Osmunda regalis, or royal fern, is a species of deciduous fern, native to Europe, Africa and Asia, growing in woodland bogs and on the banks of streams. The species is sometimes known as flowering fern due to the appearance of its fertile fronds.
The name Osmunda possibly derives from Osmunder, a Saxon name for the god Thor. The name "royal fern" derives from its being one of the largest and most imposing European ferns. The name has been qualified as "old world royal fern" in some American literature to distinguish it from the closely related American royal fern, O. spectabilis. However this terminology is not found in British literature.
Osmunda regalis produces separate fertile and sterile fronds. The sterile fronds are spreading, 60–160 cm (24–63 in) tall and 30–40 cm (12–16 in) broad, bipinnate, with 7-9 pairs of pinnae up to 30 cm (12 in) long, each pinna with 7-13 pairs of pinnules 2.5-6.5 cm long and 1-2 cm broad. The fertile fronds are erect and shorter, 20-50 cm tall, usually with 2-3 pairs of sterile pinnae at the base, and 7-14 pairs of fertile pinnae above bearing the densely clustered sporangia.
In many areas, O. regalis has become rare as a result of wetland drainage for agriculture.