I used a large piece of grapevine once and it literally rotted in my tank. I also got mine from PS. After a few months, it was really soft and when I picked it up, the bottom was disgusting. Keep an eye on it. I didn't get mold, at least not visible mold, but the decay was enough to deter me from trying it again. The other piece is now in the kids' hermit crab tank.
Oh, and mine was also heat treated, if that matters.