Yellow starthistle is an annual that is dependent on achene production, dispersal, and germination for stand renewal. Our purpose in this study was to determine the temperature relations for germination of achenes of this species. Germination temperature profiles were developed for achenes of yellow starthistle collected from 15 sites in California, Nevada, and Oregon. Each profile consisted of achene germination at 55 constant or alternating temperatures from 0 through 40 C. A total of 85 germination temperature profiles were developed by using the germination data to construct quadratic response surfaces through regression analysis. For most profiles, germination occurred at all the temperature regimes except a constant 40 C. This includes a constant 0 C and 0 alternating with 40 C. Rarely, there was no germination at 35 C and 35 C alternating with 40 C. The only evidence of afterripening requirements for achenes of yellow starthistle that we noted occurred at very cold temperature regimes. At those temperatures, the germination of dark-colored achenes without pappus increased 3 mo after harvest, and decreased for light-colored achenes with a pappus. No single temperature regime always supported optimum germination when all the profiles were combined. The most frequent optima was 2/20 C. Comparing all profiles for the Davis, CA, accession, there were 5 regimes (5 and 10 C cold periods alternating with 15 through 25 C warm periods) that always supported optimum germination. Light-colored achenes with pappus tended to have optimal germination at colder temperatures, and the dark-colored achenes at higher temperatures when seeds were tested immediately after harvest.
Nomenclature: Alfalfa, Medicago sativa L. #3 MEDSA; tocalote, Centaurea melitensis L.; yellow starthistle, Centaurea solstitialis L. # CENSO.
Additional index words: Seedbed ecology, seedbed temperatures, optimum germination, invasive species.