Purple starthistle is one of the numerous species of Centaurea that have been accidentally introduced to western North America where most have become pernicious noxious weeds. Purple starthistle is not nearly as widely distributed as its close relative, yellow starthistle, but it is more undesirable because of its growth characteristics and the number, length, and persistence of its spines. Purple starthistle reproduces only by seeds. Knowledge of the seed and seedbed ecology of weeds is important in all suppression strategies and especially for purple starthistle, which has potential biological control agents that suppress seed production. Our purpose was to determine the germination of purple starthistle seeds at a wide range of constant or alternating incubation temperatures from 0 through 40 C. The maximum observed germination ranged from 94 to 100%. Some germination occurred at least 75% or above of the 55 temperature regimes tested. Optimum germination, defined as not less than the maximum observed minus one-half of the confidence interval at the 0.01 level of probability, averaged 88 to 96%. Only one accession of purple starthistle had germination at what we classify as very cold seedbed temperatures. All accessions had near 50% germination at cold seedbed temperatures. Germination was highest at moderate seedbed temperatures and declined at warmer temperatures. The only constant incubation temperature that supported optimum germination was 20 C, and that was for only one accession. The only temperature regime that always supported optimum germination was 15/25 C (15 C for 16 h and 25 C for 8 h in each 24-h period).

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.