aaron's rod - Aaron's Rod; aaron's rod - Architectural ornament, consisting of a straight piece of rounded section moulded with leafage or scroll-work. In botany Aaron's Rod is a name for the common mullein (Verbascum thapsus). The name is taken from the O.T. (Num. 17), where Aaron's rod, taken from an almond-tree, is said to have budded. It is also popularly applied to various other tall plants which have tapering inflorescence.