I posted the following in another post on spring/repotting:
Below freezing is NOT OK for leafed out deciduous trees--even if sheltered from the wind.
This is the most dangerous time of year for deciduous bonsai. Trees that have begun to push new leaves HAVE LOST ALL THEIR ABILITY TO WITHSTAND FREEZING TEMPERATURES. Once leaf edges are visible on buds, deciduous trees have exited dormancy and lose their winter hardness.
That means if temperatures around the roots drop below freezing, roots will be killed--which can result in the loss of the tree, as it will have expended most of its stored energy pushing new growth... The smaller the pot, the greater the danger to the tree in it. Smaller pots have less soil volume and freeze more quickly than larger ones.
Freezing temperatures around new leaves will freeze tender new cells completely, turning newly emerged leaves to black mush.
The best thing to do now --in areas that haven't seen their last frost date pass--is to be VERY vigilant about nighttime temperatures. A rule of thumb--if the temp is forecast to drop to 35-38--bring the plant inside the house for the night--even if you have 30 trees. Find a temporary place for them.
Also think about when you put them back outside (hopefully it won't be more than a couple of days). Placing trees back out in the morning before the sun has had a chance to warm things up, could expose them to freezing also...