Osmunda japonica aka Asian Royal Fern

Osmunda japonica is a deciduous fern in the Osmundaceae family. Its common names include Asian royal fern and zenmai. RHS gives its native range as Pakistan to Japan. Plants For A Future lists China, Korea, Japan, Taiwan, and Sakhalin. It records the fern in damp places across Japan. This species is not native to Britain or Ireland.

The plant forms a clump of green fronds. Its rhizome may stand upright, rise at an angle, or creep a short distance. Garden plants reach about 3.3–4.9 feet (1–1.5 metres) tall. Their spread reaches about 1.6–3.3 feet (0.5–1 metre). Each frond can reach 4.9 feet (1.5 metres) tall. Individual fronds may span 1.6 feet (0.5 metres) across. The fertile fronds stand more upright and have a bronze tint. These fronds bear spores rather than flowers or fruit. The foliage falls each year. Plants take about five to ten years to reach full size.

Give this fern partial shade and shelter from exposure. It grows in loam or clay with acid or neutral soil. It prefers acid conditions and needs a steady water supply. Both poorly drained soil and moist, well-drained soil can suit it. Sun is possible where the soil never lacks moisture. Pond edges and ground beside streams or lakes offer suitable sites. RHS rates its hardiness H6, covering much of northern Europe. Plants For A Future lists USDA zones 5–9.

Use it in damp woodland beds or along shaded garden edges. It also suits bog gardens and informal cottage planting. Its green and bronze fronds provide contrast within the clump. RHS lists it as a low-maintenance plant. It reports no usual problems with pests or disease. Remove dying fronds when needed; no further pruning is listed. Grow new plants by division or from spores. Plants For A Future does not flag it as weedy.

Plants For A Future records cooked young fronds as edible. It also lists starch from the rhizome as a food. The same source reports no known toxicity for this species. However, it warns that some ferns contain carcinogens or thiaminase. That general warning does not establish their presence in this fern. Thiaminase can reduce the body's supply of vitamin B. Heat or thorough drying destroys that enzyme, according to the source. These cautions mean food use should not imply proven safety. The source lists no known medicinal or other uses.