Cystopteris fragilis aka Brittle Bladder Fern

Cystopteris fragilis is known as brittle fern or fragile fern. This small fern grows back each year from a rhizome. Its native range spans temperate regions on most continents, excluding Australia. It is native to Britain, Ireland, and North Carolina. Wild plants grow in rocky woods and on shaded walls. They also grow among rocks, often where the rock is basic. Some plants occur in wetlands.

The plant forms small clumps and spreads slowly through compact rhizomes. Garden plants stand about 0.7–1 foot (0.2–0.3 m) tall. RHS gives an upper height of 1.6 feet (0.5 m). Clumps spread about 1–2 feet (0.3–0.6 m) wide. Upright rhizomes hold tufts of pale grey-green fronds. Each frond has a lance shape and finely divided leaflets. The leaves are twice pinnate, with sharp tips and fleshy stalks. Fronds can reach 1.5 feet (0.45 m) long. The plant sheds its leaves rather than keeping them through winter.

This fern makes neither flowers nor fruit. It reproduces through spores in round sori beneath the fronds. Partial or dappled shade suits it well. NC State also reports that it tolerates deep shade. Give it fertile soil that stays moist but drains well. Shelter helps protect the fronds from cold winds that dry them. It grows in loam, clay, and shallow rocky soil. RHS lists acid, neutral, and alkaline soils as suitable. NC State notes a preference for neutral to alkaline soil. Dry soil can cause dormancy, but growth returns when moisture returns. RHS rates it H7; Chicago lists USDA zones 4–8.

Use it in shade gardens, rock gardens, or moist rock crevices. It also suits native gardens and planting beneath taller plants. Close planting can make a low ground cover. NC State includes shallow water gardens among its uses. Deer tend to leave it alone. Serious pest and disease problems are uncommon, though Florida fern caterpillars can attack it. Divide clumps in spring and clear dead fronds in autumn.

Plants For A Future lists the root as emergency food. Its low food rating does not support routine eating. That source reports no known toxicity for this species. Its wider warning about ferns does not establish this plant's safety. It also records past root treatments for worms. Other past uses involved plant infusions for injuries. These records describe traditional uses, not proof of safe or effective treatment.