– but it has never been studied. Moreover, natural frost temperatures, to which turions overwintering above water on the wet organic topsoil can be subjected in the temperate zone, have never been monitored. In our study during the 2012–2013 winter season, the minimum air screen temperature measured near Třeboň (ca. 49° N, 15° E), as meteorological standard at 2 m above soil surface, was as low as −15.5 °C (Dušek et al., unpubl. data), while a minimum of only −4.2 °C, even without snow cover, was monitored on the wet topsoil in our outdoor experiment. For 75 days, the topsoil temperature fell below freezing and could thus induce frost hardiness in the turions, while the control turions in the experiment overwintered at above freezing temperatures in a refrigerator that simulated natural conditions under water at the bottom of a lake or a river.