important to note that many of the cultivars are not nearly as hardy as the straight species. That being said I don't like any of the cultivars so I only have experience with regular old Chinese elm. In the past I have left mine out all winter in a small pot without a problem. I won't be doing this anymore, because I think protecting roots during summer heat and winter extremes gives far more vigorous growth come spring. Every person here will have a different opinion but after a lot of research I personally try not to let the roots get below 20F, they won't die until below 14F but that gives me a buffer. Important to note that air temperature does not equal soil temperature. And soil temperature does not equal root temperature. The plant is full of anti freeze. On a 17F night the soil could be something like 20F (in a fairly small pot, warmer in a larger one). And the roots? well I have no idea because I have no way to measure, but knowing they are almost definitely above soil temperature puts them in a safe range. I begin moving trees into shelter around 19F just to be safe. But all this is assuming they are in a decent open soil. If your tree is a tub of wet mud I would be far more protective. As the soil freezes solid into block it will mechanically destroy roots that will later root in the cold muddy pot. But to answer your question... 40F is way more protection than CE need, in fact if you keep them above that temperature I don't think they would even go fully dormant.
