Isatis indigotica Seeds | Woad
LATIN NAME: Isatis indigotica
COMMON NAME: Woad
Plant Profile
Woad behaves as a biennial, and the whole plant is organised around that fact. The first year produces nothing but a rosette of long-stalked, blue-green leaves, each one faintly powdered and slightly fleshy, laid flat over the ground from a taproot that goes down further than the top growth suggests. That rosette is the dye crop. The second year sends up a stiff, branching stem to about a metre, carrying small yellow flowers in flat-topped clusters, each with the four petals and six stamens of the mustard family.
The fruits are the most distinctive part of the plant and the reason the genus is easy to recognise. They are short, flat, pendulous pods, dark violet-brown when ripe and hanging in dense clusters, each one winged at the tip and containing a single seed. The whole plant dies after setting seed, which is the pattern of a strict biennial. Woad is a plant of open, sunny ground and deep, fertile, well-drained soil, and in its native range it occupies stony slopes and disturbed ground. It is not a delicate thing: it stands frost, poor soil and rough treatment, and in many places it escapes the dye plot and persists along field margins for years.
Science & Biotechnology
The leaves contain indican, a glucoside that hydrolysis and oxidation convert into indigo and its red isomer indirubin, and this is the chemistry that made woad a European industry for centuries before tropical indigo displaced it. Indirubin turned out to be far more interesting than the dye trade required. It inhibits cyclin-dependent kinases and glycogen synthase kinase-3, and in cell culture it blocks replication of Japanese encephalitis virus at concentrations around fourteen micrograms per millilitre, apparently by interfering with viral attachment, while protecting mice from a lethal challenge. Related work shows it restores interferon signalling through the MAVS pathway in influenza-infected cells.
A synthetic derivative, meisoindigo, has been used clinically in China for chronic myeloid leukaemia, which makes indirubin one of the clearest cases of a dye-plant alkaloid becoming a drug lead. The root, Ban Lan Gen, and the leaf, Da Qing Ye, carry a different and broader reputation in Chinese medicine, and both have been examined for antiviral and antipyretic activity. There is a taxonomic wrinkle worth knowing. Isatis indigotica is treated by some authorities as a synonym of Isatis tinctoria, the European woad, and by others as a distinct Chinese species with a different chromosome complement and a different alkaloid balance. The debate is not settled, and it matters commercially because the two are sold under the same names.
Uses & Benefits
Woad has two careers, and they are almost unrelated. The first is as a dye. The leaves are fermented, the resulting paste is dried into balls, and the dye is reduced in an alkaline vat to a soluble form that fixes onto wool and silk as a fast, slightly greenish blue. The old European process was foul-smelling and labour-intensive, which is exactly why indigo from the tropics replaced it. The second career is medicinal. Ban Lan Gen is one of the most widely taken Chinese herbs for the early stages of colds and sore throats, usually as granules, and Da Qing Ye is used for more developed heat conditions with rash.
Both are cold and bitter, and neither is a tonic. They are taken for a short course at the start of an illness, not as a daily supplement, and they are considered unsuitable for people with weak digestion or a cold, deficient constitution. Woad also yields a yellow dye from the same leaves if the processing is stopped early, and the seed yields an oil that has been used in lamps and soap. In the garden it is a useful plant for the dye bed and the wildflower strip, and its flat yellow heads are visited by small bees and hoverflies.
Growing from Seed
Sow woad where it is to grow, because the taproot makes transplanting a gamble. Late summer sowing suits it best: the plants overwinter as rosettes, which is what you want for a leaf crop the following spring, and they flower and set seed in the second summer. A spring sowing works too but produces a larger stem and less leaf before the plant runs to flower. Scatter the seed thinly on prepared ground, cover it lightly, and water until it is up, which usually takes ten days to two weeks. Thin the seedlings to about twenty-five centimetres apart.
The species self-sows freely once established, and a patch left to its own devices will maintain itself for years, moving slowly around the garden as the seed falls. It wants full sun and drains well on almost any soil, and it is hardy enough to shrug off hard frost. Cut the leaves for dye in the first autumn or the following spring, before the flower stem rises. If seed is the object, let a few plants run to fruit and collect the pods when they turn brown but before they shatter.
Our Take
Woad is worth growing for the dye alone, and the medicine is a bonus that comes from the same ground. It is a biennial, so plan the patch around that rhythm rather than fighting it.
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