Osmunda claytoniana

Interrupted Fern

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

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

Name:

• 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)

Taxonomy:

• 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.

Description:

• 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.

Identification:

• 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

Distribution:

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

Habitat:

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

Fire:

Associates:

History:

Uses:

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

Reproduction:

• By spores and vegetatively by rhizome

Propagation:

• By rhizome division,

Cultivation:

• 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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