Earthgirl is right,
My elms never lost their leaves this year, despite the temperatures falling to freezing point on many nights over the winter season, and virtually all have been below 7c (which should be low enough for them to drop leaf).
Last week the weather was really mild, 4 nights in a row the night temperature was 12c. Some new growth was beginning to push on my Chinese elms (although its been pushing new growth all winter
) and so I was contemplaiting repotting them. Fortunately I decided to err on the side of caution and didn't touch them, mainly because of it being so early in the year. The past few nights, the temperature dropped to -3c which could have harmed the freshly pruned roots.
If you are going to repot the trees and do any root work upon them, just ensure they are well protected from frosts and hard freezes, by bringing them indoors to an unheated room at night if the forcast is for cold weather.
All the best,
Aaron