- Type
- Roots & Stems
- Difficulty
- Moderate
- Season
- Spring
- Sowing
- From rootstock
Rehmannia
Rehmannia glutinosa
Catalpol and stachyose · a root sold raw, dried or steamed
Rehmannia is a perennial grown for its root, and a herbal market lays it out under three separate names: saeng-jihwang, geon-jihwang and suk-jihwang. The first is the root as it comes out of the ground, the second is that root dried, and the third has been steamed and dried over and over until it turns black. What goes into the familiar preparations, ssanghwa tea or sipjeondaebotang, is usually the black one. The Donguibogam lists it in the Herb section as 生地黃, records that it is planted everywhere, and says the root is lifted in the second and eighth months of the lunar calendar and dried in the shade. It tells the reader to float the lifted roots in water and keep the heavy ones that sink, and it adds a handling rule: never let copper or iron touch the root while digging. On composition, the iridoid glycoside catalpol and the storage sugar stachyose are both reported, and analyses continue to show how far steaming and drying shift that profile. A home plot can grow it from root cuttings set in spring and lifted in autumn, but on ground where water stands the root simply rots. Since it is sold as a medicinal material, anyone using it as food does best to pick a package labelled for food use.
Year-Round Calendar
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Where It Grows
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Origin
Native to the middle Yellow River. The old growing area is around Wen County and Wuzhi in Jiaozuo, Henan, the former Huaiqing Prefecture. Shanxi and Shandong are large producers as well. Harvest October, November
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Major producer
Grown in Jeongeup and Sunchang in North Jeolla, and across South Chungcheong and North Gyeongsang. Harvest October, November
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- Major producer A little is grown for medicinal use in Nara Prefecture and a few other places.
It is native to the middle reaches of the Yellow River in China, and its cultivation, too, is in practice confined to three countries: China, Korea and Japan. Roots from the Huaiqing Prefecture area of Henan have long been graded apart under the name Huai dihuang. Saeng-jihwang (fresh root), geon-jihwang (dried root) and suk-jihwang (steam-processed root) differ only in how the same root has been prepared; they are not different crops.
- Optimum
- 20~26℃
- Range
- 10~30℃
- Hardiness
- Half-hardy
This is a plant for full sun and well-drained sandy loam. It stands up to summer heat well, but if the roots sit in water they simply rot. It overwinters as roots, and where the ground freezes deep the planting roots are damaged, so they are lifted and buried for storage.
In Korea Central Korea and south
Warm, free-draining sandy loam suits it best. It is grown in Jeongeup and Sunchang in North Jeolla, in South Chungcheong and in North Gyeongsang. A field that has carried it once is not planted with it again for several years.
How to grow
- Light
- Full sun all day. In shade the root never thickens
- Soil and depth
- Free-draining sandy loam at least 30cm deep. Where water stands the root simply rots
- Spacing
- 30cm between rows, 20cm between plants, on a ridge raised 20cm or more
- Watering
- Watch it only until it has rooted. Once established it prefers to run on the dry side
- Feeding
- Compost worked in before planting as a base dressing. Extra nitrogen in summer gives leaf at the expense of root
- Harvest
- Lift the roots in October or November once the leaves have died back. The Donguibogam gives the second and eighth months of the lunar calendar. Knock off the soil, wash, and sort by thickness
What makes the difference. Keep the thickest, soundest roots as fresh rehmannia and dry the rest. For next year's planting stock, choose root sections with a bud and bury them somewhere cool
Why this one is harder. A bed that has carried rehmannia is not replanted with it for several years. Grown in succession the roots stay thin and losses to rot climb. In a small plot, move it each year or keep it in a large container
Common problems
- Root rot You will see Leaves yellow, the whole plant wilts, and the lifted root is soft What to do Avoid ground where water stands and raise the ridge. Lift affected plants and do not replant that spot
- Spider mites You will see The underside of the leaf roughens and the colour fades to a dull grey What to do Spray water underneath the leaves to raise humidity and remove badly affected leaves
- White grubs You will see Chewed patches on the root surface, with holes visible at lifting What to do Turn the bed before planting and pick the larvae out; never dig in half-rotted compost
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
The old record — saeng-jihwang (生地黃). The Donguibogam lists it in the Herb section as 生地黃. It sets down a cold nature, a sweet taste (bitter by another account) and no toxicity. It gives 地髓 and 芐 as alternative names, and on provenance says it is planted everywhere and that roots from yellow earth are best. The root is lifted in the second and eighth months of the lunar calendar and dried in the shade; lifted roots were floated in water, those that floated called 天黃, those half-floating 人黃, those that sank 地黃, and the heavy ones that sank were kept. Steamed and dried the nature turns warm, simply dried it stays neutral, so the text separates the grades by processing, and it adds the prohibition on letting copper or iron touch the root. This is a historical medical record and differs from modern judgment.
Analyses of change through processing. Repeated analyses report that catalpol falls and 5-hydroxymethylfurfural rises as the root is steamed and dried. The result is that fresh, dried and steamed rehmannia differ in composition even though they come from the same root. This is an analysis of composition and has nothing to do with preventing or treating disease.
Nutrition
- Catalpol (the marker compound of the root) — one of the iridoid glycosides
- Stachyose (the storage sugar of the root) — an oligosaccharide of four linked sugars
- 5-hydroxymethylfurfural (rises through steaming and drying) — a compound formed when sugars are heated
- Water (most of the fresh root) — the reason the weight falls so far on drying
Catalpol Analyses have repeatedly reported the isolation of catalpol, an iridoid glycoside, from the rehmannia root. It serves as the marker compound for judging quality. Heat lowers the level, so a fresh root and a steamed one read very differently. This entry stays with the compositional record.
Stachyose and sweetness The root stores sugar as stachyose, an oligosaccharide. Analyses attribute the black, sticky, faintly sweet character of the steamed root to what happens to that sugar under steam. Human digestive enzymes break oligosaccharides down poorly, so a large amount tends toward gas in the gut.
What steaming changes Repeated cycles of steaming and drying have been reported to raise 5-hydroxymethylfurfural in the root. It is a compound formed when sugars are heated, and it is used as an index of how far the processing has gone. The same root reads as a different composition table at each stage.
Reading a composition table The fresh root is mostly water and the dried root has lost much of its weight, so the same 100g gives entirely different numbers. Before comparing figures, check whether the data is on a fresh, dried or steamed basis.
Pairings
○ Jujube and licorice — Herbal markets bag rehmannia together with jujube and licorice. The sweetness of the jujube makes the deep, dark taste of the steamed root far easier to drink. Long boiling turns it muddy, so drop the heat once it comes to the boil. Jujube · Licorice
○ Ginger — The heat of ginger covers the earthy note of the fresh root. Two or three thin slices are enough; more and only the ginger comes through. Ginger
○ A cold soak — Rather than dropping dried rehmannia straight into hot water, soak it in cold water for half a day and then bring that same water to the boil. Keep the soaking liquid rather than pouring it away.
△ Brewing it daily on your own judgment — Rehmannia is a material that belongs to prescriptions. This entry goes as far as choosing it at market and handling it; what to brew, how much and how often is a question for a Korean medicine doctor or a pharmacist. That holds all the more if you are taking other medication or are pregnant.
△ Unlabelled material — Herbal markets set medicinal-grade and food-grade material side by side on the same table. For food use, take the package that states a food category, an origin and a manufacturer. If the bag carries no labelling, leave it.
△ Copper and iron vessels — The Donguibogam says copper and iron must not touch rehmannia as it is dug. The rule still holds up in practice: handle dark roots in stainless steel or glass, and do not leave them sitting in a cast-iron pot, and the colour holds better. It is an old handling rule that has stayed in place.
Varieties
- Saeng-jihwang (fresh) (processing) — The root as lifted, wet and yellowish. Not a cultivar but a processing stage
- Geon-jihwang (dried) (processing) — The fresh root dried. It hardens and darkens. Again a processing stage
- Suk-jihwang (steamed) (processing) — Steamed and dried repeatedly until black. The method handed down calls for nine steamings and nine dryings
- Cultivated lines (root cutting) — Korean growers carry forward lines that set a thick root, propagated from root cuttings rather than seed
Source: Donguibogam, Herb Section 生地黃 · Rural Development Administration (Nongsaro)