White horehound (Marrubium vulgare L. # MAQVU) is one of the few perennial, herbaceous alien weeds to successfully colonize semiarid to arid rangelands in the Great Basin. Seeds of white horehound, collected from diverse habitats within the Lahontan Basin of northwestern Nevada, were incubated at constant or alternating temperatures ranging from 0 to 40 C. Germination was low and largely limited to alternating temperatures (10 to 25 C alternating with 35 to 40 C). At least a 15-C range in diurnal fluctuation was required for optimum germination (defined as not lower than the maximum observed minus one-half of the confidence interval with 1% probability). Cool-moist stratification at 2 C for 4 to 8 weeks enhanced germination at optimum temperature regimes by as much as 70% and increased overall average germination by 35 to 40%. Cool-moist stratification caused a shift in temperature optima for germination from widely fluctuating temperatures to constant 15 or 20 C. The magnitude of this shift varied among sources of seeds.