The ability to extract indigo pigment from any indigo-producing plant species relies on the presence of a single chemical compound, indican, produced within the cells of the plant’s leaves.
Indican is the indigo pigment precursor, and is produced within the leaves of over 100 different species of plants world-wide. It is essentially colorless (and invisible to the naked eye) until it reacts with oxygen, at which point it becomes indigotin, or indigo pigment. The more indican a plant produces, the more indigo pigment it will yield for the dye pot! Some species, such as indigofera naturally produce higher amounts of indican than others.
That being said, growing practices can also greatly impact the amount of indican a given plant produces in a year, and in general, healthy, happy indigo plants harvested at the right time will offer the highest pigment yield potential.
This series of articles will focus on Japanese indigo (Polygonum tinctorium/Persicaria tinctoria) because this is the primary species I have grown and researched over the past seven years, but the same basic goals apply to whichever species you are able to grow.
• grow happy, healthy plants
• increase indican production
• increase leaf/vegetative production
• time harvest and post harvest processing to maximize pigment yield
•Optimize processing techniques to achieve maximum pigment conversion
Starting From Seed
Start Japanese indigo seed indoors 4-6 weeks before the last frost, and plan to transplant out after the soil has warmed and all danger of frost has past. It is important to plant fresh seed harvested the previous season, as germination rates drop significantly in seeds older than one year, even with the best storage methods.
Seeds can either be sown in plug trays with a few seeds per cell, or densely in an open flat and divided out*. Germination is greatly increased by warm temperatures, so if you have access to a heat mat for seed starting, consider using it! (I would recommended setting it to around 75 degrees). If you encounter a particularly cool spring or live in a cold coastal climate, consider potting up your seedlings to grow on in 3″ pots to get a head start before placing them out in the spring.
*If you have bed space in a hoop house or greenhouse that is warm and protected from frost, seeds can also be sown densely in an incubator bed, and then dug and divided up to plant out in the field. However, having grown Japanese indigo on the cool and foggy central coast, I advocate for starting seeds in plug trays in cool-summer climates. These plants appreciate heat, and soil in plug tray cells will warm faster than those in open flats. Additionally, in my experience seedlings are not as tolerant of root disturbance (which is inevitable when teasing seedlings apart that have been grown close together) during cool weather as they seem to be here in the warmth of the central valley.
Promoting plant health and leaf development
Again, indican is produced in the leaves of indigo plant species. This means that growing practices should focus on encouraging vegetative plant growth.
Japanese indigo is a heavy nitrogen feeder, so begin watering seedlings once a week with diluted fish emulsion as soon as the first true leaves appear and you will help your plants get off to a vigorous start. When planting seedlings out in the garden or farm, make sure to prepare the soil by forking it deeply and incorporating a healthy amount of organic matter and a balanced fertilizer. Space seedlings 12″ apart.
Japanese indigo can grow in both full sun and partial shade, but requires good drainage and a consistent supply of moisture to thrive… Water generously in drier climates, and mulch plants heavily with straw or leaf litter to retain water and suppress weed growth.
In addition to providing adequate water, if you continue to feed plants weekly or every other week with diluted fish emulsion (which provides nitrogen) during the first month of the growing season, you will be generously rewarded with lush growth and a marked increase in leaf production (and consequently, available indican!). Do not underestimate this step–proper fertilization can mean the difference between a plant that produces .5lb of vegetative growth for harvest and a plant that produces close to 5lbs!
While apical dominance in Polygonum/Persicaria species is relatively weak, and plants will naturally send out leafy side shoots for harvest, pinching out the growing tips when seedlings are 6-8 inches tall will greatly speed vegetative production. Pinching results in healthy, bushy plants that produce a greater abundance of leaves for harvest in a much shorter period of time. If you live in colder climates, or get a late start planting your indigo this can be especially valuable! An added bonus is that pinched tips can be used to create the first fresh-leaf vat of the season, ensuring nothing goes to waste.
The photo below shows the magic of pinching–the small plant in the center was left alone for comparison, and is surrounded by plants that were pinched only five days earlier… you can see how quickly the plants that are pinched respond by directing energy into vigorous side shoots and lots of new leaves.
Finally, to whatever extent you are able, avoid stressing your plants. Japanese indigo responds to stress (inadequate nutrients/moisture/soil depth and prolonged cold temperatures) by flowering prematurely, which means fewer leaves will be produced for dyeing before the plant begins to put its energy into producing flowers and seeds.
Monitor your indigo plants regularly for signs of flowering. When you see the first buds beginning to form, in is time to harvest!
Coming up next in part two, we will look at tips for harvesting and handling your indigo to ensure maximum pigment yield.
The purpose of this article is to provide information and tips for growing Japanese indigo with the intent of maximizing the pigment yield potential of your crop. I am hoping the information provided here will be useful to both those starting out, and those of you who are already growing indigo in your gardens.
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