The frequency of plume formation in fully-developed thermal convection is determined experimentally. Because the fluid has a temperature-dependent viscosity, the cold and hot thermal boundary layers have different thicknesses and viscosities. As a result, plumes are released from these layers with different frequencies. There appears to be a direct temporal interaction between the cold and hot boundary layers. In particular, whereas there is a single frequency for cold plume formation, hot plumes form with multiple frequencies, one of which is the frequency of cold plumes. The measurements thus suggest that cold sinking ``plumes'' in the mantle can induce hot rising plumes.