I have a 20cm young seedling kotobuki. I realised the between branches distance is quite far. What should I do to encourage branch growth? Or should I wait until I have my desired height? I’m going to repot into a air-pot. Please advice!
I would let it grow, well fertilized, in full sun. Next spring it may pop some buds low on the trunk and/or closer to the trunk. Then you can consider what you are going to create. You likely will not want that long straight trunk. If you don't get anything popping on it below, one of those two branches will have to become your next trunk section, the other your bonsai's first branch and the trunk from there up will be the sacrifice that will get lopped off once the bottom trunk section is a thick as you want in your final bonsai. But there might be more interesting possibilities if a lower bud or two shows up next year.
Meanwhile, you might want to fiddle with wiring and bending an upper branch or two. You might also see if you can bend the trunk. I do think the trunk and branches quickly become quite stiff and hard to bend, but fiddle a bit instead of just taking my unreliable word for it. If you find that you can readily bend either or both, there are even more possibilities for you to sort through in deciding what you can and may do with it (i.e., your development plan and objectives).
Kotobuki are supposed to backbud easily. I would be concerned with letting it grow and letting the trunk thicken for a couple years at least. Then pruning back to a new leader. This is to create movement and taper and also induce back budding lower on the trunk.
With my Kotobuki the branches are very flexible and have no issue waiting to wire.
I’ve had mine for two years and I do have backbudding already but the new leader is very slow to thicken. Slow growers in general so I would feed and let grow till it at least doubles at the base.
The new leader on the one is in the second full growing season . I have been shaping and candle pruning the lower branches to keep them in check while the top I’m letting grow.
Remove the brown and partially brown needles. Dispose of in sealed plastic bag. Remove dead needles to a good distance off your property. I think you have needle blight. Treat with Cleary's 3336 or Daconil. Repeat as recommend, especially before you put away for winter, and again at least twice in spring.