Dendritic morphology was investigated in a directionally solidified magnesium-zinc alloy using synchrotron X-ray tomography and electron backscattered diffraction. Unexpectedly, primary dendrites grew along ⟨213¯¯¯1⟩⟨213¯1⟩ , rather than the previously reported ⟨112¯¯¯0⟩⟨112¯0⟩ and ⟨224¯¯¯5⟩⟨224¯5⟩ directions. Further, seven asymmetric sets of side branches formed, instead of six-fold symmetric arms, evolving with three coexisting morphologies per trunk of: traditional, seaweed structure, and free growth. The anomalous growth is attributed to the imposed thermal gradient and zinc-induced interfacial energy anisotropy variations.