This page may contain affiliate links. As an Amazon Associate I earn from qualifying purchases.
How to grow Japanese indigo and extract pure indigo pigment from its leaves. Includes tips on growing it from seed, harvesting, and processing the leaves for indigo pigment powder. This cold water method of indigo pigment extraction results in blue indigo powder, a natural blue dye. It stores well for up to five years and can be used for coloring fibers, cloth, wood, and handmade soap. DIY video included.
As a natural soapmaker and someone fascinated with useful plants, it was inevitable that I’d grow indigo. I use deep blue indigo powder to create naturally blue soap, whether that’s light blue or deep denim blue. Indigo is also an incredibly popular natural blue dye for wool and other fibers, as well as finished fabric and textiles. It’s color-fast and natural, and many natural dyers specialize in indigo alone. While there are a few pigment extraction methods and dyeing methods, I’d like to share the simple cold-water fermentation method I learned this year as a beginner. It works a treat, and I hope that it helps you, too.
Indigo Pigment Extraction
Indigo refers to various shades of deep blue to violet as a color, but it’s also a natural blue dye and the name we give to plants that are the source of the blue dye. You wouldn’t know that indigo leaves have such a vibrant blue color in them from looking at them. However, if you bruise a leaf and leave it to wilt, you can start to see a blue hue coming through. This is indican, a water-soluble precursor to indigo present in the plant’s cellulose tissue that’s turned into indigotin prematurely (for our purposes). Indican is a glucoside molecule made up of glucose and indoxyl. It’s the indoxyl part of the molecule that becomes indigo, and we can isolate it by fermenting indigo leaves. Wild yeasts eat the glucose part of the molecule for you.
That’s how, through fermentation in water, indoxyl is released from the leaves and is left behind. The water at this point is a vivid blue with a beautiful metallic surface and is referred to as mermaid liquid. We’re not done yet, though. Indoxyl is then converted into indigotin by exposure to oxygen, and this happens by using a whisk or pouring the liquid from one vessel into another. Being alkalized with soda ash helps speed the process up dramatically. The indigotin can then be precipitated from the water by allowing it to settle to the bottom. Then, it’s collected and dried into a powder. While this all sounds very scientific and mind-boggling, it’s not difficult to do at home as a beginner. I’ll take you through the steps later on.
The indigo pigment extraction process begins with growing indigo plants. You need fresh leaves to get a good yield, so this is not one of those cases in which you can order dried indigo leaves to make indigo pigment. Many natural dyers grow and work with Indigofera tinctoria, also called true indigo. However, it’s not one that wouldn’t do well in my garden. True indigo is a tropical plant that prefers hot and humid climates, and the hottest it’s ever been on our wet and windy island was 28.9°C (84°F). If you have hot summers, perhaps consider it and know that the extraction process described further below works for it, too.
Last winter, I discovered another type of indigo that would grow better for me. I learned about Japanese indigo Persicaria tinctoria, also called Chinese indigo or Dyer’s Knotweed, from Bailiwick Blue in Guernsey, where I then sourced my seeds. This plant has been grown and used as a natural dye for thousands of years in China and Japan and performs well when grown with protection in Britain. It contains the same vibrant blue color that true indigo does and has fared well in my polytunnel this year.
Japanese Indigo Growing Conditions
Growing Japanese indigo has been relatively easy, and I think that’s because if you know what a plant needs and give it to them, then you can’t go wrong. Japanese indigo loves sunny, warm, sheltered places with fertile, moist soil. Though it can stand a little shade, it will not grow well if the soil is ever dry, so the bed I grew them in had an inch of compost as a mulch. I also had the bed planted with automatic olla irrigation to serve as water reservoirs for the indigo’s roots. It seems to have paid off.
In my northern British climate, I grow it in my polytunnel, where it stays warm and protected from the wind. It could do well outdoors a bit further south, especially in urban areas like London, where it’s warmer and more like the temperatures of Seattle in the USA. Even though Japanese indigo is more cold-tolerant than true indigo, it’s still a sub-tropical plant. If you can successfully grow tomatoes outdoors, then I imagine that you could grow indigo, too. Also, though I have seen Japanese indigo grown in pots, it seems to do much better if in open ground or raised beds. It likes to spread out its roots under the soil.
Germinating Japanese Indigo
Sow the indigo seeds in free-draining in early March. I sowed mine quite thickly but shallowly in a small pot. A more conventional way would be in a row in a seedling tray with a propagator dome. The seeds then need warmth, moisture, and light to germinate. It can take two to four weeks for them to sprout, and during that time, keep the potting mix damp and warm. I had mine set inside my heated propagator with a grow light over them. You could set seedling trays on a heat mat, too. After they germinated, I let the seedlings grow in the small pot before pricking them out into 1.5″ modules. These I grew inside the house for another six weeks until planting them out in my polytunnel.
Since sharing my indigo video on YouTube, I’ve had several people say that they’ve tried growing indigo, but the seeds didn’t germinate. If this has happened to you, please be aware that indigo seeds have very low germination rates after the first year. There is some anecdotal evidence that older seed goes into a form of dormancy rather than dies. However, it’s not clear how to break it out of that dormancy to germinate readily. So, always begin with the freshest indigo seeds possible.
How to Grow Japanese Indigo
Plant out Japanese indigo in late May and/or well after your last frost. Nighttime temperatures should be above 10°C (50°F). Space them out at least a foot apart, but I think 18″ is better. I planted mine that distance, and they quickly filled in the gaps and became a thriving mass of indigo leaves. They don’t need support since you’ll be harvesting from them regularly after they put on enough height and leaves.
Japanese indigo can form roots from the many leaf joints along each stem. I even noticed roots forming from them while still high up in the air! What this means, though, is that you can readily propagate more Japanese indigo plants from cuttings. You can also bend the stems down and pin them to the surface of the soil so that they can form roots there. Both are handy tips in case you don’t have many indigo plants to begin with.
Harvesting Japanese Indigo
Begin harvesting Japanese indigo about a month after you plant it. This tends to be when you can cut a stem at least three inches above the soil, and what remains still has some leaves left. You might measure it that first harvest, but after that, it grows abundantly. I was grabbing up bunches of stems at a time and cutting them so that about six inches of stem remained. Japanese indigo continues growing and filling out after each cut, providing that you keep the bed well-watered and the soil is fertile. I just noticed that George from Bailiwick Blue just took her twenty-second harvest of indigo yesterday, in the third week of September! As long as new green leaves are forming, you can harvest indigo leaves for natural dyeing.
Natural Indigo Dye
Indigo is a brilliant blue dye that works well with silk, wool, cotton, and many other fibers and fabrics. It’s also one of the rare dyes that don’t require a mordant, and it creates light-to-dark blues that don’t fade with washing or exposure to light. The indigo dyeing process usually begins with creating an indigo vat and then soaking pre-wetted fabric or fibers in it. There are several ways to create an indigo vat, and I recommend the book Wild Colour by Jenny Dean if you want to learn more about indigo dyeing. It gives indigo growing, processing, and dyeing instructions alongside other types of homemade plant-based dyes.
It’s a fascinating process that begins with the blue powdered dye created from indigo pigment extraction. It’s mixed in pH-adjusted water and changes from blue to a pale greeny-yellow color through natural chemistry. You then leave the fabric or yarn to soak in this “indigo white” liquid for a period of time. It doesn’t turn blue until exposed to the air and is quite magical to see! There’s also a way to make indigo dye with fresh leaves.
Extraction of Indigo for Other Uses
While indigo pigment isn’t a safe food colorant, you can also use it to stain wood. I’ve seen artisans do that to make ornamental pieces and jewelry. There’s a way to use indigo to make ink, and it’s also a natural soap colorant for making blue soap. Speaking of denim, indigo traditionally is what makes blue jeans blue! Synthetic indigo dyes are now used for them, though.
Also, if you want to buy finished indigo, be aware that two types of powder are sold. Blue indigo powder, which is processed indigotin, and a green powder that is not a blue dye. Indigo leaves that have not been fermented are cut, dried, and sold in their green form as hair dye. Unfermented indigo leaves can dye hair black when used alongside henna.
Extracting indigo extract from indigo leaves is very similar to the woad extraction process. Woad Isatis tinctoria is a native European plant that can create the same blue dye as indigo but in lower concentrations.
Step-by-Step Indigo Pigment Extraction
- 1 Clear bucket with lids
- 1 Bucket same size or larger than the above but doesn't need to be clear
- 2 Bricks
- Stick for stirring
- Large metal sieve
- Clear pitcher or jug 2-3 litre (half gallon)
- Measuring spoon one teaspoon size
- pH strips or pH meter
- Greaseproof paper a type of waxed parchment
- Baking tray or baking dish with raised edges
- Glass Jar for storing the pigment
Fermenting the Indigo Leaves
- Begin with harvesting enough indigo to fit halfway to three-quarters the way in your clear bucket. Cram it all, leaves and stems inside. Indigo dye is only in the leaves but it takes time to strip each stem so you don't have to bother.
- Take the bucket outside and go through it all, making sure that you remove any insects, other creatures, and any obvious impurities. You can rinse the leaves, too, but it's not necessary. In the photo I show an immature moth that I rescued.
- Pack the indigo leaves back in the bucket and weigh them down with something clean and heavy. I used two clean bricks, but large stones work well, too.
- Fill the bucket with clean, cold water¹, covering the tops of the leaves by at least an inch. If you have heavy or highly chlorinated water in your region, it would be best to use rainwater.
- Cover the bucket with a lid, and leave it in a warm place for several days.
- After three days, kept at room temperature (20C/68F) or above, have a look inside the bucket. You're trying to ferment the leaves just enough² to give you a beautiful indoxyl solution, also called mermaid liquid. The water that you added to the indigo leaves should now have a stunning coppery sheen on the surface with vivid turquoise-green (some say antifreeze color) liquid underneath. The fermented leaves and liquid should have bubbles around the edges and smell faintly to sickly sweet.
- When you've achieved this, take the brick(s) and leaves out of the liquid, squeezing all of the liquid from the leaves. Gloves are optional and the leaves can be composted.
- Next, strain the liquid through a fine mesh sieve a few times to get any small pieces of leaves or debris out.
- The clean indoxyl solution needs to be aerated now³. Pour it from one bucket to the next twenty or more times⁵. Try to pour from as high as you can without spilling. This adds more force and more air to the liquid.
- You'll see the color become darker during this time to a greeny-teal with pale green froth or foam.
Adding the Alkali
- Next, add about three teaspoons of pickling lime to the bucket. It's also known by its formula name, calcium hydroxide, and is a safe chemical made from limestone. Stir the mixture well with a clean stick or other implement.
- When you think the pickling lime is dissolved, take the solution's pH. You can use a clean pH strip or a pH meter. You're trying to get the solution up to pH 9 to 10, and if it's lower than that, add another teaspoon of pickling lime and repeat.
Aerating the Indoxyl Solution
- The indoxyl solution now needs more aeration to help the indoxyl to convert to indigotin, the blue dye pigment that we're aiming for. So that means more pouring of the liquid back and forth between buckets. Do it twenty or more times.
- You stop when the liquid is deep blue in color and the froth on the top is thick and blue, too. The more you pour and aerate, the more indigo pigment you're likely to get.
Settling the Indigo Pigment
- Put the lid on again and leave the liquid to settle in a safe place where it won't be disturbed. Now it's time to wait.
- You can move to the next step when a thin dark layer settles to the bottom of the bucket and the liquid above is a clear tea color. Japanese indigo is infamous for taking more time to settle than true indigo, so you'll need to be patient. It can take a week or sometimes much longer.
- The aim now is to isolate the dark layer of indigo pigment and remove the liquid and any crusty bits that form at the surface (if present) from the bucket. I think the best way to do this, when using a safe alkali such as carbon hydroxide, is to siphon it off with a small winemaking hose. Siphon it into a second bucket, trying not to disturb the indigo layer at the bottom, and pour the liquid somewhere outside⁶. See notes below on this.
- You are left with a dark layer of indigo sludge at the bottom of the bucket, along with a little tea-colored water above.
Cleaning the Indigo Pigment
- Although optional⁷, you can clean the indigo pigment now. I think this is best done by using a clear container that's smaller than the bucket now, which is why I've recommended a clear pitcher or jug in the materials list.
- Fill the jug halfway with clean water, then gently pour it into the bucket containing the indigo sludge. Gently swirl it around to pick up the indigo, then pour the dark liquid back into the smaller container. Cover so that nothing can fall in.
- Leave the container to settle again, before siphoning off the liquid again. Japanese indigo takes a long time to settle, another several days to a week or more, so it may be that the liquid that you siphon off may be green before you continue to the next step. Siphon it off and leave it to settle on it's own and you'll eventually get a small amount of indigo pigment at the bottom.
Drying the Indigo Pigment
- After the last siphoning of liquid from the sludge of indigo, it's time to dry it out for storage⁸. Line a dish or tray with raised edges with greaseproof paper. Basically waxed kitchen paper that won't allow water through.
- Pour the indigo sludge onto the paper. Use a rubber spatula to get as much as possible from the container. If you've reserved the liquid siphoned off the indigo previously, please read the note below⁹.
- Leave the indigo sludge to dry out in a warm place until its bone dry. It takes about a week in my warm greenhouse and is finished when there is no tackiness and the indigo is pulling away from the paper.
Storing the Indigo Pigment
- With a spoon, gently loosen the ring of indigo that formed around the edge of where the sludge was poured.
- Then, fold the paper up, remove it from the dish, and gently roll over it several times with a rolling pin to break up and crush the indigo.
- Pass the indigo through a fine metal sieve over a bowl, to create a fine powder. Use a spoon to push and grind it through.
- Then, pour the powder from the paper and into a jar for storage. The powder keeps indefinitely, but its dye potential is said to decrease after five years.