By Ashley Walker
Copyright Natures Rainbow. March 2021
What is Chinese Woad?
The origins of all Woad are thought to be in central Asia. However, Chinese Woad is generally considered to be a subspecies or variety of Isatis tinctoria (European Woad). A review paper published in March 2020 concluded that despite conflicting views it was the same species. It does have some distinct differences to European Woad both morphologically and genetically so if it can be counted as the same species then it’s evident that the Chinese population has diverged considerably from the European. I detect a political influence on the argument as European papers are more likely to state that the two plants are the same species and Chinese papers more likely to say they are different species. If I had to make a choice I personally would go with the Chinese (see here) but hey, I’m not a botanist.
Chinese Woad is grown mainly as a medicinal herb and has attracted recent attention as a possible remedy for COVID-19. The roots, and sometimes the leaves, are used as an anti-inflammatory, anti-bacterial and anti-viral medication. See here and here. Its use as a dye plant has come to the attention of western natural dyers, as it produces more indigo per unit weight of plant material than European Woad.
We have been growing Chinese Woad for about four years and discovered that its promise is far from easy to realise. So let’s just begin by looking at the differences between tinctoria and indigotica.
Physical Differences – Morphology
Chinese Woad is a much smaller plant than European Woad. It may be possible to make it grow larger in sub-tropical or tropical climates and I would certainly like to hear from anyone who has done this. The leaves are a different colour to European Woad – more a pale greyish-green. Photos often do not show this difference well. The structure of the leaves is markedly different too. Just look at this photo comparison of the upper and lower leaf surfaces.
Habit and propagation
In our temperate climate Isatis indigotica behaves much more like an annual or weak perennial and nearly always flowers in its first year. In fact once it has started flowering it will often continue all through the year and even through winter (unless killed by frost) until it dies, usually in its second year. Never-the-less it is still classified as a biennial. Perhaps it behaves like a biennial in its native China. I have been unable to find out. According to Philip John and Luciana Angelini in the Handbook of Natural Colorants (2009), it is a very sensitive plant which will flower if exposed to night time temperatures below 5ºC. They also report it as less resistant to being harvested, managing only four harvests to tinctoria’s seven when grown in Southern Spain. The most I have managed is two harvests. I can also add that any disturbance to its root during transplantation can also cause it to flower.
Typically it will grow from seed quickly to produce a rosette of leaves just like tinctoria. However after 2 – 3 months it sends up a flower spike and the large rosette leaves begin to die off. For the dyer this is very annoying because it catches you out, particularly if you have been used to growing tinctoria which happily produces huge leaf rosettes without any flowers in sight during its first year.
Resistance to pests
Indigotica is particularly prone to pest attack. Cabbage white butterfly caterpillars eat it, flea beetles chew holes in it and aphids suck its sap. Pretty much anything that likes eating cabbage family plants will eat indigotica. I even tried eating it myself to see what all the fuss was about but I can tell you it tastes just as disgusting as tinctoria.
Indigotica is less tolerant of frosts than tinctoria and will be damaged or killed outright if the temperature falls much below about -3˚C. In the winter just gone (February 2021) we had a week long cold spell with temperatures down to -5˚C and our plants were killed off completely.
Genetics and diversity
Not that you are likely to notice without the facilities of a genome analysis laboratory, but the genes of indigotica and tinctoria are quite different. According to one paper (see here) it is more genetically different to tinctoria than other Isatis species.
Genetic diversity is the variation found between plants of the same species/variety. Indigotica shows less genetic variation than tinctoria (see here). This is noticeable in the outward appearance of the plants. Tinctoria plants show some considerable differences sometimes.
A strategy for growing a useful quantity of Isatis indigotica in temperate climates
To get a useful amount of indigo dye from the young leaves of indigotica a special strategy must be used in the UK and other similar growing zones. (note: our growing zone is 8 and in China it is grown in zones 6 to 7).
The objective is to grow as many plants as possible and prevent them from flowering. When buying seed you may only get a few seeds in the packet because most suppliers sell the seed for herbal medicine rather than as a dye plant – so unless you have access to bulk supplies of seed the first thing you need to do is grow your plants for seed until you have enough.
You will need about a thousand seeds. This may take a whole year. Sow the first lot of seeds indoors or under cover in April and plant out in June. Most of these plants (probably all) will flower and be producing seeds throughout the rest of the summer. No need to worry about any cross fertilisation with any European Woad because their flowering times are very unlikely to overlap. European Woad flowers in April and will have produced seed by the time your Chinese Woad starts to flower. Collect the seeds as they ripen until you have enough and keep in a cool dry place until the following year.
Once night time temperatures are unlikely to fall below 5˚C (typically early June in Hertfordshire) prepare a well-fertilised bed with a minimum size of 1m2. Sprinkle the seed quite thickly and rake in gently. Water every day until the seed germinates. Sowing directly into the ground minimises disturbance to the tap root. Transplanting seedlings which have been grown in seed trays or modules may damage roots and stimulate flowering.
Keep your plants happy for the next two months by watering regularly and adding fertiliser if the plants need it. By August the plants will be ready to harvest. They will still be small (much smaller than European Woad). Pick the leaves taking care to avoid uprooting the young plants. Cutting with scissors may be the best method. You can then use whatever method suits you best to extract the indigo (hot or cold) from the leaves. 1m2 of young plants will provide enough indigo to dye a pleasing amount of fibre. If you are lucky you may get a second harvest before the plants go to seed. Collect seed for the following year.
Note: there are many other indigo-bearing Isatis species which are closely related to tinctoria. A considerable number can be found growing in Turkey (See here). One of particular interest is Isatis glauca, a perennial woad which will grow in the UK.
Other growers experience
I managed three harvests last year from your Chinese Woad seeds – grown in lots of small pots, pinching out flower stems as soon as I spotted them. – Nicky Anderson
Research notes and references
For anyone interested in the Woad group of plants I have included below just a selection of the many scientific publications with a few abstracts on the subject. (My comments in blue).
Hybridisation in Plants and Animals
In the animal world closely related species (e.g. horses and donkeys) are generally easier to differentiate as they generally do not interbreed, and if they do (mules) the hybrid animal is often sterile. However, in the plant world closely related species very often do interbreed to produce hybrids which usually have viable seed. It is this fact which can often cause confusion in separating different plant species and subspecies.
In 2018 I wrote a short account which can be seen here. https://www.naturesrainbow.co.uk/category/chinese-woad/
Optimal experiments on the field cultivation of chinese Isatis Indigotica Fort.
KE Shao-ying, LIU Dong-lian, Zuo-dong, LIU Qing-hai
No doubt the Chinese researchers here were more interested in harvesting the root than the leaves which may account for the preference for a late harvest. Unfortunately the only publicly available section of this paper is the abstract as follows:
“The results showed that the optimal sowing time was from April 15 to 25th in Yutian county of Hebei province .The optimal space was 10 – 20 cm between plants, and 30 cm between rows. The optimal harvest time was from October 15 – 20th.”
Hebei province is around latitude 40ºN, the same latitude as Spain. Plant hardiness zone is around 6-7. This means it was grown in an area experiencing colder winter temperatures than the UK but a longer day length. At this latitude the sun is visible for 15 hours and 1 minute during the longest day and 9 hours and 20 minutes during the shortest day. In our more northerly latitude (London) we have 16 hours and 30 minutes for the longest day and 7 hours and 49 minutes for the shortest day.
Note: Hebei province is in Northern China. Beijing is close by. The Sars Covid 2 virus was first reported in Hubei Province In Central China.
Analysis of Genetic Diversity of Cultivated Populations of Isatis indigotica.
LI Yong-chaoa, REN Yong-xinb, CHEN Hong-enb, YANG Jinga, ZHOU Xiu-renb
Abstract: The genetic diversity of botanic characters within populations ranked from high to low as plant height, leaf area, leaf dry weight, root dry weight. The phenotypic difference among the 3 populations was not significant while the diversity within population was rich. The content of indigo and indirubin of S1 was 5.67 mg / g and 10.93 mg / g, of S2 was 4.74 mg / g and 9.68 mg / g, and of S3 was 5.32 mg / g and 9.27 mg / g respectively, the content of the 2 ingredients in the 3 populations was very significantly different(P 0.01) respectively.
A high degree of genetic diversity is revealed in Isatis spp. (dyer’s woad) by amplified fragment length polymorphism (AFLP)
More genetic evidence that Indigotica and tinctoria are different species.
Genetic diversity in populations of Isatis glauca Aucher ex Boiss. ssp. from Central Anatolia in Turkey, as revealed by AFLP analysis
Özlem Özbek, Elçin Görgülü & Şinasi Yıldırımlı
This paper on Isatis Glauca a perennial Woad and indigo bearing plant from Turkey.
Hardy and available in UK.
Morphological and agronomical characteristics of some wild and cultivated Isatis species
Süleyman KIZIL 2006
This paper also useful as it compares various other Isatis species. And says Glauca and tinctoria produce comparable amount of indigo.
High yielding indigo sources in native Isatis (Brassicaceae) taxa from Turkey
Indigo dye has a distinctive blue color and is one of the oldest known dyestuffs. In order to identify new Isatis genotypes for indigo production, 14 Isatis taxa collected from native area were grown under field conditions in Kahramanmaraş, Turkey. The effects of taxa and harvest times on leaf, indigo and indican yield were analyzed and compared with culture species (Isatis tinctoria and Isatis indigotica) by using spectrophotometer and HPLC. High indigo yield (87.13 kg/ha) was obtained from Isatis constricta and I. tinctoria subsp. tomentolla (Mersin location) had high indican yield (12.25 mg/g) in fresh leaves. The present work identified high indigo yielding taxa that may be used for genetic improvement in order to re-introduce Isatis species in the agricultural systems of Mediterranean regions. Especially I. constricta can be evaluated in terms of higher indigo yield than the native and culture taxa. I. constricta is an endemic plant in endangered category. This situation reveals that the need for the conservation of the species. To the best of our knowledge, there is no previous study reported about indigo contents of the studied 11 Isatis spp. so far
Isatis species is biennial, herbaceous shrub belonging to family Cruciferae and is represented by 40 taxa, of which 24 are endemic widely found in Turkey under natural conditions. East and South-eastern Anatolia regions represent 31 species and 14 sub species.
Differences in leaf yield and indigo precursors production in woad (Isatis tinctoria L.) and Chinese woad (Isatis indigotica Fort.) genotypes
Luciana G.Angelini Sabrina Tozzi Nicoletta Nassi o Di Nasso
Abstract: Isatis tinctoria L. (woad) is one of the earliest known sources of indigo in Europe where it was cultivated since the Middle Ages. Isatis indigotica Fort. (Chinese woad), widely distributed in China, had been used from ancient times as indigo-producing plant and medicinal plant. Both species produce indigo precursors indican (indoxyl β-d glucoside) and isatan B (indoxyl ketogluconate) in their leaves. In order to identify new suitable crops for indigo production in Italy, 17 woad lines were studied under field conditions in Central Italy (Pisa, 43°40′N, 10°19′E) from 2001 to 2003. We analyzed the effects of year, genotype, and harvest times together with their reciprocal interactions on leaf yield and indigo precursors production. Woad lines were then compared with seven I. indigotica lines in a field crop experiment set up in 2003. Extraction and quantification of indigo precursors were accomplished by HPLC-ELSD. Isatan B and indican content, as well as equivalent indigo and fresh/dry leaf yield, were compared between species and among genotypes.
In I. tinctoria wide variations in phytochemical and agronomic traits were observed among genotypes, with significant differences in isatan B (1–2 g kg−1 FW), indican (0.3–0.7 g kg−1 FW) and leaf yield per harvest (11–22 t FW ha−1). In I. indigotica significant differences were observed in indican (0.3–0.6 g kg−1 FW) and fresh leaf yield per harvest (10–20 t FW ha−1). Chinese woad showed higher isatan B than woad (4.9 and 1.5 g kg−1 FW, respectively). In both species isatan B represented the major precursor, particularly in I. indigotica. The ratio indican:isatan B recorded was 1:5 in woad against 1:14 in Chinese woad, leading to significantly higher +55% equivalent indigo in the latter. Interestingly, I. tinctoria showed good adaptation to Mediterranean climate conditions with high re-growth capacity after harvest and elevated biomass production. Conversely, I. indigotica, although its higher indigo precursors content/leaf weigh, appeared to be more affected by climate conditions and produced −25% leaf yield per hectare per season. The present work identified high indigo yielding genotypes that may be used for genetic improvement in order to re-introduce Isatis species in the agricultural systems of Mediterranean regions.
Transcriptomic Analysis Reveals Differential Gene Expressions for Cell Growth and Functional Secondary Metabolites in Induced Autotetraploid of Chinese Woad (Isatis indigotica Fort.)
Yingying Zhou, Lei Kang, Shiying Liao, Qi Pan, Xianhong Ge,
Abstract: Isatis tinctoria L. and Isatis indigotica Fort. are biennial herbaceous plants belonging to the family of Cruciferae that are used as a source of natural indigo and show several morphological and genetic differences. Production of indigo (indigotin) precursors, indican (indoxyl β‐D glucoside) and isatan B (indoxyl ketogluconate), together with seed germination ability were compared in Isatis tinctoria and Isatis indigotica grown under six different light conditions (darkness, white, red, far red, blue, yellow light) at 25°C. Light quality influenced both germination and production of indigo precursors in the two Isatis species. Different responsiveness to far red and blue light was observed. Indeed, a detrimental effect on germination by blue and far red light was found in I. tinctoria only. Different amounts of isatan B were produced under red and far red light in the two Isatis species. In I. tinctoria , the level of main indigo precursor isatan B was maximal under red light and minimal under far red light. Whereas in I. indigotica far red light promoted a large accumulation of isatan B. The photon fluence rate dependency for white and yellow light responses showed that the accumulation of indigo precursors was differently influenced in the two Isatis species. In particular, both white and yellow light enhanced above 40 μmol m−2s−1 the production of isatan B in I. indigotica while only white light showed a photon fluence dependency in I. tinctoria. These results suggest a different role played by the labile and stable phytochrome species (phyA and phyB) in the isatan B production in I. tinctoria and I. indigotica. I. indigotica , whose germination percentage was not influenced by light quality, demonstrated higher germination capability compared with I. tinctoria. In fact, I. tinctoria showed high frequency of germination in darkness and under light sources that establish high phytochrome photoequilibrium (red, white and yellow light). Germination in I. tinctoria was negatively affected by far red and blue light. I. indigotica seeds appear to be indifferent to canopy‐like light (far red). Our results provide further insights on the distinct behaviour of I. tinctoria and I. indigotica that belong to two different genetic clusters and different original environments.
An Improved Method for the Large-Scale Processing of Woad (Isatis tinctoria) for Possible Commercial Production of Woad Indigo – 1998
Kerry G. Stoker, David T. Cooke, David J. Hill
Abstract: The increasing use of alternative crops has meant the introduction of new technologies to process their products. In this work, an alternative method is presented for the extraction of natural indigo from woad (Isatis tinctoria) based on a technique used to extract indigo fromIndigoferaspp. This method does not rely on the old fermentation procedure used throughout Northern Europe and is cheap, clean and efficient. Evolved from laboratory-based tests, it involves steeping the leaves at low pH in warm water and extracting the indigo at a higher pH, followed by ultra-filtration of the product, which is then left to air-dry. Problems encountered during the development of the technique and how they were overcome are discussed.
Isatis tinctoria L. (Woad): A Review of Its Botany, Ethnobotanical Uses, Phytochemistry, Biological Activities, and Biotechnological Studies
Jasmine Speranza, Natalizia Miceli, Maria Fernanda Taviano, Salvatore Ragusa, Inga Kwiecie, Agnieszka Szopa and Halina Ekiert
MDPI Open Access Plants Journal
Abstract: Isatis tinctoria L. (Brassicaceae), which is commonly known as woad, is a species with an ancient and well-documented history as an indigo dye and medicinal plant. Currently, I. tinctoria is utilized more often as medicinal remedy and also as a cosmetic ingredient. In 2011, I. tinctoria root was accepted in the official European phytotherapy by introducing its monograph in the European Pharmacopoeia. The biological properties of raw material have been known from Traditional Chinese Medicine (TCM). Over recent decades, I. tinctoria has been investigated both from a phytochemical and a biological point of view. The modern in vitro and in vivo scientific studies proved anti-inflammatory, anti-tumour, antimicrobial, antiviral, analgesic, and antioxidant activities. The phytochemical composition of I. tinctoria has been thoroughly investigated and the plant was proven to contain many valuable biologically active compounds, including several alkaloids, among which tryptanthrin, indirubin, indolinone, phenolic compounds, and polysaccharides as well as glucosinolates, carotenoids, volatile constituents, and fatty acids. This article provides a general botanical and ethnobotanical overview that summarizes the up-to-date knowledge on the phytochemistry and biological properties of this valuable plant in order to support its therapeutic potential. Moreover, the biotechnological studies on I. tinctoria, which mainly focused on hairy root cultures for the enhanced production of flavonoids and alkaloids as well as on the establishment of shoot cultures and micropropagation protocols, were reviewed. They provide input for future research prospects.