Reproductive phenology in yellow starthistle (Centaurea solstitialis)

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Abstract
Exotic annual weeds on western U.S. rangelands, including yellow starthistle, owe at least part of their invasion success to prolific seed production. Occurrence of flowering in relation to soil moisture and development of competing vegetation heavily influences reproductive output. Yellow starthistle flowers much later than associated Mediterranean invaders, which avoid summer drought by early flowering and senescence. This study used successive planting dates and four densities to investigate the influence of temperature, photoperiod, and intraspecific competition on reproductive phenology in a population of yellow starthistle 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, flowering, and achene dispersal stages. A thermal time model adequately predicted phenology in this population of yellow starthistle. Plants emerging between October and July required about 1,240 degree days for 50% of the plants to reach bud stage, and an additional 500 and 900 degree days to flowering and achene dispersal, respectively. The relatively long time from emergence to mature achenes in this species influences management options such as prescribed fire, livestock grazing, and management of competing vegetation. Insensitiviry to photoperiod and lack of a vernalization requirement allow late germinating plants to reproduce if moisture is adequate.