REPRODUCTIVE PHENOLOGY IN YELLOW STARTHISTLE (CENTAUREA-SOLSTITIALIS)

Exotic annual weeds on western U.S. rangelands, including yellow start histle, owe at least part of their invasion success to prolific seed p roduction. Occurrence of flowering in relation to soil moisture and de velopment of competing vegetation heavily influences reproductive outp ut. Yellow starthistle flowers much later than associated Mediterranea n invaders, which avoid summer drought: by early flowering and senesce nce. This study used successive planting dates and four densities to i nvestigate the influence of temperature, photoperiod, and intraspecifi c competition on reproductive phenology in a population of yellow star thistle from Lewiston, ID. A base temperature for seed germination of 2 C was determined by experiments at constant temperatures. Under the conditions of this study, density, photoperiod, and vernalization did not regulate reproductive development, as measured by onset of bud, fl owering, and achene dispersal stages. A thermal time model adequately predicted phenology in this population of yellow starthistle. Plants e merging between October and July required about 1,240 degree days for 50% of the plants to reach bud stage, and an additional 500 and 300 de gree days to flowering and achene dispersal, respectively. The relativ ely long time from emergence to mature achenes in this species influen ces management options such as prescribed fire, livestock grazing, and management of competing vegetation. Insensitivity to photoperiod and lack of a vernalization requirement allow late germinating plants to r eproduce if moisture is adequate.