Analysis of 38 tree ring chronologies yields evidence for enhanced drought conditions every 18.6 years in the North American interior for the past millenium. Cross spectra between a drought area index and 26 temperature records confirm the analysis of Currie (1979, 1981d) as regards a lunar nodal MN term in these data. Discussion of the MN tidal constituent, which appreciably modulate amplitude and phase of four main constituents (M2, K1, O1, Mf), is presented, and implications for agriculture are surveyed.