What is a rotten plant for?

[Alice]

Does it do anything? Tried to put it in the recycling machine since it's trash, but it won't accept it.

[three kobolds in a trench coat]

give it to grobus

[ChrillBill]

Not as far as I know. And Krobus, like everyone else, will not appreciate it as a gift.

[Keeper Of Monsters]

The only use for it - for one of Mr. Qi special orders, one with different colored items. There you can submit it as yellow or orange items (not sure atm which one).
Other than that - it's a literal trash.

P.S. Ah yes, forgot: if you are into crafting clothing you can use it in dye pots; but that's it.

[MightyJean]

Originally posted by Keeper Of Monsters:

The only use for it - for one of Mr. Qi special orders, one with different colored items. There you can submit it as yellow or orange items (not sure atm which one).

According to this discussion[stardewcommunitywiki.com] on the wiki it's orange.

[Maya-Neko]

Its main purpose is most likely to just remind you, that you've forgotten to store your jack-o-lanterns properly before winter.

[I Kinda Fail]

as Maya-Neko said, it's a jack-o-lantern that turns to mush at the season change. I think you can put it down for a full season (like Fall 1-Fall 28) but Fall 28-Winter 1 will turn it to mush.

[ChrillBill]

There is also the case with the golden Lewis statue getting replaced with a rotten plant if you place and leave it (the statue) in town. I've heard that the statue can end up in Marnie's room after being removed from town. I guess that way she will never have to feel alone or abandoned by Lewis... o.o