Q. Is it correct to tie the aravos branches for the hoshaanos for Hoshana Rabba with rubber bands or red string? Do they have to be tied at all?

A. Mishna Berura (664: 17) mentions that some maintain the branches do not have to be tied, however Remaz disagrees and rules they should be tied and that is our minhag. Magen Avrohom (ibid. 4) debates whether it is correct to tie them with leaves of an unused lulav, since something done for the purpose of beautifying the object does not constitute a chatzitza. However, Pri Megodim opines that this may apply only to the mitzva of lulav since there is a mitzva to have it tied to the other minim, but not to the arovos and it may thus constitute a chatzitza. Therefore, he and others suggest to tie them with ties made out of the aravos branches themselves.
Piskei Teshuvos (664: 4) writes that the minhag is to tie them with lulav leaves or with other means and keep the tie high so one can hold the branches bellow it. He mentions that the minhag of the Steipler Gaon zt’l was to tie them with a string or a rubber band and hold the branches bellow the tie.
Horav Shlomo Miller’s Shlit’a opinion is similar.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a