I have seen in many Shuls on Hoshana Raba after using the Hoshanos people throw the used Hoshanos on top of the Aron Kodesh. What is the reason and source for this Minhag?
Nitey Gavriel (Sukkos pg. 379 footnote 15) brings the custom in the name of the Malbushei Yom Tov to Levush 664:4 and Nitzutzei Zohar Parshas Tzav, who explain that it is in order to leave the sparks of judgment behind at the conclusion of the days of judgment and not take them back home. The custom is also brought in the Bikurey Yaakov (S"K 16). The Nitey Gavriel himself writes that he heard the reason to commemorate the arovos in the Beis Hamikdosh that they would place on the mizbeach.