In Slavic folklore, which flower is said to bloom only on Midsummer's Eve?

In Slavic folklore, the flower said to bloom only on Midsummer’s Eve is the fern flower.

The fern flower is mythical rather than botanical: true ferns do not produce flowers. Folklore says it appears for only a brief moment during the night of the summer solstice, often at midnight, and that finding it brings happiness, wealth, or special powers such as understanding animal speech.

The legend is especially associated with Kupala Night, known in Poland as Noc Kupały and in Ukraine as Ivan Kupala. Depending on region and calendar, the festival may be observed around June 21–24 or, in some Eastern European traditions, in July. Young people traditionally searched forests for the magical bloom, linking the story with love, fertility, and fortune-telling.

The myth may have been encouraged by plants whose spore-bearing fronds look flower-like, or by flowering plants with fern-shaped leaves. In Polish tradition, adder’s tongue was sometimes identified with the legendary bloom, despite being a real fern relative.