Q. Is it true that some have a minhag of drinking beer on Tu Bishvat?
A. Yes indeed, and it is based on the fact that we eat fruits of the seven species (Shivas Haminim, Devarim 8: 8) on Tu B'shavat, the day that marks the Rosh Hashana for trees. Wheat and barley, belong to that group. (See Nitey Gavriel - Purim 5: 2 he mentions that the Belzer Rebbe maintained that tradition).
Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit'a