- Type
- Herbs & Aromatics
- Difficulty
- Hard
- Season
- Summer
- Sowing
- From seed
Chinese indigo
Persicaria tinctoria
Indican — the dye plant that yields blue from its leaves
Chinese indigo is an annual of the knotweed family grown for the blue drawn from its leaves. Sown in spring and cut in summer, the leaves dye cotton and hemp. What the leaf holds is indican, a colourless glycoside; steeped and fermented, then made up with lye, the vat takes the cloth, and blue rises as the cloth meets the air and indican becomes indigo. Repeated dipping deepens the colour for the same reason. The Donguibogam lists the seed in the Herb section as 藍實 and gives the Korean name as jok ssi. It identifies it as the seed of 大藍, the cultivated indigo of the day, records the harvest of seed in the fifth and sixth lunar months, and notes that the seed resembles that of a smartweed but is larger and black. Indigo dyeing has carried through to the present as the craft of the nationally designated dyeing masters. Chinese indigo is not grown to be eaten, so this entry goes only as far as raising the crop and setting the vat.
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Where It Grows
- Main producer In central and southern China it has long been grown under the name liaolan, meaning knotweed indigo.
- Main producer Tokushima is known for Awa ai, the indigo of Awa, and the growing area is the waterside fields of the Yoshino River basin.
- Main producer The indigo dyeing of the Naju area in South Jeolla is well known.
It is native to East Asia, and is thought to have come out of China and spread to the Korean peninsula and Japan. Most of the world's natural blue dyestuff comes from Indigofera tinctoria, grown in the tropics, a plant of a different family from Chinese indigo, and the blue in use today is almost all synthetic indigo. Growing areas for Chinese indigo are therefore distinguished not by output but by where the dyeing tradition has survived.
- Optimum
- 20~28℃
- Range
- 12~35℃
- Hardiness
- Not hardy
- Day length
- Short-day
Summer heat and a generous supply of water thicken the leaves and deepen their colour. It is an annual that dies with the frost, so leaf cutting is concentrated into the single summer season. The flower stalks rise in late summer as the days shorten, so the plants are cut before that.
In Korea Nationwide
How to grow
- Light
- Full sun all day. In shade the leaves thin out and the colour comes weak
- Soil and depth
- Moist, rich ground. It shrinks in dry soil, so a bed suits it better than a pot
- Spacing
- 20 to 30cm between plants, 40cm between rows. Crowded, the lower leaves go slimy
- Watering
- Water often through summer so it never dries. After a cut, water generously and the regrowth comes fast
- Feeding
- Work in a base dressing, then top-dress once after the first cut. Too much nitrogen gives leaf without colour
- Harvest
- July and August, cut before the flower stalks rise, leaving about 20cm of stem. Watered well, the regrowth allows a second cut in the same year
What makes the difference. Steep the cut leaves before they dry; dried leaves yield less colour. Fermented liquor mixed with lye and left to settle gives the indigo paste
Why this one is harder. Setting the vat is harder than growing the leaf. The strength of the lye, the temperature and the state of fermentation need watching daily for the colour to rise, and a vat that has died is hard to bring back
Common problems
- Aphids You will see Colonies under new leaves, which turn sticky What to do Wash them off with a jet of water; if they persist, use an egg-yolk oil spray. Damaged leaves mean lost colour
- Leaf-eating caterpillars You will see Holes in the leaves with black frass beneath What to do Check the undersides morning and evening and pick them off. The cut leaf is the raw material, so every hole is a loss
- Powdery mildew You will see A dusting of white settles on the upper leaf surface What to do It worsens where planting is dense. Thin the stand for airflow and remove affected leaves
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 — namsil (藍實). The Donguibogam lists the seed in the Herb section as 藍實 and gives the Korean name as jok ssi. It records the nature as cold, adding that some call it chilly, the taste as bitter with a note that some call it sweet, and no toxicity. It identifies the seed as that of 大藍, the indigo then under cultivation, sets the seed harvest in the fifth and sixth lunar months, and notes that the seed resembles a smartweed seed but is larger and black. This is a historical medical record and differs from how it is judged today.
From indican to indigo. The leaf holds colourless indican; an enzyme cleaves it and the fragments pair up in air to form blue indigo. The chemistry is established, and it explains why a freshly cut leaf is not blue. This is an account of how the colour is made.
Soluble form and oxidation. Indigo is insoluble and will not enter fibre unaided. Fermentation converts it to a soluble form, and the moment cloth leaves the vat the air fixes it again inside the fibre. Repeated dipping deepens the colour for that reason. This is an account of the dyeing process.
Nutrition
- Indican (held in the leaf) — a glycoside that carries no colour of its own
- Indigo (indigotin) (forms on contact with air) — a blue pigment insoluble in water
- Enzyme (acts as the leaf is bruised or fermented) — the leaf protein that cleaves indican
Indican What sits in the leaf is not blue pigment but indican, a colourless glycoside. As the leaf is bruised or fermented in water an enzyme cleaves it, and the fragments pair up in air to form indigo. That is why a freshly cut leaf is not blue.
Indigo does not dissolve in water Indigo is insoluble, so on its own it will not enter cloth. Fermentation and lye return it to a soluble form, the cloth goes into the vat, and the moment it comes out the air fixes it again, locked inside the fibre. Repeated dipping deepens the colour for the same reason.
Not a food plant Chinese indigo is not raised for the table. It belongs in this guide because it is still a field crop grown for dye and because it appears in the old medical text. Preparations as a green or a tea are not covered here.
Cutting time and colour Indican is taken to be highest in the leaf before flowering, so the crop is cut before the flower stalks rise. Water it well after cutting and the regrowth gives a second cut. Dried leaves yield less colour, so steep them as soon as they are cut.
Pairings
○ Lye — Lye leached from wood ash is what sets the indigo vat. Alkali is needed before the fermented indigo will dissolve into a form the cloth can take. Ash from rice straw or bean stalks was used.
○ Cotton, hemp and ramie — Plant fibres take indigo best. Silk and wool come out differently, and synthetics barely take it at all. Wash the size out and rinse several times before dyeing. Ramie (Mosi)
○ Air and time — The colour rises while the cloth hangs in the air between dips. Watching it turn from yellow to green and then to blue is watching the oxidation happen. Giving that stage its time, rather than hurrying it, is what sets the depth of the colour.
△ Never treated as a food plant — Chinese indigo is grown for dye. It is not dressed as a green or brewed as a tea. The settled sediment and the vat liquor are not food either, and dyeing equipment should never share shelves with kitchen vessels.
△ Lye and vat liquor — Lye is strongly alkaline. Prolonged bare-handed contact damages skin, and a splash in the eye is dangerous. Wear rubber gloves and an apron, and keep the vat out of reach of children and pets. Fermentation smells strong, so work where air moves.
△ Confusion with other indigo plants — Plants called indigo sit across several families. Woad is a crucifer, true indigo a legume, Assam indigo an acanthus relative, and each is grown and worked differently. Check the botanical name Persicaria tinctoria when buying seed.
Varieties
- Chinese indigo (Persicaria tinctoria) (species) — An annual of the knotweed family. This is the species used for indigo dyeing in Korea
- Woad (Isatis tinctoria) (species) — A crucifer. The European source of the same blue
- True indigo (Indigofera tinctoria) (species) — A leguminous shrub. The mainstay of the indigo trade
- Assam indigo (Strobilanthes cusia) (species) — An acanthus relative, used in southern Asia and Okinawa
Source: Donguibogam, Herb Section 藍實 · Korea National Arboretum species information system