How to use this key. Read each of the major sections (I-V) in order to see which section your fern fits into. If your fern doesn't fit, go to the next section or couplet. More information on key usage at the bottom of the page.
Uncommon or rare species that key here: Adiantum aleuticum, Adiantum viridimontanum, Azolla cristata, Botrychium minganense, Botrychium neolunaria, Botrychium pallidum, Botrychium tenebrosum, Marsilea quadrifolia.
1B - Blade horseshoe shaped, lax to somewhat arching - Northern Maidenhair Fern Adiantum pedatum
1A - Fertile leaf always present and above sterile leaf on same stalk; Semi succulent; Pinnae fan shaped or linear pinnate - 2 (Moonworts - are notoriously variable and difficult to ID to species, a technical key should be consulted)
2B - Sterile lowest pinnule fan-veined; Lowest pair of lobes noticeably larger and more complex than the next pair; Lobes varying from oblong to fan-like; Stalk of leaf attached near base of plant (sometimes toward mid-height); Plants primarily of open areas such as fields, shores, and banks - Least Moonwort Botrychium simplex
2A - Sterile lowest pinna pinnate-veined; Pinnae linear to narrow-ovate, the apex obtuse to acuminate - 3
3B - Sterile blade equilateral, parallel to ground - Narrow Triangle Moonwort Botrychium angustisegmentum
3A - Sterile blade elongated, ascending - Daisy-Leaved Moonwort Botrychium matricariifolium
Uncommon or rare species that key here: Asplenium rhizophyllum, Ophioglossum pusillum
Uncommon or rare species that key here: Sceptridium oneidense
10C - Stipe grooved; Coarse pinnae; Frond usually large - more than 18 inches - Bracken Fern Pteridium aquilinum
10B - Stipe dark, thin, wiry; Frond typically less than 12 inches - Northern Oak Fern Gymnocarpium dryopteris
10A - Stipe fleshy; fertile branch above leaf - 11
11C - Leaves sessile with basal leaf sheath open; More than 12 inches tall - Rattlesnake Fern Botrypus virginianus
11B - Terminal pinnule significantly larger than lateral pinnules; Pinnules variable - lanceolate to trullate, denticulate to lacerate, acute to acuminate at apex - Cutleaf Grapefern Botrychium dissectum
11A - Terminal pinnule the same as or only slightly larger than lateral pinnules; Lobes rounded to obtusely pointed, entire to weakly crenulate; leathery - Leathery Grapefern Botrychium multifidum
Uncommon or rare species that key here: Asplenium platyneuron, Asplenium viride.
20D Blade at least somewhat webbed, not fully divided - 21
21B - Nearly once cut though may have deep lobes on pinnae; Pinna margins wavy - Sensitive Fern Onoclea sensibilis
21A - Nearly twice cut; Webbed between at least the middle pinnae; Bottom pinnae reflexed to stipe - 38D Beech Fern Phegopteris sp
20C - Stipe green with many coarse brown scales; Pinnules with auricles - Christmas Fern Polystichum acrostichoides
20B - Stipe green mostly smooth; On rocks - 22
22B - Leaf blade averaging 5.8 cm wide (range of 3.2-8.2 cm); Blade widest at the base, thus the blade elongate-deltoid in outline; Rhizome scales averaging 1.1 mm wide, mostly golden brown throughout; Sporangiasters usually > 40 per sorus (range of 25-120); Leaves mostly lobed to apex, without an attenuate, unlobed tip - Appalachian Polypody Polypodium appalachianum
22A - Leaf blade averaging 4.5 cm wide (range of 3.0-5.8 cm); Blade widest near the middle, thus the blade oblong to narrowly lanceolate in outline; Rhizome scales averaging 1.5 mm wide, mostly brown, with a dark central stripe; Sporangiasters usually < 40 per sorus (range of 7-69); Leaves mostly with an attenuate, unlobed tip - Rock Polypody Polypodium virginianum
20A - Stipe dark, thin and wiry; Pinna rounded; Usually on rock - Maidenhair Spleenwort Asplenium trichomanes
Uncommon or rare species that key here: Cryptogramma stelleri, Dryopteris filix-mas, Dryopteris goldiana, Physematium obtusa, Woodsia alpina, Woodsia glabella.
30D - Frond double tapered (short stipe and 3 or more smaller basal pinnae) - 31
31D - Blade widest above middle; Often more than 24 inches tall; Rachis smooth with deep groove - Ostrich Fern Matteuccia struthiopteris
31C - Blade widest near middle; Brown scales on stipe - 32
32B - Blade widest near middle; Brown scales on stipe - Braun's Holly Fern Polystichum braunii
32B - Growing exclusively on rock; Small; Frond covered in fragrant glandular hairs; dead curled leaves persisting at base for several years; Uncommon - Fragrant Wood Fern Dryopteris fragrans
31B - Blade widest near middle; No scales on stipe - New York Fern Amauropelta noveboracensis
31A - Blade widest below middle; Sparse, narrow, nearly black scales on stipe; Sori comma-shaped - Northern Lady Fern Athyrium angustum
30C - Stipe with hairs only - no scales - 33
33D - Stipe woolly in spring becoming smooth in summer - 35A
33C - Profuse hairs on blade turning rust colored in summer; Usually plant of dry rock - Rusty Woodsia Woodsia ilvensis
33B - Long white hairs on stipe - Silvery Spleenwort Deparia acrostichoides
33A - Sparse white hairs on stipe; Usually more than 15 inches; Basal pinnae cut 2.5X or more; Scattered growth often forming large patches; Sori in 'cups' - Hayscented Fern Sitobolium punctilobulum
30B - Stipe smooth - 34
34B - Growing in groups or crowns of usually more than 4 fronds - 35
35B - Blade fully bipinnate; Pinnules entire, oval and petioled - Royal Fern Osmunda spectabilis
35A - Stipe green or pastel-colored; Usually woolly esp. in spring - 36
36B - Blade, pinna apex acute; Fuzz on pinnae axil underside - Cinnamon Fern Osmundastrum cinnamomeum
36A - Blade, pinna apex blunt - Interrupted Fern Claytosmunda claytoniana
34A - Fronds usually growing individually (occasionally in groups of only 2 or 3) - 37
37C - Stipe dark, bulbous at base; Plants of wet ground; Usually more than 15 inches; Not in crowns or clumps - Virginia Chainfern Woodwardia virginica
37B - Brown hairs on costa; Plants of wet ground; Sterile veins forked; Stipe nearly as long at the blade and dark toward base - Eastern Marsh Fern Thelypteris palustris
37A - Usually more than 15 inches; Basal pinnae cut 2.5X or more; Scattered growth usually forming large patches; Sori in 'cups' - Hayscented Fern Sitobolium punctilobulum
30A - Stipe with scales - sparse or profuse - 38
38D - Rachis webbed at least between the middle pinnae; Bottom pinnae reflexed weakly to stongly toward stipe - 39
39C - Rachis webbed including between 1st and 2nd pinnae; Bottom pinnae with length to width ratio 2-3:1 - Broad Beech Fern Phegopteris hexanoptera
39B - Rachis webbing does not extend to the lowermost pinnae; Bottom pinnae strongly reflexed to stipe, with length to width ratio 4:1 and often less - Long Beech Fern Phegopteris connectilis
39A - Rachis webbing does not extend to the lowermost pinnae; Bottom pinnae with length to width ratio 5:1 or greater - Tall Beech Fern Phegopteris excelsior
38C - Stipe with scattered long, narrow, nearly black scales; 2 or more basal pinnae shorter than those above; Pinnae symmetrical; Sori comma-shaped - Northern Lady Fern Athyrium angustum
38B - Stipe with coarse brown scales; Fronds usually in a distinct crown - 40
40C - Growing exclusively on rock; Small; Frond covered in fragrant glandular hairs; dead curled leaves persisting at base for several years; Uncommon - Fragrant Wood Fern Dryopteris fragrans
40B - Blade somewhat narrow with little to no taper except near tip; Basal pinna nearly equilateral - 41
41B - Fertile pinnae twisted perpendicular to plane of blade; Pinnules lacking bristle tips - Crested Woodfern Dryopteris cristata
41A - Pinnules bristle tipped; Rachis with glandular hairs - Boott's Woodfern Dryopteris ×boottii
40A - Blade at least fully bipinnate - 42
42B - Basal pinna isosceles; Fertile blade nearly planar - Clinton's Wood Fern Dryopteris clintoniana
42A - Blade and pinnae triangular; Basal pinna asymmetrical; Scales dense; Sori marginal - Marginal Wood Fern Dryopteris marginalis
38A - Stipe green to straw with scattered tan scales - 43
43B - Blade glabrous; Usually on rocks; Basal pinnae often cut more than 2X - 44 (these 2 species often hybridize)
44B - Margins of pinnae with sharp teeth; Basal pinnules sessile or nearly so; Pinnae typically perpendicular to rachis, not curving toward blade apex; Dark pigment confined to petiole base - Fragile Fern Cystopteris fragilis
44A - Margins of pinnae usually with rounded teeth; Basal pinnules short stalked; Pinnae typically at acute angle to rachis, often curving toward blade apex; Dark pigment of petiole base usually extending up to base of blade - Upland Fragile Fern Cystopteris tenuis
43A - Hairs on blade - 45
45B - White glandular hairs; Bulblets; Blade narrow, to 3 feet long; Usually on rocks - Bulblet Fern Cystopteris bulbifera
45A - Brown hairs on costa; Plants of wet ground; Semi-tapered toward stipe; Lowest pinnae reflexed to stipe; Veins not forked - Bog Fern Coryphoteris simulata
Uncommon or rare species that key here: Dryopteris ×triploidea
If your fern doesn't key here try Basal Pinnae Twice Divided 30A.
50C - Stipe with white hairs; usually more than 15 inches; Scattered growth usually forming large patches; Sori in 'cups' - Hayscented Fern Sitobolium punctilobulum
50B - Stipe with scattered long, narrow, nearly black scales; semi-tapered with 2 or more basal pinnae shorter than those above; Pinnae symmetrical; Sori comma-shape - Northern Lady Fern Athyrium angustum
50A - Stipe with coarse brown scales at base; crown growth form; Sori round-reniform - 51
51B - Glandular hairs on indusium, costa, rachis - 52
52B - Lowest, innermost pinnule shortest; Basal pinnae asymmetrical - Intermediate Wood Fern Dryopteris intermedia
52A - Blade somewhat narrow with little to no taper except near tip; Basal pinna nearly equilateral - Boott's Wood Fern Dryopteris ×boottii
51A - Lacking glandular hairs on indusium, costa, rachis; Lowest, innermost pinnule longest - 52
53B - Lowest, innermost pinnule twice width of first upper pinnule & significantly displaced - Mountain Wood Fern Dryopteris campyloptera
53A - Lowest, innermost pinnule same width and opposite of the first upper pinnule - Spinulose Wood Fern Dryopteris carthusiana
Read each of the major sections (I-V) in order to see which section your fern fits into. If your fern doesn't fit, go to the next section.
Most of the choices in the sections contain 2 choices. If there are 3 or 4 choices, they are indicated by the letters C or D respectively. Treat these as you did the major sections - if your fern doesn't fit in the first category go to the next.
For blade division we mean how the fern is 'cut' based on bottom most pinna. By divided we mean 'cut' at least 3/4 of the way to stipe or rachis.
We have put variable or ambiguous species in more than 1 section.
This key does not include rare or introduced species. Hybrids are also excluded which may be problematic for Wood Ferns (Dryopteris) as at least two hybrids are not uncommon in Maine.
Suggestions or improvements? Errors? Email Jeff at info@DigitalNaturalist.info.
last updated 2024-Mar-13