also called: Persicaria tinctoria
A frost tender annual that likes fertile soil, heat and humidity but will grow almost anywhere. You should use fresh seed from the previous year's harvest (though I have germinated 2 year old seed).
Start seeds indoors 6-8 weeks before setting out after all danger of frost has passed. Harden off and transplant out on 1 foot centers. Keep the ground moist, mist the leaves on particularly hot and dry afternoons. Good fertility and plenty of water will yield more color than poor ground and drought.
When the plants are 1-2 feet tall and a bruised leaf will turn navy blue, it is ready for the first harvest. Cut the stems several nodes up from the ground (it will re-sprout at those nodes). You may be able to harvest the re-growth several times. Harvest and make up your dye vat on the same day.
There are many ways to make an indigo vat. This one has worked for me (with thanks to "The Dyer's Garden" by Rita Buchanan).
Strip the leaves from stalks and cram them into a gallon glass jar (or a plastic bucket or a stainless steel or enamel pot -- any non-reactive container). Fill the jar with water and place it in another pot on a trivet or some jar lids (you are creating a double boiler). Over the course of one hour (or longer), slowly bring the temperature to 160 degrees F. Do not (ever) exceed 180 degrees F.
Strain out and compost the leaves, pour the brown liquid into a bucket or other large non-reactive container. Add ammonia (buy the NON-sudsing kind) at the rate of 1 fluid oz/gallon of dye liquid to make the vat alkaline. The goal is to maintain a pH of 8-9 throughout. Now pour this liquid back and forth between 2 buckets for 5 minutes to get as much air into the vat as possible. The liquid will turn blue and a bit foamy. Allow the vat to cool some and return it to the double boiler (which is itself cooling). The goal from here on out is to maintain a temperature of 110-130 degrees F.
There is one more step before you can dye. The vat must be reduced (the air removed) and turned to "indigo white" which is actually a sort of chartreuse green. To do this, add Sodium Hydrosulfite (you can buy this as Rit Color Remover from the grocery store or a craft store – once opened it goes bad from exposure to air and moisture) at the rate of 1/2 teaspoon/gallon of dye liquid. Stir gently, remembering not to get more air in the vat. It might take another hour to reduce, you will see the vat change color.
Pre-wet your yarn or fabric. It should be thoroughly saturated, then the excess water squeezed out. Now you may slide skeins or fabric into the vat (still, no air), swish them around gently under the surface and leave them for about 20 minutes. Draw the yarn or material out of the vat (gently squeezing out the excess but not splashing --no air into the vat), and shake it around to oxidize. Magically, before your eyes, it will turn to yellow to green to blue. For darker shades repeat the process (oxidize for 1/2 hour between dips) until the vat is exhausted. You may want to add more ammonia to maintain the pH, or more hydrosulfite if you get too much air into the vat, or more heat to maintain the temperature.
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