I’m aware that some bonsai trees are ‘possible’ to grow indoors but that gives me the impression that a ideal environment would be outdoors. Am I wrong? If I’m not are there any bonsai species that have nothing to lose inside.

No, there are no tree that is better off indoors than outdoors. However if you grow tropicals in temperate climat, obviously you need to protect them from frost during winter. For most species 5-10 degrees is better than in living room conditions.

But all trees should be outdoors in summer.

Trees evolved without the ability to migrate, so each species has grown very accustomed to local climate, including temperature, humidity, day light length, daylight spectrum, wind, insects, local soil composition, soil chemistry, and more. There is practically nowhere that indoor conditions are the same as outdoors - that is the reason we moved from caves to building, built fireplaces, etc.

My tropicals come inside every winter and suffer greatly. Half of them have died over the years, and the rest do poorly all winter. Indoors I cannot supply the wind and humidity they are used to. Outside in summer humidity here is >90%. Indoors it is mostly lower than 30% this winter. They are all suffering and I can't really do a lot about it, except not get anymore tropicals.

There are many who post here saying they are going to try keeping outside trees inside, skipping winter dormant spells. I have not seen a single post the next year saying the tree lived. They really need to be outdoors in the sun, wind, and rain.

Ivy, have you tried a grow light during the darker growing months? I have good succes with that.
I am not using real or purple grow lights. But just normal white bright light led spots 1500lumen 6000kelvin. I have them on during the day, about 12hours in nov/dec/jan/feb.
House is always around 60% humidity. House heating not warmer then 19,5 and at night not lower then 17. All tropicals are outside when the weather allows it.

One of the easiest i find are the ficus species. For example ficus retusa tigerbark. I have a few in my southfacing window, and even in winter they grow slowly. Did shed about 10leaves per tree when moving back indoors.
If you have no southfacing window, then definitly get a good led light on it so the tree can still be 'healty' during the darker days.

Wow! 60% humidity indoors in winter. May be good for your plant, but not for your house. Here home insurance dont cover damage on houses with more than 50%. In reality most houses is below 30% in winter due to heating systems drying out the air.

I have seen a few successful indoor bonsai setups. All seems to be placed in the coldest parts of the house, enclosed in grow tents for high humidity and have a lot of artificial lights. But that is really not living room conditions? Plants hardy to ones area kept outdoors at all times is the easiest beginners bonsai.

My ficus bonsai grows perfectly indoors. It does not require special care, but it is better to let it get direct sunlight. In a shady place, the ficus will not be comfortable. The temperature is kept at a relatively constant level. Ficus can withstand low humidity due to its thick leaves. It is an ideal plant, it seems to me, for someone who is constantly short of time (like me), but wants to have a bonsai in his house))) It is very important to protect it from drafts and exclude sudden temperature changes, so when the summer temperature outside and inside the apartment is the same, I take it to the balcony.

Tropfrog wrote:

Wow! 60% humidity indoors in winter. May be good for your plant, but not for your house. Here home insurance dont cover damage on houses with more than 50%. In reality most houses is below 30% in winter due to heating systems drying out the air.

I have seen a few successful indoor bonsai setups. All seems to be placed in the coldest parts of the house, enclosed in grow tents for high humidity and have a lot of artificial lights. But that is really not living room conditions? Plants hardy to ones area kept outdoors at all times is the easiest beginners bonsai.

Indoor humidity in our house was 50-65% most of the year until last year. We had a large cinder block wall against a huge dirt wall and the constant rain soaked through the wall. Last November we had foundation work done and the humidity dropped to 30% in the first day. Also, in the next few days the tropicals that had just moved indoors all started to have issues. They all survived the winter months indoors, but the low humidity got to them. The baobab has been outdoors for two months and has just started to leaf out the way it should.

Low humidity is definitely not good for them.

Serissamain, where abouts are you in the world? I live in the UK and have mostly outdoor bonsai, but have three different bonsai on my upstairs landing windowsill - all are thriving, even my miniature forest of ficus benjamina that traditionally don't like strong draughts / air currents (unavoidable at the top of stairs). For indoor bonsai you can't go too far wrong with a ficus variety with dark green, waxy leaves, as they are usually quite hardy (the sort of plant that you see on offer in IKEA or similar in fact..) You may have to gradually move it around the house to see where it thrives in particular - if low humidity is an issue an oversized drip tray with gravel / water can help...

Just to add. In my climate ficus bonsai cannot be grown indoor in winter. Too little sun in combination with the dry air makes them suffer badly and lose brances.