ish bonsai pot or are there other types of pots (air pots, mesh pots etc) that it is best to put nursery stock into first to develop the growth before going into a bonsai pot?

Thanks so much for any advice and love from the UK!!


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If you are attempting to quickly thicken your trees then a bonsai pot is not what you want to put it in. A wood grow box, air pot or other deeper pot is what you should use (or put it in the ground).

Also your soil mixture might need to be adjusted to get more thickening. Something like pumice and bark is good for helping thickening. Akadama slows down the growth and is better for refinement.


I use an inexpensive colander with fine pumice with a stacked colander with the growth medium (usually heavy on pumice) to grow and root prune at the same time. I use organic fertilizer and Dr Earth premium with monthly chemical fertilizer to accelerate growth and girth. Works well on juniper, pine and hornbeams and zelkova… So… a colander in a colander with sunshine and oxygen and lots of nutrition make for rapid growth and the confined colander keeps the roots finer than they would be in a nursery container. I read an piece about rapid growth of pine from seed in Bonsai Today #20.
Important to not let the soil get too dry, so I use the weight as a quick check before I water the colanders. If they have dried they are much lighter than usual. The second colander slows the drying process and keeps the roots healthier in my opinion.
In praise of colanders - Bonsai Tonight
Needs winter protection in my climate. Usually end up in my unheated garage when the temp is near or below freezing.

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Thanks for the reply mate! Is there some benefit from using some Akadama in the mix to help with nutrient binding?

Thanks for the reply Bob! Trying to get my head around your setup…


Is this what you’re getting at? What is the process by which pure pumice for good growth? And what do you use for your growth medium?

Cheers mate

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If you use something organic in your mix like bark it’ll have way more nutrient binding than akadama.

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The bottom colander has fines from pumice after sifting. The top colander contains the secured tree with two parts pumice , lava one part and granite or sharp sand one part. The Pumice and lava hold the water, fungus and bacteria and nutrients to grow the tree with space and texture to keep oxygen present. The fungus and bacteria are seeded with Trubiotic from Dr Earth all purpose fertilizer. The Bio Gold or Gro Power fertilizer add long term nutrients and the chemical liquid add immediate nutrition. The fines in the bottom colander hold the moisture enough to delay the soil drying quickly. I have not been using moss or a surface treatment but I can see that surface roots would benefit from the practice. If I rotate the upper colander in the lower one every month, I have not had the problem with large roots breaking up the top container. The abrasive fines sand off the growing tips and make for many fine roots in my experience. I used bark as a substrate years ago but the roots were not evenly distributed. My goal is many fine feeder roots in a very small package. The full sun, large amounts of nutrients and fast draining substrate is my plan to get growth. The trees are wired as saplings to give as much exaggerated movement as possible because they will smooth out as they grow and bark up.

I save akadama for trees further along in development and refinement. If I have some left from a repot I have sterilized the mix and added to the medium, but with frequent fertilizing the binding is less important. I sterilize my used soil in plastic bags left in full sun. The pumice and lava and granite have been what I used before I became an akadama convert. The trees seemed to have fewer pathogens with a course mix, but they definitely lack refinement.

To get more vascular growth I have found that grow box with a mesh bottom works very well. The nursery industry really pushes the growth so they use a mix that contains lots of organics and drains fairly well - shredded bark or bark chunks are a fairly common component. Unfortunately for us bonsai growers this often results in lots of roots along the sides and bottom of the pot. I am now using a 50/50 mix of screened bark (3 to either 9 or 12 mm) and pumice (3 - 9 mm) in my grow boxes and small pots for growing on seedlings. I don’t have much experience with the mix, but the concept is that the bark will hold the nutrients and the pumice will improve oxygen exchange and make the soil more friable when it is time to clean up the roots and repot.

I also like the idea of a colander inside a colander to help keep the growing roots moist and allow small escape roots to be sheared off periodically. I found colanders to be too dry near the surface in my climate during the summer - moderately hot and rather dry.

Yes, surface roots suffer from heat and dryness. That is why I think moss or sphagnum moss for surface treatment could make for a better developed tree. I am still learning and my process is always open to improvement and knowledge I get from others successes and failures. I have had enough failures to make me try to find better methods!

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Thanks for all the info Bob!! Its so awesome to have a space where you can chat with people who have experience of these things and have tried and tested them over the years. For beginners like me its invaluable!

How do you determine how large a pot to use for the growing chamber? My brain wants to tell me that bigger is better but then i would presume you run the risk of water/oxygen imbalance in large areas of soil where roots haven’t grown into yet.

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Hey Marty,

Many thanks for the advice!! I asked Bob a similar question…how do you determine how big to make your grow box? Obviously you will have an existing root mass that you want to grow but until they do you will have large areas of soil without roots (if choosing a large box) which will potentially cause water/oxygen imbalance and discourage new root growth.

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You are correct there. If the container is too big and the water will just sit and cause anaerobic conditions. If you are trying to grow them, you will be doing some frequent up-potting to keep the pot right sized as the tree is growing.

I don’t know the right formula but I’d say take whatever the existing rootmass needs plus about 10-20%?

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Did you look at the in praise of colanders link? In praise of colanders - Bonsai Tonight
If you are growing from sapling or seeds you want to start with small container.
I was also surprised to see how large some of the trees were in relation to the size of the colander. As long as the trees get enough water, a surprisingly small pot can support a lot of tree.
If I am using multiple seedlings or larger stock I use a pond basket. I am currently trying to make clump Ginko in pond baskets like the demo from Bjorn Bjornholm. Kabudachi Style Bonsai, Part One - Eisei-en (
So I start small to make many fine roots. Growing in an oversized container means I have to cut away roots that grow along the sides and bottom, and that is a good way (in my experience) to stress and kill a tree. My goal is to save time and have trees that thrive. So Anderson flats, pond baskets, and colanders are my container of choice. If I have an old yamadori, it lives in a grow box with as much drainage as I can make and keep the tree stable. Stability is critical. Good drainage and oxygen being replenished is critical. However you find a way to meet these requirements are the keys to healthy trees.

When I dig a tree from either the growing bed or collected I make the grow box just big enough to hold the roots. If I don’t have the right size box I have bent the roots to fit. When getting rid of field soil I think smaller is better as long as the roots are contained. This seems to be the consensus of this thread and other discussions.

When moving from one box to another, I agree that 20-25% wider and longer than the current root mass is a good guideline. The current root mass is the mas after they have been combed out, long roots cut, and big roots cut back to promote the smaller ones. I have used larger boxes for maples, but they are generally quite shallow so they heat up in the sun fairly quickly.

For maples, I have used the plastic saucers for under plastic pots. They are typically 3 - 6 cm deep and 15 - 40 cm in diameter. I melt a fair number of 2.5 cm holes in the bottom for drainage. The trees normally fill the pot in two years with a very granular mix (mostly lava and pumice). It will be interesting to see how they do with my newer 505/50 bark/pumice mix. I am wondering if that might retain too much water in the saucer but be good in the mesh bottom grow boxes. Sounds like some good experiments.

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I plant my trees in the ground in grow bags to speed up trunk thickening and development. I go with pine bark mostly for soil medium with some additives like grow stone and manure. I think this is basically what was done in the asymmetry podcasts about field growing with telperion farms.

I would not put in a bonsai pot. Some of the other suggestions are good with building boxes or with Anderson flats. Ground is easiest and it will bulk up quickly!

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I use a suitable plant pot for the first year for freshly struck cuttings and air layers then normally they get moved into air pots, 1 or 3 ltr size depending on the variety of the tree.

You may find that as you grow ‘up’ the trees, this structure will grow out and branches become too thick for the size of the trunk. You could grow out then style when the desired size is reached.