138:2 Our practice from the prophets is that on Hoshana Rabbah, everyone takes a special arava (willow) aside from the aravos of his lulav. Everything that would invalidate the aravos on the lulav would also invalidate this arava. Therefore, a Jew should not cut it himself for his own use. However, if even a majority of leaves fall off, it is still valid. Even if only one leaf on one stem remains, it is valid. In any event, the mitzvah is best performed when this arava has many leaves and long stems. A good practice is to take five aravos (at the very least, three – Mishnah Brurah 664:16) and to bind them together with lulav leaves.
138:3 One does not take these aravos together with the lulav. Rather, when we come to “Answer Your faithful ones,” we put down the lulav and esrog and we pick up the aravos because now is the time when we are pray for water. After the Hoshanos, we wave it, then we beat it on the ground five times. This is sufficient even if no leaves fell off. (Mishnah Brurah 664:19 says that after beating on the floor, one should beat on the furniture in order to knock the leaves off, though one need not knock off all the leaves.) After beating it, we do not throw it on the ground as this would be considered disrespectful to the mitzvah. It is better to put it aside and later throw it into the fire used for baking the matzah. This way, since it has been used for one mitzvah, it is able to be used for another mitzvah.