Jumping on this thread as a watcher and a fellow black cottonwood fan hoping to compare notes. I collected some cottonwood from the Cascades at about 5000ft elevation back in 2019, and have decent recovery now. The area I collected from was loose roadside gravel on a decommissioned/unused forest road (w/ roadside collection permit).
It seems that black cottonwood is very durable. I've seen it at creeksides in the high desert, next to farms near John Day, in the Coastal range, and near my house in the Willamette Valley, which is an impressive range of climate tolerance. During the great 2021 PNW heat wave, I didn't encounter any signs of problems in my collected cottonwood, and while I shaded it during the heat wave, it is typically sat next to my pines in the sunny area, and not next to my maples. I find that its growth and health improved after moving it to a sunnier location. It consumes water very fast. I have it in mostly pumice in an old cat litter box (w/ swiss-cheesed bottom), top dressed with moss (which really helps with keeping up with the thirsty nature).
Foliage has multiple forms, I haven't studied the botany of this but informally, there is one form that emerges from spring buds usually in groups of 5 (I've been assuming it's mature form), then there are more elongated presumably-juvenile leaves that emerge after that. There's one third form that I think of as the goofy-vigor form, which is a larger and wobbly-shaped version of the juvenile form, coming out when the tree is really happy and weather is hot.
I have found that cuttings taken from my collected cottonwood are able to survive in water / wet spaghnum for weeks, and will readily root. An arm-length cutting taken from my collected tree about a month ago is still alive, having initially sat in water, then in moist sphagnum. I didn't expect it to survive the heat wave, but it did. I left it outside in shade. It lost many leaves, but after shedding those, has resumed growing with a handful of remaining functioning leaves.
The cottonwood I collected in 2019 had borers. Be on the lookout for that sort of thing. A single dose of imidacloprid knocked them out.
I had a poor recovery location for this tree in 2020, with too much shade and too large a recovery soil volume (due to the akward trunk-root setup dictating container choice somewhat). It didn't like that and had some kind of sparsely-distributed foliar disease (big fungal-looking brown/yellow spots) that I never diagnosed. By the end of 2020 it had developed many arm-length suckers with very large foliage, and I kept fertilization pretty high to prep it for a repot in spring 2021. It hasn't had any sign of any kind of disease this year after repotting into pumice and going into full sun, where I'll keep it indefinitely.
I did a test wiring this year after the first flush hardened to see how it takes to wiring. Two vigorous branches with plenty of juvenile leaves took well to being wired at that time, and recovered fully from an accidental kink-bend. Somewhat more mature apical branches with only mature leaves didn't recover as quickly or as well.
I pruned away several of the super-vigorous arm-length sucker branches (one of which became the immortal cutting mentioned above) a few weeks ago as it became obvious they were winning out over the branches I have on the trunk.
From what I've seen on this tree and one other like it, it will be very important to establish exceptionally strong branches and somehow over time minimize wounding of more mature pieces. Cottonwood seems to be a rot-happy, hollow-happy species.