Two sets of freezing temperature measurements were conducted using mini-thermocouples on different batches of turions at the state of imposed dormancy for two seasons. From the 2nd to the 13th March 2012, the measurements were conducted on turions of all 10 species that had overwintered in a refrigerator at 2.5 ± 1 °C. In Aldrovanda, U. australis, U. bremii and U. ochroleuca, the measurements were also made on turions which had been germinated in a growth chamber at 20 ± 1 °C with a 12 h photoperiod for 1–6 d before the measurements. Turions of the former three species, which germinated for 2–5 d, bore distinct symptoms of germination (erected leaflets). These turions were never subject to frost and their overwintering mimics the natural, underwater conditions at the bottom of water bodies. The second set of measurements was conducted during the 7th–8th March 2013 on the turions of 6 species that had overwintered on a wet topsoil in an empty cultivation container outdoors (see below). This type of overwintering mimics the natural situation when autumnal turions are entangled in the wet organic substrate above the water surface and are thus subject to winter frosts ( Adamec, 1999a). This can induce turion frost hardiness.