One block should do it for a 5-6' Christmas tree. The way we do it is we shove the base of the trunk in one of the holes in the block and then use 1/4" rope to tie it all up. We used to sink hundreds of Christmas trees in our lakes. We have dumped 300-400 hundred in a cove on Greers Ferry. We dove on the piles a couple of summers ago and we could hardly find any of the trees. It wasn't that the trees disappeared. It was just the fact that they are so small and insignificant compared to other types of brush pile materials. If you continue to use the Christmas trees, I would make giant piles with them and not spread them out over a large area.