Osmunda claytoniana

Interrupted Fern

Interrupted Fern, Jap Lake, BWCAW. Photo copyright 2001 by Earl J.S. Rook
Interrupted Fern,
Jap Lake, BWCAW
Photo © 2001 by Earl J.S. Rook

Flora, fauna, earth, and sky...
The natural history of the northwoods


  • Osmunda, from the Saxon god Osmunder the Waterman, the Saxon equivalent of the Norse god Thor, who hid his family from danger in a clump of these ferns
  • claytoniana, from the Latin "of Clayton"
  • Common Name from the brown fertile leaflets, which "interrupt" the green sterile leaflets on some fronds
  • Other common names include Clayton's Fern, Osmonde de Clayton (Qué), Koningsvaren (NL), oni-zenmai (Jpn)


  • Kingdom Plantae, the Plants
    • Division Polypodiophyta, the True Ferns
      • Class Filicopsida
        • Order Polypodiales
          • Family Osmundaceae, the Flowering Ferns (an oxymoron, of course)
            • Genus Osmunda, the Flowering Ferns
  • Taxonomic Serial Number: 17220
  • Osmunda ferns form spores on a modified frond. For this species, spore-bearing leaflets grow in the middle of the sterile fronds.


  • A large, robust, deciduous fern.
  • Fronds twice-cut or nearly so, 15"-40" tall
    • Petiole (leaf stalk) roughly 1/3 length of blade, winged, with light brown hairs, becoming smooth.
    • Sterile Fronds elliptic to oblong, pinnae broadly oblong, lacking persistent tuft of hairs at base; subdivided into nearly opposite, smooth edge, round-tipped, lobes.
    • Fertile Fronds with 2-4 pairs of greatly reduced, sporangia-bearing pinnae that wither early, giving appearance of no middle pinnae (hence, "interrupted" fern).
  • Rootstalk stout, creeping
    • Roots black, tangled, and thickly matted
    • Fiddleheads stout, brown, and very wooly; among the earliest to emerge in spring.
  • Sporangia greenish, turning dark brown.


  • Unmistakeable when the "gap-toothed" effect of the fertile leaflets is present.
  • Distinguished from other ferns by its large size and the spore bearing leaflets growing in the middle of the twice cut sterile fronds.
  • Field Marks
    • the largest, tallest fern clumps in the North Woods
    • the frond "interuption" created by the fertile pinnae


  • Manitoba to Newfoundland, south to Missouri, Kentucky, and North Carolina
  • Also Russia, Korea, China, Taiwan, and Japan


  • Open woodlands, damp fields, shaded roadsides
  • Rich, often alluvial, soils; neutral to mildly acidic.





  • Roots and rhizomes of Osmunda sp. provide the fibre osmundine, used as a growing medium for orchids and other epiphytes.


  • By spores and vegetatively by rhizome


  • By rhizome division,


  • Hardy to USDA Zone 3 (average minimum annual temperature -40ºF)
  • Cultural Requirements
    • Shade to part shade; full sun if constantly moist
    • Soil pH 4-6
  • Good for woodland and foundation plantings; background for more colorful, flowering plants. Leave plenty of room.
  • Available by mail order from specialty suppliers.



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Last Updated on 26 February, 2004