Polygonum tinctorium Japanese indigo preferred by dyers in Maine. A tender annual, indigo thrives in fertile soil and likes heat and humidity. Best started indoors 6–8 weeks before the last frost, transplanted out on 1' centers and kept moist. When the plants have grown 1–2' tall and bruised leaves turn navy blue, they are ready for a first harvest. If you cut the stems several nodes up from the ground, they will re-sprout and you can harvest the re-growth, perhaps several times more.
Traditionally, dyers use dried leaves for the classic indigo color. If you intend to dye with fresh leaves, prepare the dye bath the same day as the harvest and expect a lighter shade of blue. Annual. ~250 seeds/g. ②
Susanne Grosjean, of the Wednesday Spinners group in Downeast Maine, says, “You need only three dye plants: indigo for blue, madder for red and weld for yellow. Three primary colors, all excellent color-fast dye plants.”