An Impartation of Color: Japanese Indigo (Polygonum tinctorium)

This was my third summer growing Japanese Indigo, also know as Chinese Indigo, or Dyer's Knotweed. The Japanese refer to it as "Ai." For centuries, Japanese Indigo has been widely used in Asia as a source for blue dye. The leaves of this plant contain indican, a glucose substance, which once reduced (made soluble), results in beautiful shades of blue and aqua on both protein (animal) and cellulose (plant) fibers.

While this annual typically prefers a warm climate, I have found that it grows quite well here in the Pacific Northwest. I start my seeds indoors in the spring, and as soon as chance of frost has passed (usually around the first of June), I transfer my seedlings to a raised bed on the southfacing side of my house. Here the plants get plenty of full sun for most of the day and are somewhat protected from the west winds blowing in off the Pacific Ocean. Japanese Indigo needs alot of moisture, so I soak my plants well at least every other day in summer, twice a day (morning and evening) when the weather is particulary hot. My raised bed drains well and assures that over-watering does not become a problem.

When a bruise on the leaf leaves a deep blue mark, this is an indication that the indican has developed in the leaves, and they are ready to harvest.

It is usually August, here in the northwest, when I make my first harvest, however, this year a long, cold, wet spring delayed the growth of my plants. They struggled along in the cold for some months and didn't really begin to put on any real growth until well into August, when we finally had a couple of weeks of nice warm weather. September gave us some warm weather as well. This helped tremendously, although my plants never did grow as tall or lush as previous years. As autumn quickly set in, along with colder temperatures, I worried that I may not get a very good harvest of leaves, and my hope shifted more for my plants to produce some blooms for seed. While it is considered best to harvest leaves for dyeing before the blooms appear, I feared disturbing my plants might prevent my getting seed for next year. It was October before the blooms began to appear and past mid-October when I made my first harvest.

October 2011 - blooms just beginning to appear

Using the fresh leaf dyeing method described by Vivien Prideaux in her book "A Handbook of Indigo Dyeing," I filled two one-gallon plastic jugs with leaves, filled the jugs with water and set the jugs on trivets in pots on the stove, heating them slowly over the course of an hour to 160 degrees F. Removing the jugs from the heat, I strained the leaves through a mesh bag, squeezing all the liquid from the leaves, and gathering the clear liquid into a pot. The liquid was a beautiful deep blue color.

To this I added 2 tsps. of baking soda to make the vat alkaline (pH 7.5-8), which changed the color of the liquid to a yellow-green. A couple of years ago when working through this same process, I had to leave the vat at this stage for a day or two. When I returned, this is what it looked like:

Next, I aerated the liquid by pouring it back and forth between two pots. I also used a handheld beater to help with the aeration until the liquid became more yellow in color with a light blueish foam on top. This process can take 10 minutes or more.

Next, I reheated the liquid to 120 degrees F., then added 1 tsp. (1/2 tsp. per 1 gallon of liquid) of sodium hydrosulphite (Rit Color Remover) to the vat to reduce the indigo, and maintained the heat for about 15 minutes before removing the vat from heat. Two skeins of pre-wetted wool yarn (3 1/2 oz. each), a small silk scarf, and a little silk gift bag were entered into the dye vat and left to soak for 20 minutes. While in the pot, the fibers became a nice yellow color, although you can see from the picture how wherever the silk scarf pooched above the liquid and came in contact with air, it's color turned aqua.

Following the 20 minute soak, the fibers were pulled from the vat, excess liquid squeezed out and held in the air to watch the magic begin. The yellow fibers turn blue when exposed to the oxygen in the air. Here are the results:

100 % wool skeins

100 % wool skein

Silk scarf

Tie-dyed silk gift bag

Japanese Indigo will result in different shades of blues and greens depending on the fiber being dyed. Following are some photos from my dyeing experiences with my leaves from the previous two years. Superwash sock yarns often result in much deeper colors.

Superwash Sock Yarn (2009)

L to R: natural white, over tan, over yellow dyed w/carrot tops

Superwash Sock Yarn (2009)

L to R: over yellow dyed w/carrot-tops, natural white, over tan

100% Wool (2010)

Lace neck cowl knit from Indigo dyed wool yarn above. (2010)

Japanese Indigo on cotton produces a beautiful, crisp, sky blue.

100% Cotton Baby T-shirt (2010)

At a summer retreat in 2010, we dyed silk scarves with some powdered indigo from India. We tied dry garbanzo beans into our scarves, and did other fun things to create beautiful designs. The results were a stunning deep blue!