Cystopteris protrusa aka Lowland Brittle Fern

Cystopteris protrusa is a small fern. It belongs to the spleenwort family, Aspleniaceae. Names include lowland brittle fern and southern fragile fern. Its native range spans southeastern Canada and the United States. North Carolina State lists Ontario. It also lists much of the eastern United States. The fern grows in rich woods, valleys, and on slopes. It also grows beside streams. In North Carolina, it grows in the mountains and Piedmont.

This perennial lives for years. It is deciduous, so it sheds its fronds. Creeping rhizomes, or ground stems, give rise to airy growth. Garden plants reach 0.5–1.5 feet [0.15–0.46 metres] tall and wide. Missouri Botanical Garden gives a usual height of 0.5–1.25 feet [0.15–0.38 metres]. Growth is slow. The fronds stand upright or arch. The rhizome tip grows past the newest frond. This trait gives the fern its scientific name. Green fronds form a rosette, with finely split blades. Their forms range from bipinnate-pinnatifid to tripinnate-pinnatifid. These terms describe how the blades divide and split. The stipes, or frond stalks, snap with little force. This trait explains its common names.

Fronds emerge early in spring. Their season can vary. Missouri Botanical Garden notes new growth through spring and summer. North Carolina State says fronds may brown and fade by midsummer. This fern makes no flowers or fruit. It breeds by spores held in round groups called sori. Each group is a sorus. A bladder-shaped cover, the indusium, covers part of each sorus. These covers look swollen when young. The sori turn brown or black as spores mature. Spores ripen from late spring into midsummer.

Grow it in part shade, dappled light, or deep shade. It needs loose soil rich in organic matter. The soil must drain well. Keep it moist, but not too wet or waterlogged. North Carolina State says it prefers neutral soil. Missouri Botanical Garden says to add lime to acidic soil. Both sources say it needs little care. Missouri Botanical Garden lists hardiness in USDA zones 2–9. Gardeners can grow more plants by division, splitting a plant.

Its small size suits the front of shaded borders. It also suits woods, native plant gardens, and shaded rock gardens. Slopes and banks offer more sites to grow it. Its split fronds add fine detail near the ground. Plan for fronds that may turn brown by midsummer. Neither garden source reports major pest or disease problems. Both list resistance to rabbits. North Carolina State also lists resistance to deer. The evidence does not show food uses. Nor does it show whether this fern is toxic.