My bonsai sprout is very tall

[Goodlary]

I'm hoping to get some advice on how to look after my bonsai. I am a very new bonsai owner who grew from seed in February 2024 (roughly 4-5 months old).

I have 3 sprouts that look healthy (from the pictures) but are getting very tall. The biggest one is around 60-70cm now.

I've attached pictures. Here is the background:
- I'm in the UK, near London. The bonsai is on a windowsill and collects sunlight (when we have sun) all day long.
- This is an indoor bonsai tree (I live on the top floor in a flat, so sadly unable to plant outside)

Questions I'd like some help with, please:
- Do you know what type of tree this will be?
- Is my bonsai too tall? What advice would you give to manage this before it reaches my ceiling?

My goal was to try and get that trunk a bit thicker, but as it's only 4-5 months old, I don't know if I'm on the right path.

Thank you very much in advance.

 

[Lutonian]

These are Birch trees unfortunately they will slowly die inside as they need outside conditions and they will need to go dormant for the winter. If you can only grow indoors try something like a ficus, schefflera. or some other tropical/subtropical plant and these will still preform better as bonsai if you can get them outside in the summer. apologies for the bad news

 

[ShimpakuBonsai]

1. - This is an indoor bonsai tree (I live on the top floor in a flat, so sadly unable to plant outside)
2. - Do you know what type of tree this will be?

@1. This is NOT an indoor tree.
@2. This is some kind of Betula (Birch).

 

[Shibui]

I concur with the above. Experience shows that most temperate species trees indoors do not cope well long term but as you already have them and no other options you may as well continue and maybe, just maybe you can be the one to do it.

While technically these are trees in pots and therefore literally 'bonsai', most of us would consider these 'trees in training' rather than fully fledged bonsai. In my opinion a bonsai should look like a real tree, preferably old looking. One which viewers will stop and stare at because it conjures memories and feelings.

- Do you know what type of tree this will be?
- Is my bonsai too tall? What advice would you give to manage this before it reaches my ceiling?

The others have already identified these as birches of some sort and I agree.

Whether it is too tall depends on your aims and what techniques you are applying. You can probably already see that the taller one is far thicker than the shorter trees. Growth also means trunk thickness and trunk thickness will help make your tree look older and more impressive as a bonsai. Many of us allow developing bonsai to grow free as tall as they can to get thick trunks quicker.
We manage height (and growth in other directions) by pruning. Chopping the trunk short will not only reduce height but will also force the trunk to produce side shoots which can be the beginning of your tree's branches.
How much free growth and how low to chop are questions with no definitive answers. Depends on your space, resources and what you aim to achieve. You can cut your tree at any time and it will just start to grow again. Pruning will not harm your birch tree.