Can we throw away a dried up Esrog, or does it have to go with Shaimos?
May be discarded in a respectful manner (such as placing it in a separate bag before putting in the garbage).
Best to use it for another Mitzvah, such as Besamim. Some make out of it jam for Chamisha Asar Bishevat.
Sources:
See Shulchan Aruch Harav (Orach Chayim 42:6). Shulchan Aruch Orach Chayim 154:3; Mishna Berurah 154:7.
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