The formation of turions of Spirodela polyrhiza is induced by a large number of environmental signals, already investigated under laboratory conditions. To get more close-to-nature experimental conditions, chemical composition and temperature of the water were measured during the growing season in 2002 and 2003 in a pond near Jena (50°52′N, 11°42′E). Whereas the concentrations of nitrate and sulphate (both in the millimolar range) remained fairly constant that of phosphate decreased from approximately 13 µM at the beginning of the season to 2 µM at the time of onset of turion formation (17 August in 2002, 21 July 2003). This concentration was used in experiments under controlled conditions together with the other outdoor data (day temperature, lower night temperature and photoperiod) in subsequent experiments to investigate their role in the induction of turion formation. The concentration of the nutrient media were kept constant. The following conclusions were drawn. (1) Low phosphate concentration appears to be the decisive factor in inducing turion formation. Growing fronds take up phosphate, and turion formation is then induced towards the end of the season. (2) Lower temperatures during the day (18 vs 25°C) and especially during the night (18 vs 15°C) evidently enhance the effect of the turion-inducing factor phosphate by increasing the yield. (3) At much higher anthropogenic phosphate concentrations low temperature takes over the function of inducing turion formation. (4) Whereas much lower concentrations act directly to induce the formation of turions regardless of the season.