northwestern lady fern

not listed

NNR - Unranked

not listed

FAC - Facultative

Northwestern lady fern is a relatively large, delicate, perennial fern that rises from a short-creeping rhizome and fibrous roots. The rhizome is sometimes semi-erect and often branched. It usually has the bases of dead stalks still attached. It often forms small colonies.

Sterile and fertile fronds are similar in appearance. The fronds are deciduous, erect, arching, 11½″ to 36″ long, and lacy in appearance. They form a compact, somewhat circular but irregular cluster.

The leaf stem (stipe) is 6″ to 17¾″ long, shorter than the leafy portion (blade). It is stout and light green or straw-colored for most of its length. It is flattened at the base and becomes deeply grooved toward the top. At the base it is dark reddish-brown or black, swollen, and has 2 rows of teeth. The stipe is covered with scattered chaffy scales. The scales are brown or dark brown, lance-shaped or egg lance-shaped, ⅜″ to ¾″ long, and 1 ⁄16″ to 3 ⁄16″ wide.

The blade is elliptic to inversely lance-shaped, 12″ to 30″ long, 4″ to 14″ wide, about 2 times as long as the stipe. It is pinnately divided into 30 to 40 pairs of leaflets (pinnae). It tapers to a point with concave sides along the tip, and is widest below the middle. The 4th or 5th pair of pinnae is the largest pair. Only the lowest two pair are shortened. The lowest pair is conspicuously angled downward. The central axis of the blade (rachis) is pale green and usually hairless, sometimes sparsely glandular or hairy, sometimes with a few scattered scales.

The pinnae are oblong lance-shaped, short stalked or stalkless, and taper to a narrow point with concave sides along the tip. The longest pinnae are up to 8″ long or longer, becoming shorter as they approach the tip. They are deeply, pinnately divided, with 12 to 20 segments (pinnules) per side.

The pinnules are variable. They are linear to oblong, angled at the tip, and asymmetrically wedge-shaped at the base. They are stalkless and may extend along the central axis of the pinna (costa) at the base. They are often deeply lobed (pinnatifid), cut up to halfway or more to the midrib (costule). The upper and lower surfaces are medium green to yellow-green and hairless. The veins visible on the underside are forked. They are free, meaning they do not rejoin but extend all the way to the margin. The margins are minutely toothed, tough, and firm, but elastic.

The rachis, costa, and costule have a V-shaped groove on the top. The groove of the costule connects with the groove of the costa, which connects to the groove of the rachis, which connects to the groove of the stipe.

The reproductive structures are born on the underside of the pinnules near the base. There are several clusters (sora) of spore-bearing cases (sporangia) arranged in one row on each side of the midrib halfway between the midrib and the margin. The sorus is hooked at the end or horseshoe-shaped with the hook curled over a vein. It is covered with a protective veil (indusium). The indusium is the same shape as the sorus. It is irregularly toothed and has a fringe of long hairs along the margin. The hairs are as longer or longer than the indusium is wide. The indusium is attached to one side of the sorus.

There are 64 yellow spores per sporangium.

Fronds 11½″ to 36″ long

Common wood fern (Dryopteris intermedia) fronds are evergreen, not deciduous. The sori are round. The indusia are kidney-shaped and are attached at the base of the notch.

Northern lady fern (Athyrium filix-femina var. angustum) scales on the stipe are much smaller, no more than ⅜″ long, and 1 ⁄16″ wide. The blade is 1 to 1½ times as long as the stipe and broadest below the middle. The 4th or 5th pair of pinnae is the largest pair. The sorus is usually straight, sometimes hooked or horseshoe-shaped. The indusium has a fringe of shorter hairs along the margin. The spores may be yellow or brown.

Spinulose wood fern (Dryopteris carthusiana) fronds are evergreen, not deciduous, and are usually smaller, no more than 30″ long. The sori are round. The indusia are kidney-shaped and are attached at the base of the notch.

Moist. Bottomland forests, moist woods, meadows, thickets, swamps, streambanks. Dappled sunlight to light shade.

July to October

Sources

3, 4, 29, 30.

Native

Rare or absent in Minnesota

There is just one report of northwestern lady fern in Minnesota. It is from the Bell Museum of Natural History in Minneapolis. That record has no date or location information.

The genus Athyrium was formerly placed with the spleenworts in the family Aspleniaceae. Today, most place it in the new family of lady ferns and allies, Athyriaceae. Some taxonomists, including ITIS, place it with the cliff ferns in the family Woodsiaceae. However, the more broadly defined Woodsiaceae would be invalid (paraphyletic). Some taxonomists, including Flora of North America and USDA PLANTS, place the genus with the wood ferns in the family Dryopteridaceae. Neither classification is widely accepted.

Most sources list northern lady fern as a variety, Athyrium filix-femina var. angustum. iNaturalist and USDA Plants list it a subspecies, Athyrium filix-femina ssp. angustum. Some list it as a separate species, Athyrium angustum.