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Kate's Key to Some Common Ferns of PEI

I love ferns, but find the technical keys are not very user-friendly for non-botanists. So, here is my key to some of our most common ferns for the layperson! Fellow botanists beware: I know this is simplified and the terminology is not technically correct in some cases. But if it helps people identify and enjoy ferns, I'm OK with that!


(Apologies to those who read my blog on mobile devices; this key is best viewed on desktop or laptop).


Oak Fern (Gymnocarpium dryopteris)

1. Fern frond three parted, appearing branched….....................…………………………….……………2

Fern frond unbranched..……………............................………..................…………………………………….………3


2. Large fern of open areas and edges, leaf edges inrolled on the

underside……………….......................................................Pteridium aquilinum (Bracken Fern)

Small fern of woodlands, leaflets (=pinnae and pinules) often drooping from

midrib (=stipe)…………………………………...…......……..Gymnocarpium dryopteris (Oak Fern)


3. Fern frond with lowest two leaflets (=pinnae) pointing down and/or out

Phegopteris connectilis (Northern Beech Fern)

Not as above……………........................................................……………………………………………….…………..……..4


4. Fern with somewhat scaly midrib, leaflets not divided but thick and leathery

with a distinctive, thumb-like lobe near the midrib (=stipe)

Polystichum acrostichoides (Christmas Fern)

[Fern with a dramatically scaly midrib, leaflets divided, thumb-like lobe not as

obvious..................................................................Polystichum braunii (Braun’s Holly Fern)]*

*Rare, known from only a few locations on PEI. Worth keeping an eye out for.

Not as above……….........................................................……………………………………………………………………….5


5. Fern frond widest in the middle and with leaflets much reduced in size

continuing along the midrib nearly to the ground

Paratheylpteris noveboracensis (New York Fern)

Not as above…….........................................................……….…………………………………………………………………6


6. Fern with sori on the undersides of some leaflets, no “flowers” (=fertile

fronds)….......................................................................................................................................................……...7

Fern without sori, “flowers” (=fertile fronds) very different from vegetative

fronds................................................................................................................................................................….11


7. Sori horseshoe-shaped, curved, or elongated…….....................……………………………………...…8

Sori more or less round……..............................................………………………………………………………………9


Round sori on the undersides of Spinulose Wood Fern (Dryopteris carthusiana)

8. Sori horseshoe-shaped or curved, leaflets alternate

Athyrium filix-femina (Lady Fern)

[with a red or wine-coloured midrib……….....................................……………………...……….forma rubellum]

Sori elongated and pale, leaflets alternate

Deparia acrostichoides (Silvery Glade Fern)


9. Sori at or near the margins of leaflets, midrib not scaly but lightly hairy, fern

fragrant when crushed. Often found in large patches near woodland edges,

roadsides, or in open patches in woodlands

Dennstaedtia punctilobula (Hay-scented Fern)

Sori more or less in the centre of the leaflets……......................……………………………………….10


10. Fern frond more or less triangular (widest at the bottom) and midrib scaly at

the base. Leaflets vertical, like most ferns

Dryopteria carthusiana (Spinulose Wood Fern)

[Note: this is a highly variable species in which individuals can look rather different from each other even in the same habitat, although all have the characteristics noted here. Some of the variations have now been awarded species status, so you may wish to refer to a more detailed key for this group].

Leaflets mostly the same length and horizontal, fern frond ladder-like

Dryopteris cristata (Crested Wood Fern)


11. Fertile fronds with some leaflets, more or less resembling sterile fronds…….…12

Fertile fronds with no leaflets, totally unlike sterile fronds…………………........…...………13


12. “Flower” (=reproductive structures) at the tip of fertile fronds

Osmunda regalis (Royal Fern)

“Flower” in the centre of fertile fronds….......................….Claytosmunda claytoniana

(Interrupted Fern)

[Note: vegetative fronds of Interrupted Fern look much like those of Cinnamon Fern (see below) but lack the pale tufts of hair at the base of each leaflet].


Cinnamon Fern (Osmundastrum cinnamomeum, shown here) has pale tufts of hair at the base of each leaflet; Interrupted Fern (Claytosmunda claytoniana) lacks these.

13. Fern fronds large (more than thigh-high)………………………………........................……...…………14

Fern fronds mid-sized to small (thigh-high or shorter)………...........…………………..………15


14. “Flower” (=fertile fronds) cinnamon-coloured and about as tall as vegetative

fronds. Fronds with soft, cinnamon-coloured hair at the base of the midrib

and pale tufts of hair at the base of each leaflet

Osmundastrum cinnamomeum (Cinnamon Fern)

“Flower” dark, shorter than the vegetative fronds. Fronds widest near the tip,

tapering towards the base. Both fertile and vegetative fronds with a celery-

like groove in the stem....................................Matteuccia struthiopteris (Ostrich Fern)


15. “Flower” (=fertile fronds) shorter than the vegetative fronds. Fern of wet

habitats and edges (eg. streamsides, ditches)

Onoclea sensibilis (Sensitive Fern)

Flower above a single frond……Botrychium / Sceptridium spp

(Grapeferns & Moonworts)


None of the Grapeferns / Moonworts is common on PEI, but here are a couple you may encounter:

Mid-sized fern (roughly knee-high) with a lacy, triangular frond. A fern of moist, shady forests…………………...........…………………..….Botrychium virginianum (Rattlesnake Fern)

Small fern with a leathery frond barely above ground level. A fern of drier areas including woodlands, dunes, and dry slacks….....................….……Sceptridium multifidum

(Leathery Moonwort)


Leathery Moonwort (Sceptridium multifidum)

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