Purple loosestrife is an invasive, exotic perennial weed of wetlands throughout much of the United States and Canada. Once established, it forms dense colonies to the near exclusion of more desirable vegetation. Established plants are prolific seed producers. Our purpose was to investigate the germination of purple loosestrife seeds at a wide range of constant or alternating temperatures from 0 through 40 C. Purple loosestrife seeds germinate over a wide range of temperatures. We define optimum germination as not less than the maximum observed minus its confidence interval at the 0.01 level of probability. Optimum germination occurred over a considerable range of temperatures: Only 10, 15, or 20 C for 16 h alternating in each 24 h with 35 C for 8 h always supported optimum germination. Wide ranges in diurnal temperature fluctuations were conducive to maximum germination. These ranged from a maximum of 35 degrees with 0/35 C to 5 degrees with 30/35 C. No one constant temperature supported optimum germination.
