Can anyone tell me a source for the large (25 gal?) plastic nursery pots that are used mostly for palm trees? The kind that can be taken apart. I'd like to use them inside a long planter.

I think the confusion is in that root pruning can mean different things to different people. What they are telling you is that if you are digging the plant up from being field grown, you can damage too many roots by just digging it up all at once. When people with field growing operations want to minimize the stress, you cut some of the roots and then a couple of months later, after those roots have heeled in a little, you cut a few more roots. By the third time, you dig it up and only a third of the roots are freshly cut during the actual move. This plant was in a pot and you already cut some of the roots that were going in the ground. That is all the root cutting you need to do. It is possible, depending on how many roots that was, that even that can stress your plant. If I was doing this, I'd cut some leaves. All plants have bottoms and tops and there is a ratio between the two that create the balance as it was growing. The more roots you cut, the more leaves you should remove. That is something only you know the answer to. I'd cut a few of the good leaves myself, just to make sure. they can grow back in time. As it is, after it is planted, if you see more leaves turning brown, you will know you should have cut a few more off. The plant will show you in advance what the balance is.

Mine in overwintered in quite similar conditions. Mine go into a detached garage with a Winter range of from 60 to maybe 30. The leaves stay green over the Winter. There is some minor damage to the leaves at first. Not sure if it was due to the sun or some early April sleet. I acclimate them a bit but as they taller, it gets harder to find someplace outside without sun exposure. In Winter, I water very seldomly. Maybe twice over the coarse of the Winter they would get dragged outside and hosed down. Otherwise, It would be an odd, lighter, occasional watering just to prevent them from really drying out. If there is snow around, I would pile that around the palm base for slow watering. If you are looking for containers, I ended by buying cheap garbage cans and drilling drainage holes in the bottom. Being black, they also absorb more heat, useful in the chilly weather.

If it's been that long it's a safe bet it's root bound. Especially if you are sone 10 and it gets lotls of sun and heat.