Hi robinava - I have been growing my climbing H since '06, however, not up a tree but to cover up a 'not-so pretty' re-done 4'tall x 40'long retaining wall. Like yours, I was fortunate to find a large/4.8gal H petiolaris - was warned of her slow growing habit, but I couldn't pass up a 50% sale from a reputable nursery!

My plant, had 2 thick/main branches going in opposite directions, which was perfect for my needs!!!

I posted & Chelone responded, with a thoroughly detailed/helpful tips ... complete with photos, on how I should plant this H.p. for the purpose of covering up, eventually, my wall. A few aficionados joined in & suggested, the thread as a good tutorial! Â;)

Do a search on Climbing Hydrangea in this forum & you'll find it very helpful.

If you've planted yours already, you might use thick jute, garden training-twine to tie the branches to the trunk 'til others with, tree-climbing ones, chime in.

Good luck & have fun!!! Â;)

robinava,
It's doesn't matter- it will grow how it wish to.
Just do as Dita said: tie (loosely) those two branches to the trunk and it will cling to it.