Word Origin & History
O.E. beorc (also the name of the rune for "b"), from P.Gmc. *berkjon (O.S. birka, O.N. börk, Swed. björk, Du. berk, Ger. Birke), from PIE *bhergo (cf. Ossetian barz, O.C.S. breza, Rus. bereza, Lith. beras, Skt. bhurjah, L. farnus, fraxinus "mountain ash"), from base *bhereg- "to gleam, white."
Meaning "bunch of birch twigs used for flogging" (1640s) led to verb meaning "to flog" (1830). Birch beer is 1883, Amer.Eng.