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Montana Field Guides

Spreading Woodfern - Dryopteris expansa
Other Names:  Dryopteris dilatata

Native Species

Global Rank: G5
State Rank: S3S4
(see State Rank Reason below)

Agency Status
MNPS Threat Rank:
C-value: 8

External Links

State Rank Reason (see State Rank above)
Dryopteris expansa occurs in mesic, shady forests from the valley to montane zones within many counties of western Montana. The locations of older and relatively more recent observations suggest that its distribution in western Montana has not changed. Threats have not been identified. Current data on locations, population sizes and viability, and threats are needed.
  • Details on Status Ranking and Review
    Spreading Woodfern (Dryopteris expansa) Conservation Status Review
    Review Date = 10/04/2019
    View State Conservation Rank Criteria
    Range Extent

    ScoreF - 20,000-200,000 sq km (~8,000-80,000 sq mi)

    Comment90,727 sqare kilometers

    Area of Occupancy

    ScoreE - 26-125 4-km2 grid cells

    CommentPlant occurs in 73 of the 30,590 4x4 square-kilometer grid cells that cover Montana.

    Number of Populations

    ScoreD - 81 - 300

    Comment90 observations

    Environmental Specificity

    ScoreB - Narrow. Specialist or community with key requirements common

General Description
PLANTS: Acaulescent (without stems), herbaceous perennials from a short, scaly rhizome that is usually covered with old leaf bases. Source: Lesica et al. 2012.

LEAVES: Fronds appear similar (monomorphic), up to 90 cm long. Petioles about half as long as blade. Blades are green, narrowly triangular, glandular or not, and twice pinnate at the base. Pinnules (leaflets) are deeply pinnately lobed or toothed. The first downward-pointing pinnule on the lowest pinnae is at least twice as long and wide as the opposing upward-pointing pinnule. Source: Lesica et al. 2012.

INDUSIUM: Reniform (kidney-shaped) attached off-center of the sorus. With or without glands. Source: Lesica et al. 2012.

Spores are dispersed over about a 2-month period, from August to October (Runk et al. 2012).

Diagnostic Characteristics
The broadly horseshoe-shaped indusium identifies its members as Dryopteris.

The length ratio of downward- to opposing upward-pointing pinnules on the lowest pinnae separates Spinulose, Spreading, and Male ferns (Giblin et al.[eds] 2018; Lesica et al. 2012):

Spreading Woodfern - Dryopteris expansa, native:
* Leaves 2- or 3-pinnate at base and can be more evergreen.
* Petiole is often about half as long as the blade.
* Leaves are broadly ovate to triangular in outline.
* Leaf blade is about one-half to two-thirds as wide as long (except in very robust plants).
* The first downward-pointing pinnule on the lowest pinnae is at least twice as long and wide as the opposing upward-pointing pinnule.
* Teeth on pinna margins toothed with a spine tip less than 0.03 mm.

Crested Shieldfern - Dryopteris cristata, native, SOC:
* Leaves are 1-pinnate and of two types: erect fertile fronds are taller than the sterile arching, evergreen fronds.
* Petiole is less than half as long as the blade.
* Leaves are narrowly lance-shaped in outline.
* Pinnae are deeply pinnately lobed.

Spinulose Shieldfern - Dryopteris carthusiana, native:
* Leaves 2- or 3-pinnate at base and deciduous.
* Petiole is often shorter than the blade.
* Leaves are broadly ovate to triangular in outline.
* Leaf bladed about one-half to two-thirds as wide as long (except in very robust plants).
* The first downward-pointing pinnule on the lowest pinnae is less than twice as long and wide as the opposing upward-pointing pinnule.
* Teeth on pinna margins with a prominent spine tip about 0.05 mm long.
* Leaves deciduous.

Male Fern - Dryopteris filix-mas, native:
* Leaves are 1- to 2- pinnate at the base and deciduous.
* Petiole is about one-third or less the length of the blade.
* Leaves are narrowly elliptic or lance-shaped in outline.
* Downward- and upward-pointing pinnules are about equal in length and width.

Species Range

Range Comments
Spreading Woodfern occurs around the world at northern latitudes. Within western North America, it is found mostly along the coast from Alaska to California. It is also found in the interior in Canada, Montana, Idaho, northeast Washington, and rarely Wyoming and Colorado. It occurs in some northeastern states and eastern Canada (Munger 2007).

Observations in Montana Natural Heritage Program Database
Number of Observations: 91

(Click on the following maps and charts to see full sized version) Map Help and Descriptions
Relative Density



(Observations spanning multiple months or years are excluded from time charts)

In Montana, Spreading Woodfern is found in mesic, shady forests, often along streams and beneath shrubs within the valleys to montane zones (Lesica et al. 2012). It has been associated with sphagnum bogs in Glacier National Park (Munger 2007). Across its range, Spreading Woodfern is found in cold areas because rhizomes can survive temperatures as low as -22 degrees Fahrenheit (Runk et al. 2012).

FOREST SUCCESSION [Adapted from Runk et al. 2012]
Spreading Woodfern generally increases in abundance after disturbances from logging or large-scale windthrow, especially where tree and shrub regeneration is sparse. Around 20-25 years after disturbance ferns decline as shrubs and tree saplings form a dense canopy. Around 50-60 years after disturbance ferns once again begin to dominate the understory. Around 250 years after disturbance, the final stages of forest succession create an understory diverse with forbs, shrubs, and tree seedlings, yet with a decrease in ferns.

Spinulose Shieldfern has been found to support arbuscular mycorrhizae to varying degrees (Runk et al. 2012).

ANIMAL ECOLOGY [Adapted from Runk et al. 2012]
Spreading Woodfern is a minor component of the winter diet for Blue Grouse. The fern was eaten in small quantities by Mountain Goats on their winter range in southeast Alaska.

CULTURAL [Adapted from Munger 2007]
Northern Coastal people used species of Dryopteris as food. In the spring young fronds are eaten. In the fall rhizomes and stalk bases are collected to steam or roast. Roots pounded into a paste were applied to cuts. Roots, which are flammable, were used as a slow match. Fronds soaked for several days were used as a hair wash.

Reproductive Characteristics
Spreading Woodfern reproduces by spores, rhizomes, and sporophyte crowns (Runk et al. 2012). Reproduction can produce male-only plants, female-only plants, and bisexual plants. Spores can be produced by self-pollination or cross-pollination (Munger 2007).

Spreading Woodfern can hybridize with five other species of Dryopteris, including D. intermedia and D. carthusiana (Montgomery and Wagner Jr. in FNA 1993); Runk et al. 2012). Spreading Woodfern is a diploid, which means it has two homologous sets of chromosomes per cell.

LIFE CYCLE [Adapted from Runk et al. 2012]
Spores begin to shed from sori on mature fronds in summer. Spore color can be pale or straw-colored to reddish amber brown. Spore shapes are ovoid to bean-shaped. Spore dispersal lasts for about two months, from August to October. The spores then germinate and grow a long multi-cellular filament that extends into the soil. The gametophyte, given favorable conditions, then grows further, becoming flat and heart-shaped. Male reproductive organs (antheridia) are developed first in the gametophyte. About 9 weeks after the spores are sown, female reproductive organs (archegonia) form. Eggs within the archegonia are then fertilized and new sporophytes are produced. Fronds of the sporophytes appear approximately 28 weeks after spores are sown. In approximately another 14 weeks these fronds develop a species specific appearance and can be identified.

Depending on when spores are sown, sporophytes may be produced in the autumn or spring. If sporophytes are produced in autumn, they overwinter attached to the gametophyte. Spores can infrequently stay attached through the winter and germinate as late a March. Fronds mature by June.

Spores primarily disperse by wind, but also by water. A Spreading Woodfern plant can produce 150 million spores over its life time. Most of these spores are deposited within three meters of the source plant.

The U.S. Forest Service, Eastern Region (9), prepared the "Conservation Assessment for Spreading Wood Fern (Dryopteris expansa) C. Presl Fraser-Jenkins & Jermy" to compile published and unpublished information (USFS 2003).

  • Literature Cited AboveLegend:   View Online Publication
    • Flora of North America Editorial Committee. 1993. Flora of North America north of Mexico. Vol. 2. Pteridophytes and gymnosperms. Oxford Univ. Press, New York. xvi + 475 pp.
    • Lesica, P., M.T. Lavin, and P.F. Stickney. 2012. Manual of Montana Vascular Plants. Fort Worth, TX: BRIT Press. viii + 771 p.
    • Munger, Gregory. 2007. Dryopteris spp. In: Fire Effects Information System, [Online]. U.S. Department of Agriculture, Forest Service, Rocky Mountain Research Station, Fire Sciences Laboratory (Producer). Access on January 29, 2019. Available at
    • Runk, K., M. Zobel, and K. Zobel. 2012. Biological Flora of the British Isles: Dryopteris carthusiana, D. dilatata, and D. expansa. Journal of Ecology, Volume 100(4), pp. 1039-1063.
    • U.S. Forest Service. 2003. Conservation Assessment for Spreading Wood Fern (Dryopteris expansa) C. Presl Fraser-Jenkins & Jermy. February.
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Spreading Woodfern — Dryopteris expansa.  Montana Field Guide.  .  Retrieved on , from