AusBonsai

Hello,

I'm definitely not ready to put anything into bonsai pots or soil, but I am thinking of re-potting what I have into bigger pots for some growth and thought it would be good to begin transitioning the soil slowly in the right direction.

They are 1 to 2 year old seedlings currently in fairly small pots. Lemon, lemon scented gum, weeping fig, dwarf pomegranate, and a kind of unidentified conifer.

What would be a good soil mix for growing these in bigger pots for a while, and for a first transition toward potential bonsai/prebonsai?
What kind of pot size is suitable at this stage?
Could several trees go in one pot together?
Would this also be a good time to cut off any tap roots?
Is mid-spring an okay time to do this?
(Spring because I'm in Europe, some are starting to get some nice growth)

Note:
I'm not necessarily asking for specific ingredients/recipes, although that would also be welcome. Happy just for some general advice on particle size, organic to inorganic ratio, etc.

Everyone has their own preferences for potting mix.
Some basics:
You can use less perfect mix in deeper pots. The shallower the pot the more critical the mix becomes for drainage and aeration.
Larger particle sizes provide better conditions for roots and reduces the chances of over watering and root rots but it will need more regular watering.
You can grow trees in almost any mix. It just requires adjusting your care to the mix you use.

Pot size:
Larger pots generally allow more growth so faster development.
Some plants do not tolerate being put into really large pots. The roots cannot fill the area and the unused mix seems to go sour or toxic or something and then the plant just doesn't grow well. Best to just go up one or 2 sizes each time instead of straight from a little pot to a huge tub.
trees will grow in the same pot but generally trees seem to do best one in each pot so there's less root competition. Maybe if the tub was large enough several in a pot would still give good growth. There's also a range of possibilities. Many people need to do things that may not give maximum growth because of circumstances like lack of space or pots, etc. You just get whatever you can with the resources available.

Repotting time:
Plants can be up potted at almost any time with minimum root disturbance.
If you want to prune and improve the roots you'll need to get most of the old soil off and prune, sometimes prune quite a lot of roots. That's best done when the plant can best tolerate such root reduction. Best times of year depends on the species. Deciduous winter - spring before they start growing; Conifers and many other evergreens late winter through spring, maybe as late as mid spring; Aussie natives and tropicals like figs best in warmer weather Mid spring - mid summer. many growers are also repotting after mid summer now too.
Seedlings can be transplanted almost any time, even if they are growing. That first transplant is the time I cut the roots quite hard as they seem to have no problem with growing new roots when young.

It is far more convenient if pics are loaded direct to the threads so we don't have to go to third party sites. Files must be under 1 mB for Ausbonsai.

Thanks for the reply!

I was pretty confused about why bonsai has to use such coarse soil (or, if that's a better kind of soil, why isn't it recommended for every pot plant), but I think I just came to an understanding of why that is.

shibui wrote: ↑May 20th, 2020, 5:57 pm You can use less perfect mix in deeper pots.

I thought about this and remembered an article published by Ken Thompson. He was busting the myth of improving drainage by adding pebbles, rocks, or pieces of broken clay pot. (It appears in his book The Sceptical Gardener, not bonsai related but very enjoyable read by the way, I recommend it). He mentions the idea of the perched water table and a bit about how that works.

I found a really good explanation of that here too: https://lee.ces.ncsu.edu/2018/12/rocks- ... -problems/

My understanding is that there's a downward force applied by the mass of water in the pot that flushes out the water. There is an upward force generated by the capillary action of the soil (that sponge effect, sucks some water back up). The balancing out of these forces determines the depth of the perched water table: the stronger that suction, the higher the mass of water it can support and so the deeper the water table.

In the link above they say "The area above the water table is the unsaturated zone and is where the plant’s roots have space to grow well."

And obviously a coarser/rockier material with larger particle size will have less spongy suction.

So in a shallow pot, the depth of water table will take up a higher percentage of the pot height. With the wrong soil, there'd be little to no 'unsaturated zone'.
A deeper pot will still have the same depth of perched water for a given soil type, but it will take up a much smaller percentage of the total pot height, leaving an unsaturated zone where healthy roots can grow.

I guess it gets a bit more complicated that this because it doesn't mention aeration, and then the root growth themselves affect the way the drainage behaves, but this explains a lot of things that previously seemed a bit obscure to me. Including why deeper pots are more forgiving.

(ps. I just searched for a few of the right terms in the forums and found even more discussion and explanations but haven't yet had the time to read it all...)