Thanks guys. I've been doing this cold hardy citrus thing for 6 years now, and never had barrels bust like this. I think it usually gets warm enough to keep them from fully freezing, but the past few weeks were just too much prolonged cold. The idea of paper/ foam barrier around edges of barrel would allow for expansion, but I think we need the water as. Close to the tree as possible to benefit from the heat given off as water freezes. Also, lowering the freezing point of the water may work against the effort of warming the tree since we're making it harder for the water to freeze and therefore harder for water to give off the heat from freezing, no? What a science project, huh?