It sounds like that was a plant that was not well taken care of before you got it. I have re-potted hundreds of bamboo and what you describe as the condition of the roots is something I have only seen in a couple of instances where I have had a plant die on me and noticed the roots were mushy when I emptied the pot months later. But, to me, mushy roots on an otherwise seemingly healthy plant is weird. I see you are in Zone 10, which is fine for B. oldhamii, but I'm wondering if it got shipped in from someplace else where the temperature dropped below freezing, or if it was sitting in a flooded area for several weeks. In any case, this is not a symptom particular to this species.

I don't think your plant is going to die, but I do think it is in shock from abuse prior to your acquiring it, and being re-potted and having the rootball disturbed and trimmed since you got it. Not that what you did was a bad idea, but I think the best thing right now is to not mess with it anymore and to simply give it good light and regular water. It may take months for it to get better, and in the meantime could lose some foliage or culms. But don't give up on it.

From what I can tell, the grower was located in Lodi, CA, which is zone 9 to my zone 10. It gets colder there in the winter, but hotter there in the summer. But I wouldn't expect that to be a big difference.

Perhaps it got over-watering by the Home Depot staff.

In any case, I cut off a few of the dessicated leaves that were mostly destroyed from winter wind burn, but other than that, I'm basically leaving it alone.

watchnerd-
Good luck...and report back when it gets better.

I forgot to add: It's really tough to overwater bamboo in a pot. About the only way I can imagine damage from overwatering is if it was in standing water for a couple of weeks. So...regardless of what happened to it in the past, don't be afraid to keep it moist.

Update:

I trimmed off about 10-15% of the leaves. I picked the scraggly, burnt, and dessicated ones that got damaged during the winter. They were likely to fall off over the next month or two, anyway.

Since doing so, the leaf-curl-while-raining issue has gone away.

My theory:

The hypothesis about too many leaves relative to the root mass causes transpiration issues appears to have been correct.

Your hypothesis may be correct, but there are several other factors that can cause leaf loss in this situation. Anyway, there is nothing wrong with getting rid of stuff that is clearly dead or dying. Hopefully it is on the mend now.

kudzu, it wasn't losing leaves (or at least not very quickly). Just inexplicable curling when it shouldn't have.

Curling is typically a result of not enough water. If you were keeping it moist, then this was most likely a result of the plant being shocked from the transplantation process or a result of impaired function of the rhizome system due to damage.

Yes, which is what I stated in the first post.

watchnerd-
The distinction I was trying to make is that, with an adequately watered bamboo, there are two main causes of the foliage not looking good: 1) a root/leaf-mass imbalance, or 2) transplant shock, which is a temporary thing and requires no trimming of drooping leaves...just patience while the plant recovers. Since it is hard to know which it is, I generally let the plant alone to self-correct, although I, too, would remove any tanned leaves that are ready to drop.

What I was thinking, but didn't make clear, is that it would be premature to remove any branches. I've had bamboo that completely defoliated after being divided and transplanted...and came back with new leaves on all the branches in two months.

Gotcha.

Just to clarify, I didn't remove any branches, just leaves. And only the damaged ones.

It seems to have worked.