Dryopteris expansa

Spreading Wood Fern

Spreading Wood Fern, Photo courtesy Wisconsin State Herbarium and Sue R. Crispin
Spreading Wood Fern
Photo courtesy Wisconsin State Herbarium
and Sue R. Crispin

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


  • Dryopteris, from the Greek, drus (drys), "oak" and pteris (pteris), "fern", "fern of the oak wood"
  • expansa, from the Latin expando,"to spread out, spread apart, to expand"
  • Common name from its spreading growth habit, as in photo above.
  • Other common names include Arching Wood Fern, Crested Wood Fern, Northern Spreading Wood Fern, Northern Wood Fern, Northwestern Spreading Wood Fern, Dryoptère Dressée (Qué), Northern Buckler Fern (UK), Nordbräken, Nordlig Lundbräken (Swe), Sauetelg (Nor), Finbladet Mangeløv (Dan), Isoalvejuuri (Fin), Feingliedriger Dornfarn (Ger)


  • Kingdom Plantae, the Plants
    • Division Polypodiophyta, the True Ferns
      • Class Filicopsida
        • Order Polypodiales
          • Family Dryopteridaceae, the Wood Ferns
            • Genus Dryopteris, the Wood Ferns
  • Taxonomic Serial Number: 17534
  • Also known as Aspidium spinulosum var. americanum, Dryopteris assimilis, Dryopteris dilatata subsp. americana, Dryopteris spinulosa var. americana, Dryopteris spinulosa var. dilatata, Nephrodium expansum, Thelypteris spinulosa


  • A large, lacy, woodland fern.
  • Fronds monomorphic, slow to die back in winter, to 12"× 36".
    • Petiole (leaf stalk) 1/3 length of leaf, scaly at least at base; scales scattered, brown with dark brown stripe.
    • Blade green, deltate-ovate, thrice-cut and lacy.
    • Pinnae (primary leaflets) more-or-less in plane of blade, lanceolate-oblong; basal pinnae triangular and slightly reduced in size.
    • Pinnules (secondary leaflets) with toothed edge; lowest pair equal to or longer than adjacent pinnules, basal basiscopic pinnule (lowest downward pointing subleaflet on lowest pinnae) longer than basal acroscopic pinnule (lowest upward pointing subleaflet on lowest pinnae).
  • Rootstalk erect or ascending, producing offshoots.
    • Roots black, wiry, and widely spreading; highly variable.
  • Sori midway between midvein and margin of segments.


  • Identifiable as a Wood Fern by its larger size, thrice-cut fronds, and woodland habitat.
  • Distinguished from the closely related and nearly identical Intermediate Wood Fern (Dryopteris intermedia) and Spinulose Woodfern (Dryopteris carthusiana), with which it once shared the common species designation of Dryopteris spinulosa, by the first downward pointing secondary leaflet (basiscopic pinnule) being longer than the downward pointing subleaflet next to it AND that first large downward pointing subleaflet is attached at a point much closer to the second upward pointing subleaflet than the first.
  • Field Marks
    • thrice-cut, lacy fronds
    • relative lengths and orientation of pinnules (subleaflets) on lowest pair of pinnae (leaflets)


  • Alaska to Newfoundland and Greenland, south to California, Idaho, Wyoming, Minnesota, Wisconsin, and Michigan.
  • Also northern Asia, Europe.


  • Cool moist woods and rocky slopes.






  • By spore and vegetatively by rhizome.


  • By rhizome division.


  • Hardy to USDA Zone 3 (average minimum annual temperature -40ºF)
  • Occasionally available by mail order from specialty supplier.



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Last Updated on 26 February, 2004