If this is the same site you describe in the discussion on clematis in that forum, I'm sorry to say that this is probably not going to work for you with a climbing hydrangea, either. First, these get to be large, woody and eventually quite heavy vines. While they can be trained in an espaliered form, they will need some sort of very sturdy support to hold up the main stem or woody trunk - that bare opening between the two ends of the pergola is not gonna work. And they do not vine but cling to rough surfaces via rootlets or fine hair roots that form along the stems.....hard to attach to any sort of wire, although it can be used as sort of a guide. And they grow very slowly initially, taking several years before establishing and beginning to put on any serious vertical growth. And finally, once growth begins in earnest, the pruning necessary to keep the vine restrained to just the side of your pergola and along the espalier framework will remove any flowering potential.

Since no one else has answered yet, I'll jump in with my thoughts on this. Can you possibly plant the climbing hydrangea in the ground? These plants may take 3 years to get "going", but grow to be huge over time ... 30-50 feet! A vine this size is equivalent to a small tree and, I believe, will need an adequate root system to support it. You'd need a rather large pot. However, since it is winter hardy to USDA Zone 4, you shouldn't have any trouble overwintering it in a container, as long as it is not subjected to repeated and extreme fluctuations in soil temperature (as in a dark pot absorbing the sun's heat on a mild winter day, followed by a severe cold spell). If the container is in the shade, no problem. If the container will be in the sun, I would protect it with mulch or some other means of insulating it from extreme temperature fluctuations - the root ball, not the vine. I have a climbing hydrangea growing in my back/side yard, in dappled sunlight, and, after the initial slow growth phase while it was establishing its root system, it has been very vigorous, floriferous, and (I hope I'm not jinxing the plant, here) pest & disease free. Missouri Botanical Garden has some good information on this plant; link below. Here is a link that might be useful: Missouri Botanical Garden: Climbing Hydrangea

I have a climbing hydrangea that is 3 years old and almost 6 ft tall, with several laterals. I have trained mine through a trellis--here's a pic: Mine also tries to spread across the ground--I just cut them off--I figure keeping them just diverts the plant's energy from growing taller and spreading farther. Since trimming them again this year, I've noticed the laterals are putting on new growth. The lack of growth of your plant may be due to placement or watering or both. My vine gets morning sun and afternoon shade and I water very frequently (every other day at the least.) I recently removed several roses from that trellis--while there, the roses were fertilized regularly, so the vine benefitted from proximity to them. Hope this helps!

Thanks Ken and NHBabs. I’ve been corrected. The walkway is actually plastic made to look like wood so it won’t rot. Whew! I like the looks of clematis, but would it survive in a more shady spot like this? I wasn’t sure if the climbing hydrangea would take over too much, so thanks for confirming my doubts. The lattice is hiding an outdoor shower area, but you’re right - a larger shrub could do the trick instead. It would take a while to fill in though. Hmm. There is another small vine on the lattice right now. I need to take a piece to the garden center for an ID, perhaps honeysuckle? I wonder if I could just encourage that vine a bit more, although I’m not sure if honeysuckle is an invasive issue here... Thanks, I appreciate the input! Curious to hear if othere’s have more ideas!