About 6 years ago I planted two dwarf plums out in the orchard. After the second year in the spring, they both appeared to be dead, but then one put out shoots from the base. I assume that they originated from the root stock that the dwarf plum was grafted to. It is now about 6 feet tall, but I'm not sure it is a plum! The bark looks right but it is covered in long thorns.
Could the original plum have been grafted onto something other than another plum?