Are you saying that you took a previously potted plant and put the roots in water? Or a separate cutting? I don't know this plant well enough to give advice but I think one would need to be clear on what's going on give you advice. If you browse threads on the house plant forum, you might find some pertinent info there, or may want to also ask there. There are 4 pages of search results in HP forum for Pachira.

Purpleinopp: Oh, yes, thank you for pointing that out. It's base completely rotted, so I cut the trunk above the point of rot. The 'cutting' was in water, but after reading some posts, it is in moist soil now.

I guess my main question is just if hardwood cuttings will root, or if I need the green part to root.

Oops, I will repost this to House Plants forum.

check out the link

Here is a link that might be useful: link

I would suggest bringing it outdoors under full sun, and keep the soil well drained. I'm not sure if your stem is rotting, but if you have plenty of live parts, they are pretty easy to root when they get stuck in soil with some type of a rooting gel, but the key is to make sure the soil is well draining and never gets damp.

Here's a link to when I propagated mine. I got 8 out of 14 of them to root as it seems like the smaller green parts don't make very good divisions due to their vulnerability to rotting off. I simply plant them, and leave them alone for the most part. It has been just over a month and they are starting to form buds to generate new leaves, but rooting usually takes another month or so. I split up my pachira aquaticas every year.

Here is a link that might be useful: Dividing up pachira aquatica