My father is building these bed's with the hopes of putting this particular tree in it http://www.fast-growing-trees.com/WillowHybrid.htm My question is this.What is the root structure like with this type of tree?The beds measure 6'x13',and are built using dry stack block construction(very strong) He is wanting to put 2 of these trees in each bed.With that type of room is there any chance the root structures of these particular trees could possibly blow out the sides of the wall?Any help would be greatly appreciated.