These characteristics, as well as a deep root system, allow yellow starthistle to thrive under full sunlight in hot and dry conditions. Vigorous shoot growth coincides with increased light availability due to senescence and desiccation of neighboring annual species. Moreover, the presence of spines on the bracts surrounding the seedhead provides protection against herbivory. This is particularly important during the vulnerable flowering and seed development stages. Senescence typically occurs in fall when moisture becomes limiting and plants are exposed to frost.
Flowers can abort development before completion.
Because the receptacle of the flowerheads contain an abundant amount of fine chaff , the seedheads will have a cotton-tip appearance . In contrast, tocalote and sulfur starthistle do not have cotton-tip seedheads after senescence. Stems of yellow starthistle degrade slowly and may remain erect for at least one year. In addition, both tocalote and sulfur starthistle have only pappus-bearing seeds that disperse once the seedhead has lost its senescenced flowers. Light In grasslands where yellow starthistle rosettes are exposed to low light, the leaves are larger and develop a more vertical or erect growth form that may reach 10 inches in height.