Does an aravah need to be hadar?
The Mishnah disqualifies all four species if they are dry. The Bartenura ascribes this to the fact that Hadar extends to all of them. The Tosfos Yom Tov adds that, based on the Gemara on 31a, Lulav, Hadas and Aravah are compared to Esrog.
be'Virkas kol tuv
According to the gemara on 31a that lulav and esrog and haddas are the same, on 34b rashi on ר טרפון אומר שלוש קטומים rashi says that haddas doesn't require hadar.
The Rashi that you quote only says that Rebbi Tarfon does not require Hadar by Hadas, implying that the Rabanan do. The problem is that the Halachah is like Rebbi Tarfon, in which case the Mishnah -- "Niktam Rosho" of Hadas -- is not the Halachah, which is indeed how the Rif rules, and presumably the same will apply to "Hadas ha'Yavesh." See Meleches Shlomo (Mishnah 2 and 4) who, citing the Ran, questions the Rif's ruling, based on the fact that "Sheloshtan Ketumin" is not synonymos with "Niktam Rosho." Consequently, even according to Rebbi Tarfon, the Hadas is subject to Hadar no less than the other three species, like the Mishnah on Daf 31a says. And the "Ketumin" of Rebbi Tarfon, the way he explains it, is Hadar.
B'Virchas Kol Tuv,