As Iron nights in are (used mostly in the plural) Norway and Sweden , the first frosty nights from about mid-August called, which can destroy a promising harvest and mean the end of the summer.
For example, Knut Hamsun uses the term in his novel Pan at the end of chapter 25.
General economic or agricultural calendar for the year 1770. Johann Benedict Mezler, Stuttgart.
Gyldendal's concise dictionary of Norwegian. Langenscheidt, Berlin / Munich / Zurich 1965, under jernnetter .
Illustrerad Svensk Ordbok, by nature and culture. Stockholm 1970, under järnnatt .