I just bought a Tardiva from the 50% off rack at Lowe's. I would like to train it into a tree. How can I tell which is the main stem that will become the trunk?
Training a Tardiva into a Tree
Select the biggest, straightest one and prune the rest out. You may have to support it for a couple of years (or more) until it thickens.
http://homeguides.sfgate.com/methods-growing-hydrangea-tree-46718.html
PS - the PG is very easy to train into tree form but harder to find these days.