I saw this plant in a park in Silistra (Bulgaria) and I don't know what it is. There were people nearby but I don't speak their language, so I couldn't ask. I tried to take a more detailed picture of the flowers, but they were all starting to wilt and not looking well on close-ups.
My mother says her parents had a plant similar to this one. The roots were taken out from soil during winter and stored inside, otherwise they wouldn't have survived the local temperature of -15 degrees Celsius (5 degrees Fahrenheit).
The pictures were taken in late September (beginning of autumn).