Leaves and stems:

Leaves (fronds) are erect to drooping, 4 to 20 inches long, up to 5 inches wide, lance-elliptic in outline, broadest near the middle, usually twice compound with 6 to 15 pairs of opposite leaflets (pinnae). Pinnae are mostly lance-oblong to narrowly triangular in outline. The largest leaflets have 3 to 14 pairs of lobes, lance-oblong with a rounded or blunt tip; edges are minutely toothed and sometimes shallowly lobed.

Veins are unbranched, branched or forked, mostly obscure except for an enlarged pore (hydathode) near the vein tip which is most easily seen on the upper surface. Both surfaces are sparsely to moderately covered in glandular hairs, more densely so on the underside, and lack any non-glandular hairs or scales.

The lower stem (stipe) is usually green to light brown, occasionally darker just at the base and is covered in scattered tan scales and sparse glandular hairs. The upper stem (rachis) is grooved, green to straw-colored with scattered scales on the underside and is moderately glandular hairy all over. Plants form a loose clump, the old stem bases persisting to the next year, broken off at varying points so are of varying lengths.

Spores:

The sori (group of spores) develop on the underside of fertile fronds starting in early to midsummer. They are circular and arranged around the edge of the pinnae lobes. Spores mature to dark brown.

Surrounding the sori is tissue (indusium) that is white with 4 or 5 broad-based lobes that encircle the sori. During the growing season there is little visual difference between fertile and sterile fronds, but the fertile fronds die back and sterile fronds remain green through winter.

Notes:

Of the 6 Woodsia species in Minnesota, this is one of the easier to identify though is not commonly encountered in Minnesota, where it reaches the northwest edge of its range. There are a mere 23 records in the Bell Herbarium, the most recent collection taken in 1981. It is more abundant in Wisconsin, particularly in the southwest part of the state, found in shaded rocky woods, bluffs, outcrops and gorges.

Blunt-lobed Woodsia is distinguished from the other Woodsia ferns by its evergreen, usually twice compound leaves, stems that are usually green to straw-colored throughout and lack a joint near the base, persistent old stem bases of varying lengths, lobed indusia that surrounds the sori, and sparsely to moderately covered all over with glandular hairs and no non-glandular hairs. Most similar are Oregon Woodsia (Woodsia oregana), which has hair-like indusia, and Rocky Mountain Woodsia (Woodsia scopulina), which has a stem reddish-brown at the base and covered all over in a mix of short glandular and longer multi-cellular hairs. Other Woodsia species may have jointed stems with old stem bases of equal lengths, have once compound leaves, hair-like indusia, have non-glandular hairs and/or lack glandular hairs.

W. obtusa is one Woodsia resembling Cystopteris ferns, which also mostly grow on rocks, but Cystopteris ferns lack the enlarged pore (hydathode) at vein tips (a distinctive trait of Woodsia), lack persistent stem bases, are not evergreen, and most lack glandular hairs. There are 2 recognized subspecies of W. obtusa: subsp. occidentalis is a southern species with smaller spores (measure in micrometers) and the lobes on the lowest pinnae usually more deeply cut/lobed rather than just toothed; subsp. obtusa is as described above and the more common of the two, and found throughout the eastern half of North America, including Minnesota.