- Type
- Leafy Greens
- Difficulty
- Moderate
- Season
- Spring
- Sowing
- Transplant
Royal Fern
Osmunda japonica
Fibre, potassium · a spring green for part shade
Royal fern is a perennial fern of the Osmundaceae. The young croziers that push up in spring are wrapped in brown wool and curled like a fist, and it is at that stage that they are snapped off, boiled, leached and dried. The plant prefers damp ground with only three or four hours of sun — the north side of a wall, the base of a large tree — so in the city it fills the shaded corner of a yard or a north-facing balcony that nothing else wanted. Kkwonggobi, whose name is close enough to cause confusion, is a separate species rather than a cultivar of royal fern; a similar name is no evidence of the same plant. Kkwonggobi is eaten as a green all the same; what really needs telling apart are the toxic ferns whose croziers look alike. Dryopteris crassirhizoma has long been handled as a medicinal plant rather than a green, and where royal fern croziers are wrapped in brown wool, those of Dryopteris are covered in large, thick brown scales. Leave any crozier that cannot be named with certainty. The Donguibogam lists it in the Vegetable section as 薇, alongside the Korean name hoechomi, giving it a cold nature, a sweet taste and no toxicity. The same entry adds that 薇 belongs to the same family as bracken and grows in the same places.
Year-Round Calendar
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Where It Grows
- Main producer In the mountains of Northeast China and of Central and South China it is gathered in bulk, boiled, dried and exported. A large share of the dried royal fern on the Korean market comes from here.
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Major producer
It grows wild in mountains throughout the country, and is gathered or field-grown in Jeju, South Jeolla and Gangwon. Harvest April, May
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Major producer
Under the name zenmai it is gathered and dried in the mountain districts of Tohoku and Niigata. Harvest April, May, June
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Its native ground is the temperate mountain country of East Asia, spanning Korea, China, Japan and Taiwan, the Russian Far East and the Himalayas. It is gathered in the hills, boiled and dried far more than it is grown in fields, so the FAO statistics carry no entry for it. Bracken (Pteridium aquilinum), whose Korean name gosari is often mixed up with gobi, belongs to a different family; royal fern differs in that its shoots are thick, sheathed in brown down, and uncoil as they rise, like a closed fist.
- Optimum
- 15~22℃
- Range
- 5~28℃
- Hardiness
- Hardy
- Day length
- Not applicable
Being a fern, it has no flowers, so it is not day length but spring temperatures and soil moisture that decide when the shoots come up. It winters over as a rhizome and stands up well to frost. It likes damp valleys and forest edges that get only three or four hours of sun a day, and when the soil dries out the shoots come up thin as thread.
Harvested 3–3 years after planting. Crowns are planted, and shoots are picked only from the third year on. Pick earlier and the crown is damaged, and no shoots come up the following year. The month of planting and the month of picking do not fall in the same year.
In Korea Nationwide
It grows wild in half-shaded valleys in mountains throughout the country, and is gathered in Jeju, the hill country of South Jeolla and South Gyeongsang, and Gangwon. Forest edges suit it better than open fields in strong sun.
How to grow
- Light
- Part shade with three or four hours of sun. Midsummer direct sun scorches the frond tips
- Soil and depth
- Leafy, humus-rich soil. It must drain, yet stay damp at all times
- Spacing
- 40 to 50cm between plants. Clumps thicken every year, so allow room from the start
- Watering
- Water before the surface dries. Once daily for pots in high summer; in the rainy season, make sure the saucer never holds standing water
- Feeding
- Lay a blanket of well-rotted compost over the soil in early spring, before the croziers rise. Mineral fertiliser is barely used
- Harvest
- In April and May, snap the croziers off at the base once they stand about 20cm tall. Leave three or four per clump to unfurl into fronds rather than taking them all. Wait until the third year after planting so the clump is not set back
What makes the difference. Boil the croziers the same day they are picked. Even one day makes them tough and darkens the colour. After boiling, soak in cold water for a day with two or three water changes; whatever is left over can be sun-dried and kept into the following year
Why this one is harder. The hard part is meeting shade and moisture at once. Shade without enough water gives thread-thin croziers; water without drainage rots the crown and nothing comes up the next spring
Common problems
- Slugs You will see Holes in the new croziers and silver slime trails left overnight What to do Collect by hand after dark and ring the pot with diatomaceous earth. In daylight, look under the pot and under nearby stones
- Spider mites You will see Leaf undersides turn rough and the colour fades to a dull grey. Worse in dry air What to do Mist the undersides to raise humidity. Cut away infested fronds and open up airflow between plants
- Aphids You will see Colonies on the crozier tips and young stalks What to do Wash off with a jet of water; if it recurs, use an egg-yolk oil spray or an approved organic product
This page describes how a crop is grown and used. It does not claim health effects for any food, nor is it intended to prevent or treat disease. Component and study notes are references only and do not mean the same result occurs in people. Consult a medical professional about health concerns.
Health Benefits
What the Donguibogam records — the name and its nature. The Donguibogam lists the plant in the Vegetable section as 薇, together with the Korean name hoechomi. It gives a cold nature, a sweet taste and no toxicity. Citing the Uihak Immun, the same entry adds that 薇 belongs to the same family as bracken and grows in the same places. This is a historical medical record and differs from modern medical judgment — today royal fern is not eaten raw but boiled and leached first.
Heat and thiaminase. Analyses report that thiaminase, the vitamin B1-degrading enzyme found in ferns, is heat-labile and loses activity on cooking. That account covers one compound, and it does not stand in for the whole reason these fronds are boiled and leached through several changes of water. Ptaquiloside, a glycoside identified in bracken (Pteridium), has been the subject of a long argument over carcinogenicity, and reports differ on whether royal fern (Osmunda japonica) carries the same class of compound. This concerns composition only and has no bearing on the prevention or treatment of disease.
Nutrition
- Dietary fibre (high among boiled-and-dried wild greens) — rehydrates to several times its dry weight
- Potassium (concentrated by drying) — a mineral common to dried vegetables
- Thiaminase (reported in ferns) (loses activity on heating) — an enzyme that breaks down vitamin B1
What drying changes Royal fern is never a green eaten as picked; it goes through boiling, leaching and drying. As water leaves, minerals and fibre concentrate into the remaining weight while water-soluble compounds depart with the cooking and soaking water. Dried fronds swell back to several times their weight once rehydrated. A single handful becomes a full dish, so anyone handling it for the first time should start with less than seems right.
Thiaminase and heat Thiaminase, an enzyme that breaks down vitamin B1, has been reported in ferns. The enzyme is heat-sensitive and loses activity in boiling water. That single compound is not the whole reason royal fern is boiled and leached, though. Ptaquiloside, a glycoside identified in bracken (Pteridium), has been the subject of a long argument over carcinogenicity, and reports differ on whether royal fern (Osmunda japonica) carries the same class of compound. Never eating it raw, boiling it thoroughly and drawing it through several changes of water is the handling rule kept in common for fern greens.
What leaves with the soaking water Soaking the boiled fronds in cold water and changing that water several times draws out both the astringency and the water-soluble compounds. Little vitamin C survives a process that is both wet and hot. What remains is fibre, minerals and texture. The soaking water is discarded, never reused.
Pairings
○ Perilla or sesame oil — Rehydrated fronds need a generous pour of oil to lose their stiffness. Perilla oil gives a heavier aroma, sesame a lighter finish. Add a couple of spoons of water partway through and cover the pan so the steam softens the fronds through to the middle.
○ Beef broth — In yukgaejang or a long-simmered beef soup the fronds soak up stock and hold their chew. Cut them into 5 to 6cm lengths rather than short pieces. Because the soup simmers a long time, do not rehydrate them to mush beforehand.
○ Rice-rinsing water — Soaking dried fronds in the cloudy water from rinsing rice is an old habit. The astringency is milder than with plain water and the colour comes out better. That water sours quickly, so soak in the fridge in summer.
△ Never eaten raw — boil it hard, then leach through changes of water — Royal fern is not a green eaten as picked. The long-standing rule for fern greens is to boil them thoroughly, then soak them in cold water through several changes before cooking. A quick blanch or a surface sear is not enough, and both the cooking water and the soaking water are thrown out rather than reused. Dried fronds are rehydrated and then boiled once more. This handling is not about protecting vitamin B1. Ptaquiloside, a glycoside identified in bracken (Pteridium), has been the subject of a long argument over carcinogenicity, and reports differ on whether royal fern (Osmunda japonica) carries the same class of compound. With that unsettled, never eating them raw and always boiling and leaching is how fern greens have long been handled. Bracken
△ Toxic ferns whose croziers look the same — In spring every fern comes up curled like a fist, which is when they look most alike. Dryopteris crassirhizoma has long been handled as a medicinal plant rather than a green, and its croziers are easily mistaken for royal fern. Royal fern croziers are wrapped in brown wool; those of Dryopteris are covered in large, thick brown scales. Kkwonggobi, whose name is close enough to confuse, is eaten as a green and carries no such risk — but leave any crozier that cannot be named with certainty.
Varieties
- Chamgobi (seedling) — The common native form. Thick croziers with dense wool
- Kkwonggobi (Osmunda cinnamomea) (seedling) — A wetland relative in the same genus. Slimmer croziers, tighter clumps
- Ostrich fern (Matteuccia struthiopteris) (seedling) — A different genus, but the fiddleheads are handled the same way. Illness has been reported from undercooked fiddleheads, so boil them thoroughly
Source: Donguibogam, Vegetable Section 薇 · Rural Development Administration (Nongsaro) · National Institute of Biological Resources