In the past, this was only really testable with blighted Cotten. There are about 20-30 different ones. If you keeps picking them up and putting them in a bag, eventually you'll get some of the same 'type' (not item type, just type).
I have a bag of about 50 I use for deco. I've always suspected that some others will stack, but I've never had enough of them to find a duplicate.
If something has changed, this would be epic. I've always thought they should be farmable anyway...
EDIT: oh crap, I just realized what you meant. I have no idea, but that would be EPIC (as opposed to merely 'epic')