This year I’ve had two reports of people in the UK growing Japanese Indigo from our seeds and experiencing premature flowering in May. I also read with interest Deb McClintock’s blog post about her early self-seeded Japanese Indigo (in mid-Texas in the USA) which germinated in early February and flowered in May. In the first two cases the plants were not remotely big enough to harvest, so this is very disappointing – doubly so since once the plants put out flower heads the amount of indigo in the leaves starts to decrease. Deb McClintock was able to harvest her “bonus crop” but she noted that it had not grown as well as her normal summer crop. Ideally the plants should grow to a large size before flowering in August/September.
So what is going on and what can be done to prevent this?
In all plants, flowering is often stimulated by the experience of stress e.g. unnaturally high or low temperatures, insufficient water or nutrients and pest damage. However, in the above examples the plants were being treated very well. My first thought was that the main cause of the early flowering was day length. In all three cases the Japanese Indigo seeds germinated in January/February when the day length is short. However, I can’t rule out low temperatures since this is the middle of winter in the UK (latitude=50deg N) and the night-time temperature in Mid Texas is going to get pretty low. Also in the UK in 2019 we experienced a Spring with extreme temperature swings and unusually late frosts.
I can find no definitive information online to suggest that low temperature is a known trigger for flowering in Japanese Indigo. But looking in my copy of ‘Handbook of Natural Colorants’ I see that day length is reported as the critical factor for Japanese Indigo, but other influences are not excluded. However it does say that Chinese Woad (Isatis indigotica) will flower if it experiences night time temperatures below 10°C (50°F).
In all three reports of early flowering, the plants received no artificial lighting and were grown in relatively frost free conditions, although they probably did experience night time temperatures well below 10°C. But of course the day length conditions in mid-Texas are considerably less variable than in the UK. (Mid-Texas latitude is approx. 35 degrees North compared to approx. 50 Degrees North here in Southern England).
The Handbook of Natural Colorants recommends sowing Japanese Indigo in April in mid Europe, with a first harvest in July or early August just before the flower heads start to form. It’s interesting to note that by this point in our summer, the day length has started to shorten but night-time temperatures are usually still over 15°C (60°F), so that should rule out cold temperatures as a trigger and points the finger at shortening day length.
However, my own observations in Hitchin for 2018 is that flowering was very late (September/October) following an exceptionally hot long Summer and warm Autumn. This is a much later flowering than normal. This is confusing because it suggests that the high temperatures prevent day length from being the dominant factor in triggering flowering. No doubt someone out there (perhaps the professional Japanese Indigo farmers) will know the definitive answer but at the present time I’m not sure if it is short day-length or temperature stress which is the most important trigger for flowering. But in either case the solution is the same.
In Southern England, no matter how warm the early Spring, don’t be tempted to sow your Japanese Indigo seeds before April, unless you are able to give them some artificial light to lengthen their day. Giving them a little warmth (particularly at night) is a good idea too. If the plants do start flowering early, pinching out the whole top of the plant may reset them back to vegetative growth. If the plants are large enough it may also work to take stem cuttings in water (cut out the flowering tips). If these measures fail, new seeds can still be sown as late as June in the South of England.
In my own case, I use heated seed trays and artificial lighting on a timer.
by Ashley Walker Copyright August 2018 Banner photograph copyright Sharon Cooper
On the 9th August, after two months with barely a drop of rain, the heatwave and drought in the South East of England may finally have come to an end. Despite regular watering the unnatural weather has taking its toll on our dye plants. For the first time our woad plants are being eaten by Cabbage White butterfly caterpillars (Large White Pieris brassicae and Small White Pieris rapae) and more recently by flea beetles (genus Phyllotreta). I guess the critters were getting desperate to find plants with a bit of juice in their leaves. The weather is having an impact on me as well, I have to water the indigo nearly every day and keeping the rest of the garden needs water too so I’m spending hours each week that I’d rather be spending on writing or dyeing.
This is the first part of a two part post on observations of the dye plants in our garden. This one deals with the plants we have been growing for more than a year. The second part will cover new plants.
European Woad – Isatis tinctoria
These Large White caterpillars managed to eat the whole woad leaf, leaving only the midrib behind.
We have grown Woad for about 12 years now and for the first time our plants have been attacked by caterpillars and flea beetles. This makes a change from the usual small black slugs which put a few holes in the leaves but seldom do any serious damage.
A cluster of Large White butterfly eggs on the underside of a woad leaf
Shiny black small Flea Beatles can slowly chew their way through a woad leaf leaving it like a sieve.
Woad flower spike August 2018 – from seed to flower in one season as a result of pampering.
I expect that the extraordinary hot weather is to blame with the butterflies and beetles probably acting in desperation. Although the flea beetles appear to thrive, the caterpillars have had a much harder time digesting the unpalatable leaves and most of the newly hatched critters have simply died, leaving a few small holes in the leaf. Only one plant had its leaves reduced to its midrib but even this one will survive as it is now putting out new growth. Interestingly it appears to be only the plants I have watered which are being eaten. There are a few plants which never get watered and these are looking fine.
A few people have asked about growing Woad in tubs or containers and this year we’ve had a few in containers ourselves and this has revealed a problem. One of our plants grown in a container in good compost and watered and fed regularly has grown large and is currently putting out a flower spike which will drastically reduce the amount of indigo in its leaves. Its very unusual to see Woad flowering in August so I can only assume we have pampered it too much – given it the ability to grow large enough to flower in one season. So if you are growing Woad in containers don’t give them too much fuss!
Chinese Woad – Isatis indigotica
Chinese Woad – about as big as it gets before flowering
Planted out in April these Chinese Woad immediately produced flower stems
We have been growing this for two years now, desperately trying to find out how to stop it flowering a few months after planting. From what I’ve read I’m in good company and this is the chief reason Chinese Woad has not caught on as a source of indigo, despite the fact that it could potentially produce as much dye as Japanese Indigo. Some of the literature indicates that botanists think Isatis indigotica is basically just a variety of tinctoria (European Woad). However, if that is so it has evolved away from tinctoria to a considerable extent. Indigotica is clearly adapted to a much warmer climate and although still nominally a biennial it behaves much more like a half hardy annual. It will flower at any time of year, even in winter, so its rosette stage is always very short and the plant never gets very big. The leaves are a paler blue-green than European Woad and its yellow flowers will continue to be produced throughout the year provided the plants are watered and taken care of. Once the plant starts to flower the larger rosette leaves die off leaving only small leaves on the plant which are probably no good for dyeing. According to the Handbook of Natural Colorants, indigotica will be triggered into flowering if the night time temperature falls below 5°C, which makes it almost impossible to grow the plant to any respectable size here in the UK. Even in Mediterranean climates the plant can only usefully be grown in the Summer. From my experience the plant will flower even if you just look at it the wrong way so I’m coming to the conclusion that it’s not worth the effort. It does grow very quickly however and if you were to grow it en masse and harvest the leaves before it flowered it might just provide a return for your efforts.
Another problem with Chinese Woad is its susceptibility to pests. Caterpillars and aphids like it very much and can easily destroy your plants. And you guessed it, significant insect damage will also trigger flowering. In a mad moment I decided to see if Chinese Woad tasted any nicer than European Woad. But the taste test settled nothing, both plants are extremely bitter and fiery. I obviously don’t have the finer tastes of Cabbage White butterfly caterpillars!
Japanese indigo – Persicaria tinctoria
Long leaf variety of Japanese Indigo with curled leaves to protect itself the prolonged hot sun of 2018
This year we are growing the same three varieties as last year – Long Leaved, Broad Leaved and an Intermediate Leaved white flowered variety. There appears to have been no interbreeding from last year. This year the difference between the long leaf and broad leaf varieties is stark. The Long Leaf plant is very vigorous with dark green leaves. The Broad Leaf variety took a long time to get going as usual and suffered from its leaves turning red. I was initially confident that the red colour was partly due to the hot sunny weather we were having in early summer – the slow growing plants were getting roasted. But after a good feed (with chicken manure pellets) the plants started growing quickly with the new foliage a nice mid green despite the continued hot sunny weather. So a bit of a chicken and egg situation: was it the lack of fertilizer that caused the leaves to redden or simply that the young plant leaves, growing slowly, were getting a longer exposure to the hot sun? The Long Leaf variety reacted differently to the hot sun with leaf curling , something I had seen last year but only on plants grown in the greenhouse.
A bed of intermediate White flowering Japanese Indigo.
Newly planted out Broad Leaf Japanese Indigo with sun reddened leaves.
Thus far we have only harvested the Long Leaf variety and used it in a little experiment comparing Jenny Dean’s extraction technique with the more often used long soak in cool water. The results will be written up in a later post. What I have also noticed is that we are currently getting a considerably better production of indigo from Woad than the Long Leaf variety of Japanese Indigo. Woad is well known for giving better results when the weather is hot and sunny. If the climate change predictions are correct and we continue to have hot summer weather then I think we would be better to return to growing mostly Woad. The Long Leaf variety of Japanese Indigo produces the least amount of indigo dye of the three varieties (see comparison here) but it does produce larger plants so perhaps still produces an equivalent amount of indigo per square metre.
Madder – Rubia tinctorum
Once again this year the madder plants are producing masses of berries. This is the third year running. In the previous 10 years or so the plants produced only a few. I have no explanation as to why this is. I’ve grown plants in different soil, in planters and in the ground and all plants are doing the same. A result of the weather?
Madder plant obtained from Southwark Cathedral in early 2018.
This year we obtained a new madder plant sourced from Southwark Cathedral dye garden. The plant is quite different to plants I have been growing up to now (all of which were derived from a single seed over 10 years ago). This new plant has paler leaves with a different shape and it flowers about 3-4 weeks later. It will be interesting to see if the root yield is also different. I’m pleased to have been able to increase the genetic diversity of our madder as I’ve always propagated by root stem cuttings or from seeds from my own plants.
Wild Madder – Rubia peregrina
Wild Madder in flower – Early July
We’ve been growing this plant for nearly three years now. It’s an evergreen but the tops do not appear to be totally hardy in the UK climate and were damaged by the winter frosts. This is the first year in which the plants (originally obtained from a wild flower nursery) are starting to look a bit happier. They are putting out new shoots from underground stems and flowering for the first time. It remains however a very slow growing perennial and I think it will take longer than Common Madder to produce a good root harvest so we are leaving it for another year.
I was given some seed from a friend from some wild plants growing on the south west coast which nearly all germinated though it did take well over a month before the first shoots appeared.
Saw wort – Serratula tinctoria
Saw-Wort plants with yellowing of leaves.
A self seeded plant with dark green leaves growing next to the transplanted ones with yellow leaves.
This native plant continues to be disappointing. Not only do the plants remain small but about half of them suffer from bad yellowing of the leaves once planted out in the garden. I have tried practically everything to remedy the problem – fertiliser, Epsom salts and seaweed extract. There are some self-seeded plants which look very healthy so I do wonder if the roots are somehow getting seriously damaged during transplanting. It also remains likely that there is something wrong with the soil itself as other plants (Genista, a red scabious and a Purging Buckthorn shrub) are similarly affected.
A comparison of our main yellow dye plants. Top is Weld, Bottom Right is Genista and Bottom Left is Saw-Wort
We did try dyeing with the Saw Wort this year and obtained a good buttery yellow. We were hoping it would be a nice lemon yellow like Weld and Genista so were a bit disappointed with that too.
Bumble bee on single type dahlia grown from seed.
Dark Red Dahlia giving pinky purple and greens. Possibly “Nuit d’Ete” or “Black Cat”
The colour of Dahlia flowers has an effect on its dye but we did not appreciate by just how much until this year when we tried using some deep red flowers to dye with. We obtained nothing like our accustomed strong yellows with acid pH and strong orange with alkaline pH. This time we got green with alkali and blue/purple with acid indicating that the dyes in this dark red flower were the same as you find in red cabbage and some other red flowers. These dyes, although very pretty, are not light fast. Over the years of growing Dahlia we have narrowed down the varieties that produce the best results for the home dyer. These are yellow or orange double flowering pom pom types. The pom pom flowers are longer lasting and produce more dye – some pom poms are very large and yield a lot of dye but bees and pollinators are unable to assess the nectaries. We have tried to stay away from these but there’s no doubt they are the best for dyers.
Tansy – Tanacetum vulgare
Tansy needs regular watering for healthy plants.
Often used by Scottish dyers as a source of yellow dye this plant has been growing in our garden for several years now but largely unused because the plant wasn’t very vigorous. There was never enough plant material to harvest and the flowers were disappointing. This year we planted a bed of Japanese indigo alongside so the Tansy benefitted from being regularly watered. The resulting Tansy flowers have been lovely so if you’re growing them keep them watered for best results.
Perennial Coreopsis – Coreopsis grandiflora varieties e.g. Golden Joy, Sun Ray, Early Sunrise
Perennial coreopsis – plant breeders benefitting the plant dyer.
Bright orange on alum mordanted wool blanket.
These are double flowering perennials with deep orangey yellow flowers which produce a lot of dye. They are not as hardy as the growers would have you believe as half our plants died during the winter and only a few have recovered enough to put on a good show this year. However, many can be easily grown from seed so are not too expensive to grow. They make excellent bedding plants and produce a fabulous orange dye from the flowers. An example of the plant breeders unwittingly aiding the home dyer.
Dyer’s Alkanet – Alkanna tinctoria?
Alkanet root. Bottom tip has had thin outer black bark removed revealing the dissapointingly white root.
This is the third year of growing and though I have not tried to extract any dye from its roots I am deeply disappointed to find that the roots are not red as they should be. I was suspicious as soon as I started to grow the plant from seed bought from the German Company Rühlemann’s. The plant seemed too vigorous with over large leaves and not hairy enough, but I persisted with it until it flowered. The flower shoots were tall (up to about a metre high) and not at all like the creeping wild flower growing around its native Mediterranean. The flowers when they finally appeared were the only part of the plant that looked like the pictures of Alkanna tinctoria seen all over the internet but the roots? The roots were white!
Doing some reading around this ancient dye plant I find that its qualities as a medicinal plant derive solely from the coloured substances in the root which were used as a dye, cosmetic and bio stain so you can imagine the way I feel after lavishing attention on this plant for the last three years only to find the roots are white! Recently I discovered one internet comment on the plant that says the cultivated version of the plant does not produce as much dye as the wild type. Well that’s some understatement. Of course it is possible that lavishing attention on the plant was entirely the wrong thing to do and I should have left it alone but it seems more likely that the growers have simply selected the seed year after year from the largest prettiest plants and in so doing have bred out the qualities that gave the plant its historical value.
Just to confuse matters Alkanna tinctoria has been and is also known as “Anchusabracteolata, Alkanna tuberculata, Alkanna lehmanii, Lithospermum lehmanii”, and has been given various common names as follows Alkanna Radix, Buglosse des Teinturiers, Dyer’s Bugloss, Henna, Orcanète, Orcanette, Orcanette des Teinturiers, Orchanet, Radix Anchusae. Rühlemann’s who sell the seed are now calling it Alkanna tuberculata. There is certainly confusion on the identity of all these plants. Are they all the same or not. If there are any botanists out there who can get to the bottom of this please please get in touch!
Philip John and Luciana Gabriella Angelini – Indigo – Agricultural Aspects. Chapter 7 of Handbook of Natural Colorants Edited by Thomas Bechtold and Rita Mussak. Wiley Series in Renewable Resourses. (Available as free download).
Rühlemann’s This German herb plant and seed supplier has a number of dye plants for sale including Chinese Woad and Long Leaf Japanese Indigo but it is primarily interested in the medical properties of the plants it sells and I get the impression they know little about plant dyeing.
Japanese Indigo, Persicaria tinctoria or Polygonum tinctoria is a frost tender member of the knotweed family. Originally from China and Vietnam it likes to grow in warm moist climates, often as a waterside plant. Our closest relative in the UK is Common Bistort or Persicaria bistorta which closely resembles Japanese Indigo but has no trace of indigo in its leaves (I did try once) though it does give a pleasant orange on alum mordanted material. Japanese Indigo will grow from seed to flower in one season and, if it does not get frosted, is capable of growing into the second year though here in the UK getting a plant through the winter is very difficult even indoors with extra lighting. We grow Japanese Indigo because it gives more indigo than Woad (approx. double the amount) and the colour is generally cleaner and more predictable than Woad.
Varieties of Japanese Indigo
There appear to be two distinct varieties though I have not seen anyone put a name to them.
See comparison here.
Pointed or long leaf Japanese Indigo. This is grown as an indigo dye crop in Germany and has probably been selected for its ability to grow in a northern climate. It is a more robust plant tolerating colder weather and flowers much earlier. It has pale green occasionally pinkish stems and white or pink flowers which grow on elongated flower stems. The leaves are large narrow and pointed.
Broad Leafed Japanese indigo
Broad or rounded leaf Japanese Indigo. This is the variety most commonly grown in gardens, perhaps because it produces a profusion of pretty deep pink or white flowers. Its stems are thick and also have a tendency to be pink. It seems less adapted to a northern climate and in poor weather struggles to grow – when first planted out, the leaves often go pink or yellow at the tops of the stems and continue like this if growth is slow. When grown in a greenhouse or polytunnel however the plants are greener, although I have not noticed any difference in indigo production between the two, (see link to article on comparison of three different strains above).
Comparison of Broad leaf and Long leaf Japanese indigo.
Growing Japanese Indigo
Some of my seeds are “tricotyledons” and these germinate with 3 baby leaves rather than the usual 2. This is an uncommon mutation and may be a sign of inbreeding.
The shiny black seeds have only a short life (about a year if stored in a cool dry place). They can be frozen in the freezer and will last longer. To freeze the seeds first dry on a windowsill then place in a sealed plastic bag with a packet of silicon desiccator for a few weeks then pop in the freezer. Note: seeds frozen in this way tend to germinate slowly and may take up to 5 or 6 weeks to germinate. Commercially bought seeds are often stored frozen so don’t be too alarmed if you get no germination’s in the first 2 weeks. Interestingly I tried drying some seeds with a hair dryer on a low heat setting from a distance and found that this treatment also put the seeds into a deep sleep.
So, using some fresh seed grow in heated seed trays indoors from early April or late March. Sow thinly and cover with a thin layer of compost. Once germinated try to give them as much light as possible as they will grow “leggy” if kept on an average windowsill. The seeds should germinate readily within two weeks but occasional seeds may not germinate for a month.
Seedlings with secondary leaves at around 4 weeks
The plants are very robust and rarely succumb to disease or pest, they will also transplant readily and can be pricked out at an early stage and potted up if you wish to do this. I usually leave them to grow in the tray until I am reasonably sure there will be no more frosts. Here in Hertfordshire that’s usually around the 6th May. By this time the plants will be quite large and the trays root-bound, some of the plants will have put out secondary roots from the stems and rooted in more than one place. Separating the plants out is therefore difficult and it is best to soak the tray in a bucket of water before trying. Do not worry about breaking the roots as Japanese indigo will re-root itself with ease. Planting in May is only advantageous if the forecast is for warm and sunny weather. If the forecast is for cold overcast weather wait until June before planting out.
Trays of dye plants waiting to be planted out including some root bound Japanese indigo.
These plants stayed much the same size for 2 months before finally starting to grow in August of 2016
These Japanese indigo plants show a typical growth pattern when planted on mass with the larger plants towards the middle and smaller plants at the edge.
Weather is critical for good plant growth as you must remember this is a semi tropical plant. Sometimes the weather never really becomes ideal, in the cool dark summer of 2012 the plants grew hardly any larger than when I put them out. The plants were pale and looked sickly and there was no harvest. This year (2016) many of the plants also grew very slowly because the weather in the early part of the season was cool, wet and cloudy. It was not until late July when the weather improved that they really started to grow. If you have plenty of seedlings you can plant them about 4 inches apart in a heavily manured soil. Both Woad and Japanese indigo require lots of feeding to get a decent sized plant. Manure is the best if you can get it, but other fertilizers will do. Soil type is not much of an issue and the plants will grow on any soil. Clay loams are probably the best. My plants are grown on a poor chalk soil and I have problems with some nutrients in the soil being used up very quickly. Crop rotation is important for me as the plants do not seem to grow well if planted in the same area as the previous year. All my indigo beds need to be given a second feed in August particularly if I cut a first crop and allow the plants to regrow. Even so, many plants regrow with yellow leaves indicating they are short of some vital nutrient.
Rows or Beds? – I mostly grow indigo in beds on mass simply because I have limited room in the garden. Every year however, I notice that the plants at the edge of the beds grow weakly and the plants in the middle are the largest of all. I do not know the reason for this but speculate that the plants thrive best when in competition with each other or there is some symbiotic relationship which allows the plants to benefit each other. I am inclined to the former as the plants also tend to grow larger when in competition with entirely different species. Either way I also speculate that growing the plants in rows is not ideal as the plants are not as close to each other.
Roots grow readily from the stem nodes.
Propagation via cuttings – If you only have a few plants and want to grow more from cuttings you are in luck as Japanese indigo is one of the easiest plants to grow this way. Just cut off the stems from about 6 inches above the ground and place in a container of water. New roots will grow immediately from the stem nodes and the cuttings will be ready to plant out in two weeks. Alternatively just stick the cuttings directly into the ground and water well (each day in hot weather) until the new plants are established.
Overfeeding – I used to think that it was impossible to overfeed Japanese indigo but in the summer of 2017 I did overdo it a bit with some fish, blood and bone fertilizer which is particularly concentrated. The plants can become scorched, deformed and yellow. Initially the damage looks very much like early frosting as the leaves die off, turning the characteristic gray blue. Later the surviving leaves turn yellow and can become deformed. If they survive they will eventually recover without any lasting problems.
Early leaf damage due to over feeding with Fish, Blood and Bone fertilizer.
Yellowing of leaves due to overfeeding.
Weeks later plants still showing damage – stunting with deformed and yellowing leaves
Under-watering – Easily done if you go on holiday or just forget during hot sunny weather. Once again the effect on the plants is similar to frosting and overfeeding.
Frost Damage – After the first frosts of the Autumn the upper exposed leaves are usually damaged first and turn a dark blue-gray. The indigo has become fixed into the leaf and can only be extracted if the leaves are placed in a reducing dye bath. (Reducing agents are fructose, spectralite (Theourea dioxide), sodium dithionite or a fermentation bath).
Early Frost damage on Japanese indigo showing characteristic blue gray areas where the plant tissue has died.
Maintenance and harvest
As a waterside plant Japanese indigo is used to having its roots in water and it follows that it will grow very poorly in dry well drained soils. In the summer months of June to August I have to water my plants nearly every day because I have a light chalky soil. Once the plants achieve total ground cover they act as a shade and help keep the soil damp but they still need water on a regular basis.
Take the first cut about 8 inches above ground leaving plenty of leaves on the plant.
In experiments growing some Japanese indigo amongst other plants. I was surprised how well it responded to the competition, growing taller and greener. In a good year it is possible to take two or even three harvests of the plant. The stems are cut about 6 to 8 inches above the ground and the plants soon grow back.
For an added boost to your harvest once you have stripped the leaves off the stems the now leafless stems can be placed in buckets of water with a dollop of liquid fertilizer and they will grow quickly back. The stems alone have no indigo content.
Stems of Japanese indigo after leaves have all been stripped off.
The stripped stalks when placed in buckets of water with added liquid fertilizer will rapidly regrow and provide you with an extra harvest.
Indoors or outdoors and growing for seed
Growing in a greenhouse it was possible to plant the seedlings out earlier but even then a mild frost in early May damaged and killed some of the plants.
Frost damaged seedling in the greenhouse in May 2016.
Japanese indigo does not need to be grown in a greenhouse or polytunnel unless you live far to the north but it clearly prefers being indoors growing lushly with greener, larger leaves and does not produce flowers until later so has a longer season.
Japanese indigo needs a long growing season to flower and usually only comes into full flower in October. It is self-fertile so will produce seed in a sealed greenhouse but will flower sooner outside in full sun. However, If frosts or bad weather are forecast before the seed has set be ready to dig a few up and transfer to greenhouse or poly-tunnel or bring indoors in pots. In the North it may be best to grow Japanese Indigo in a greenhouse or poly-tunnel. But because it flowers later it may not be possible to get seed unless the greenhouse is heated.
Some growers have reported that it is possible to get plants through the winter by bringing them into a well-lit and warm area where they will flower early the following year and produce seed.
The different varieties of indigo respond differently to greenhouse growth as the photo below shows. This particular variety flowered at much the same time in or out of the greenhouse. The leaf curl may be a response to the extra heat.
The long leaved variety growing in the greenhouse with curled leaves.
This miserable looking plant flowered in June and as you can see has been highly stressed with stunted growth and pale leaves with a pinkish blush
There are always a few plants that come into flower before the rest and it is best to leave these alone when you harvest. A few years ago I began an experiment to try and breed a variety of early flowering plants so I could be sure of getting some seed even in a bad year. This went according to plan and I ended up with plants that flowered in early September and some in August. However, once the plants start to flower the amount of indigo in the leaves starts to reduce and in full flower the yield is very poor. This led to lower harvests overall. Another strategy you can use for getting seed is to grow a few plants in a dry bed only giving them enough water to keep them alive. These plants will become stressed and will flower earlier – they may look miserable but the seed will produce nice healthy plants next year. Interestingly when the plants are in full flower they are very attractive to bees particularly honey bees. I wonder if honey can be obtained from the German fields of Japanese Indigo, as is possible with the fields of Woad in Norfolk.
Seeds from the flowers in the greenhouse. As bees and most pollinating insects could not get into the greenhouse it looks like the the flowers are self fertile.
When the flowers go brown they can be cut and hung up or laid out to dry and some of the seed will fall out. The remainder can then be rubbed out. Separating the seeds from the “chaff” is a skill all by itself. Once you have removed the seeds and dried flower material from the stalks, place the whole lot in a tray and shake from side to side. All the heavy seeds will settle to the bottom and if you are careful you can blow the chaff from the top. This can be a dusty business so you must be careful not to breath it in. Some of the seed will retain an outer layer of brown chaff bound to the seed; this does not impair germination.
Take a few handfuls of leaf and crush them up in a suitable container or blender. Add some white fibre immediately to the mix and leave for an hour. The colour obtained is rather dull and some of the colour may wash out. What remains will be fast and will give you a good indication of how good your indigo is.
When new to dyeing with Japanese Indigo and Woad it is common to wonder how indigo was discovered as the plants apparently do not produce anything obviously blue though occasionally leaves will die and turn a dark grey or blue black. I suspect that someone observed that when crushed the fresh leaves will release indigo but it is only noticeably blue if you then add some white fiber to them. This is a good test to see if your indigo is ready to be harvested.