I have a chinese elm bonsai plant. I used to keep in indoors for the winters, but this year we have moved and I don’t think we’ll have enough sunlight during the winter inside the house, so I decided to leave the plant outside for the winter. The plant was outside from the spring. Now the autumn is in full swing and all the trees here are almost without leaves, but my elm doesn’t seem to shed its leaves at all. This troubles me, as the temperatures will drop soon and the tree can’t stay outside with the leaves still on. What should I do? Should I take it inside?
If you live in Central Europe, you probably get cold winters, which means your tree cannot stay outside all winter anyway. Chinese elm varies a little in hardiness depending where it was grown in the first place, but all are really only frost hardy to varying degrees and should be removed inside when the weather becomes very cold. And yes, they are late to lose their leaves - more info in the link below
From Wikipedia page on Chinese Elm:
The leathery, lustrous green single-toothed leaves are small, 2–5 cm long by 1–3 cm broad, and often retained as late as December or even January in Europe and North America.