Using the Willow of the Four Species for the Ritual of Beating the Willow
Question
Is it permitted to use the willow from the Four Species for the Hoshanot ritual, on the grounds that it is being used for a mitzvah, or might this be considered disrespectful to the Four Species that were used to perform a mitzvah?
Answer
Peace and joyous holidays,
From a halachic standpoint, it is permitted to use the willows from the lulav for the custom of beating the willow. However, I have not found anyone who would write that there is an advantage in this, according to the principle 'if it was used to perform one mitzvah, it should be used to perform another mitzvah' [perhaps because beating a willow is not considered a mitzvah, but rather a custom instituted by the prophets].
Source
Shulchan Aruch, Orach Chayim, 664