One of the most critical phases in the life cycle of plants encompasses early stages, since success in seed dormancy overcoming and germination as well as in seedling establishment represents a key process filtering the participation of species in plant communities. In this study, the requirements for dormancy breaking and germination of seeds of Helleborus foetidus were analyzed. The seeds were dormant with underdeveloped embryos (0.54 mm) at the time of dispersal and radicle emergence did not occur until embryos reached full size (3.6 mm). The light–temperature requirements for embryo growth and radicle emergence were studied correlating experiments under near-natural conditions with others under laboratory-controlled conditions. The embryos completed their growth and the radicle emerged when seeds were warm → cool stratified in darkness under a thermal sequence meeting late summer to early autumn temperature conditions. Conversely, cold stratified seeds did not germinate at winter temperatures (5 °C). The highest seed germination response (i.e., 75%) occurred in 6-month-old seeds incubated at 15/4 °C in darkness after being warm + cool stratified for 3 months (i.e., 32/18 °C for 1 month + 28/14 °C for 1 month + 25/10 °C for 1 month). In outdoor conditions, the embryo grew during late summer–autumn and the radicle emerged in late autumn, but the shoot did not emerge immediately because it was physiologically dormant too, so it required cold winter temperatures for dormancy breaking. Seedling establishment commenced at late winter–early spring, which is a more suitable season for seedling survival. In conclusion, the seeds of Helleborus foetidus exhibit deep simple epicotyl morphophysiological dormancy (MPD), which is ecologically well adapted to temperate forest regions. This is the second report of this level of MPD in Ranunculaceae.
