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).
Nomenclature: Purple starthistle, Centaurea calcitrapa L. #3 CENCA; yellow starthistle, Centaurea solstitialis L.
Additional index words: Invasive weeds, seed and seedbed ecology.