Hi in Pgh....I'm near New Stanton. I'm sure they form on new wood, I have 2 trees that just bloomed for the 1st time and it was definately on new wood. I'm not sure if they bloom on old wood also. They are so pretty once they get large and are all covered in blooms.

Mamabird-
I was searching for info on mimosas and came across this older posting you had contributed to. How old were your 2 trees that bloomed for the first time? I planted a mimosa a couple of years ago and was really hoping to have blooms this year but nothing yet.....and I see older trees all over town bloomin like crazy. The tree is about 5 feet tall now...I'm beginning to worry that maybe there are male and female mimosas, and I have a male.
Any thoughts?