Weeping cherry trees keep dying.

by Kylie
(Melbourne)

We have had two weeping cherry trees planted into our front garden and both have died. Do you have any information on how big the root base should be when planting? We believe the ground below is quite rocky and could have some granite. Could this be the problem, and would it mean no weeping cherry trees would grow in these conditions? We were thinking of digging a half wine barrel into the ground and putting the tree into this, hoping to eliminate any heat that could be causing them to die. Could this work?
Regards
Kylie