- Type
- Leafy Greens
- Difficulty
- Hard
- Season
- Spring·Summer
- Sowing
- Transplant
Water Shield
Brasenia schreberi
Mucilage polysaccharide — a jelly-wrapped shoot that grows underwater
Water shield is a perennial aquatic plant whose oval leaves float on the surface of still ponds. The part you eat is the young shoot that has not yet broken the surface, still wrapped head to toe in a clear, jelly-like mucilage. In the Joseon dynasty it was sent to the king as tribute, but its habitats have shrunk so badly that Korea now lists it as a Class II endangered wild species. Taking or transplanting plants from a wild stand is prohibited under the Wildlife Protection and Management Act and carries criminal penalties, so both the food and the planting stock have to come from cultivated supply. Almost all the water shield sold as food comes from cultivation in Japan and China, and growing it at home means nursery-propagated rootstock and a tub of water rather than a pot of soil.
Year-Round Calendar
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Where It Grows
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Main producer
It is grown in ponds at West Lake in Zhejiang Province and at Lichuan in Hubei Province, and the volume produced here is the largest. Harvest April, May, June, July, August, September
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Main producer
Mitane Town in Akita Prefecture is known as a producing area for junsai (the Japanese name for water shield). Pickers work from small boats, taking the shoots from the water one at a time. Harvest May, June, July, August
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- Wild population A few plants remain in old ponds in Gangwon, South Gyeongsang and Jeju. It is a Class II endangered wild species under the Ministry of Environment, so gathering it from wild sites is prohibited.
- Wild population It grows widely in the acidic lakes and marshes of the east and the south, but is not harvested for food.
The only species in its genus, this water plant has a wide distribution, growing scattered across East Asia, North America, Oceania and parts of Africa. Yet it is harvested and traded as food only in East Asia, and there in only two countries, China and Japan, so its natural range and its production areas diverge sharply. In Korea the wild stands have declined and there is almost no commercial production.
- Optimum water temp.
- 20~28℃
- Range
- 10~32℃
- Hardiness
- Hardy
What decides the growing season is not air temperature but water temperature. New shoots come up from late spring, once the water rises above 20℃, and they stop growing once it passes 30℃. In winter the leaves rot away and the plant carries on as rhizomes in the mud underwater; even when the surface freezes over, the rhizomes survive. It grows where the water is clear and barely moving, like an old pond 30~60cm deep.
In Korea Nationwide
A few wild stands remain in old ponds in Goseong in Gangwon, Miryang in South Gyeongsang, Jeju and a handful of other places. As a Class II endangered wild species under the Ministry of Environment, it may not legally be dug up or transplanted from a wild site; to grow it you must obtain artificially propagated rootstock and raise it in a water-filled container. Set the rootstock in during April~May, and pick the young underwater shoots from May~July.
How to grow
- Light
- Give it a spot with six hours of sun or more. The floating leaves have to catch light before the submerged shoots will thicken.
- Soil and depth
- It will not grow in a soil pot. Lay 10cm of paddy soil or loess in the bottom of a water tub and bury the rhizome shallowly.
- Spacing
- One or two rhizomes in a tub 50cm across. Once the leaves cover the whole surface, divide and replant the following spring.
- Watering
- Keep the water 30 to 60cm deep. Do not change it out; just top up what evaporates. Let tap water stand a day before using it.
- Feeding
- Almost none. In spring, push a single slow-release pellet made for aquatic plants into the soil and leave it at that. Fertilizer dissolved in the water feeds the algae before it feeds the plant.
- Harvest
- From May to July, reach under the surface and pick young shoots that have not yet emerged. Look for a thick jelly coat and a leaf still furled. Drop them into cold water and blanch them the same day.
What makes the difference. Above 30C the shoots stall, so shade about half the tub through midsummer. Wild plants are a Class II endangered species and cannot be collected. Source rootstock from artificially propagated stock, and for eating, buying cultivated or imported shoots is the realistic route.
Why this one is harder. It cannot live in soil. You need a container holding at least 30cm of water, and the shortcut of collecting it from the wild is closed off by law.
Common problems
- Aphids You will see Black specks clustered under leaves that have risen above the waterline What to do Knock them off with a strong jet of water. If fish share the tub, do not use any pesticide.
- Aquatic snails and apple snails You will see Round holes in the leaves and chewed leaf margins What to do Lift out every one you see. Eggs often arrive with fresh soil, so dry new soil out once before adding it.
- Filamentous algae You will see Green threads tangled across the surface and around leaf stalks What to do Wind them off with a stick and shade roughly 30% of the tub. Never dissolve fertilizer straight into the water.
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. Water shield appears in the Donguibogam's vegetable section as sunchae, recorded as cool in nature, sweet in taste, and free of toxicity. The entry also describes the plant itself, a pond herb whose leaves float on the surface. This is a record from a seventeenth-century medical text, not a modern medical judgement.
Research on the mucilage polysaccharide. In vitro studies that isolated the mucilage polysaccharide have reported observed antioxidant activity. Those results come from an isolated compound in the lab; clinical evidence in people who eat the vegetable is not yet substantial.
Nutrition
- Mucilage polysaccharides (The jelly layer sheathing the young shoots) — Creates the slippery texture
- Water (Over 95 %) — Almost no calories
- Polyphenols such as gallic acid (In leaf and stem) — Darken the colour on oxidation
The jelly coat: a mucilage polysaccharide Each young water shield shoot is sheathed in a clear mucilage. It is a polysaccharide built from sugars such as galactose, arabinose and fucose, and it works as the plant's own protective layer underwater. On the plate that layer is the whole point: what you notice first is not chew but a slippery glide down the throat. The thicker the coat, the higher the grade.
Over 95% water Water shield is more than 95% water, with almost no carbohydrate or fat. A full bowl carries essentially no calorie weight, which is why it goes generously into cold summer soups and fruit punches. It is also low in protein and vitamins, so treat it as texture and refreshment rather than as nourishment on its own.
Gallic acid and other polyphenols Gallic acid and related polyphenols have been identified in the leaves and stems. That is also why blanched or long-standing water shield darkens slightly: those compounds oxidize. Soaking prepared shoots for a long time washes the compounds out along with the mucilage, so keep the rinse short.
Pairings
○ Omija (magnolia berry) infusion — Sunchae-hwachae floats water shield in a red infusion of omija berries. The tartness gives an outline to a vegetable that has almost no flavor of its own, and the sight of jelly-wrapped shoots suspended in red liquid is why it was served to summer guests. Chill the infusion first and add the shoots last, or the mucilage loosens. Schisandra
○ Vinegar and vinegared soy sauce — On its own water shield tastes of very little, so it needs acid to come alive. A light toss in vinegared soy sauce or vinegared gochujang makes the slipperiness stand out. Do not let it sit in the sauce; dress it right before it goes to the table.
○ Water dropwort and cucumber — In a cold soup, water-heavy vegetables like water dropwort or cucumber sit beside water shield without competing. You get the herb's fragrance, the cucumber's crunch and the shoot's slide as three separate sensations in one bowl. All three also want the broth cold, so nothing has to be heated. Cucumber · Water Dropwort (Minari)
△ Hot broth — The Donguibogam records that water shield is cooling yet nourishing, but that eating it hot blocks the qi from descending and does real harm. In the kitchen the practical version is simpler: dropped into boiling broth, the mucilage dissolves and the texture that makes water shield worth eating is gone. Turn off the heat and add it at the very end.
△ Eating a lot, or eating it often — The same passage carries a caveat: do not eat it in quantity, and do not eat it over a long stretch. That reflects a seventeenth-century view of the plant as cooling, but the practical read still holds for anyone with a weak stomach or who runs cold, who may want to avoid large servings in cold broth.
△ Foraging it from the wild — Water shield is a Class II endangered wild species in Korea. Digging or pulling it from a natural habitat is prohibited by law and carries penalties. If you want to grow it, buy rootstock from a supplier working with artificially propagated plants; if you want to eat it, buy cultivated or imported stock.
Varieties
- Mitane cultivated line (Transplant) — Grown around Mitane in Akita, Japan; it accounts for most of Japan's cultivated water shield.
- Xihu line (Transplant) — Long grown around Hangzhou in China. Used in clear soups.
- Korean wild population (Transplant) — A few wetland populations remain in Korea. Collecting them is prohibited by law.
Source: Donguibogam, Vegetable Section (蓴菜) · National Institute of Biological Resources, endangered species records