Persicaria (syn Polygonum) tinctorium. Tender perennial for blue dye
This plant has been used for centuries in the East to obtain shades of blue the same as Indigo and Woad, but is easier to grow and you need far smaller quantities of leaves than for these other indigo plants, which have the same chemistry. But it's not easy to get hold of! The seed is said to be viable for only a short time; but we have a small amount of both seed available in winter and plants available in Spring and Summer . (This is not the same plant as Japanese Knotweed, although it is in the same family - it cannot become invasive as it dies down in winter.) It is grown as an annual in the USA, but it does persist in the UK if potted up and brought under cover for the winter, when you can go on using the leaves for dye.
Part to use in dyeing; leaves, which can be harvested many times from the same plants.
Added chemicals in place of mordant required; Soda Ash and Spectralite (see Mordants section) - process fully described in Rita Buchanon's book (see Books section), or instructions sent with plants if requested.
Plants only available in spring and summer; fresh seed available from November-December.