In the past, if you went to the American Bonsai Society website there was a good article, rather old now, the first version was published back in 1985 or so, about growing bonsai under Fluorescent Lights. It was written by Jack Wilke. I went there today to post the link and the ABS has the resources area protected by password, you have to be a member to get the article, if it is still there, the whole site has been re-designed. It may be gone. But I saved a copy.
The article was written quite some time ago. It is accurate, it is what Jack Wilke does, or did for 40 or more years. I don't know if Jack is still around, he would be up there in years, at least 80 something, maybe older than 90.
Keep in mind Jack was growing his trees under T-12 fluorescent lights, the 1970's technology.
I have grown orchids under lights for over 35 years. I have periodically kept trees under lights with them. Chinese elm can certainly be grown under lights with good results. Even the rough bark varieties. Right now my Chinese elm is growing in the back yard, and it winters in my back yard. They are perfectly hardy in zone 5b. My Chinese elm came from Brussels some 10 years ago. There is no need to grow them under lights if you have a back yard. However, if you do not have access to a back yard, they can be grown under lights, on a window sill or on a combined set up where light from a window is supplemented by fluorescent light.
Now growing under lights takes a modest amount of horticultural skill. It is easier to grow trees outdoors, than it is to grow trees under lights. But it is not rocket science. If you want to grow the elm indoors, you can, key is you must be consistent about care. You must check daily to see if the trees need water, but only water if the potting media has dried to the point of being barely damp a half inch to one inch under the media surface. The finger is the best water meter invented. No gadget is more accurate.
I have had quite a number of trees in Brussel's proprietary potting mix. There is nothing "wrong" with it. The only problem is it may require a slightly different watering schedule than your other trees in other mixes. If you use your finger, check for when it has dried to barely damp, then water, you will have no problems with it.
How much water? Always take your tree to a sink, or hold it over a bucket. You FLOOD the pot with water. You want water to run freely through the drainage holes. Then let the tree drain 10 minutes on the sink side board, Then return the tree to the windowsill. You can not dribble a little water in one corner of the pot and expect it to diffuse around the pot. Bonsai media is coarse particles with out the ability to wick water around. You much take the tree to the sink and flood the pot with water to avoid getting "dry pockets". Let drain, return to where you grow it.
Hope these comments help. It really is possible to grow Chinese elm indoors. It is more difficult to grow indoors than outdoors, in that one much be consistent in their care. But it can be done.
