This climber will not attach to slick surfaces, like aluminum siding - it needs a textural surface for the rootlets to adhere to - wooden siding, tree bark, a fence. You can use the iron lattice for support but you will have to manually attach the hydrangea to it. And unless it is a BIG trellis, a happy climber will rapidly outgrow it. They get to be 40' or more!

ps. I though the whole advantage to aluminum siding was that it didn't need painting.......just wondering :-)

No aluminum siding doesn't have to be painted ever (key words "doesn't HAVE to"). I just wanted to. I didn't choose the siding - it was there when we bought our house 20 yrs ago and it's fine but I've gotten bored a couple times with the color since that time....so a change was fortunately possible.
Thx again for your advice gardengal!