I protected mine a bit when it got below freezing for a week or so at a time... I have some shelves that I made into a makeshift greenhouse with bubble wrap, that I use in the winter. I left it outside then... it stayed between 2 an 10 degrees C in there most of the time. some people say that chinese elms do not need any protection... I would normally agree with that..but if it spent most of it's life indoors, you might want to consider some protection if it gets really, really cold... how cold are your winters?

I live in Karlsruhe, Germany. We have oceanic climate. Usually between -5C - 0C degrees but some winters we experience very cold weeks below -10C. My maples and conifers do not need any protection at that temperatures as I know(I read that they can survive through -15C)? So, before night temperatures go below 5C I move the elm inside for couple months.

And about my other plants, I can make a DIY greenhouse in my balcony if you think they also need protection if it is -10C outside.

I wouldn't only take my word for it on which plants you protect. Moving the elm inside could potentially have adverse effects. if it has been outside the whole summer and the beginning of fall it should be fine.

www.bonsai4me.com/SpeciesGuide/Ulmus.html

Elms can stay outside. If you get real cold (colder than say -5 to -10) consider protecting by putting it in an unheated area, say your garden shed. I keep mine outside. And at bonsai museum Dusseldorf I have seen some very good elms out in the -10 last winter too..