Crops Woad (Isatis tinctoria)

Young plants

Woad is a biennial or short-lived perennial, producing a strong taproot. Young or overwintering leaves may be dark or purplish, but most basal and stem leaves are mid-green, shiny on the upper surface but often hairy underneath.

Plants at rosette stage

The lower leaves are stalked, usually of a narrow oval shape, often pointed and sometimes with toothed margins; stem leaves are smaller and narrower, clasping the stem, on which they develop singly.

Plants in flower

Stems develop in early spring, sometimes several per plant, growing to a height of up to 1.5 m; they branch freely, producing trusses of small bright yellow flowers.

Ripening seedpods

The flat seedpods, each containing one or two seeds, change from light green to a shiny purple-black colour: when ripe, they are easily detached and may be carried for some distance by the wind.

Seedpods

Seed was traditionally sown in spring, being placed about 5 cm deep in rows some 25 cm apart, the objective being to establish a population of about 15 plants per square metre Woad was always known as a 'hungry' crop and it is suggested that it should be fertilised as heavily as winter oilseed rape. No crop protection chemicals can yet be recommended, but some herbicides used on oilseed rape may be suitable: weed control is important to avoid contamination of the harvested leaves. Formerly, leaves were gathered 4 or 5 times a year, wrenched from the plant by hand, but it is expected that a modified spinach harvester will be used on contemporary sowings: a total production of up to 25 t per ha of fresh leaves is anticipated. Current research in the UK is examining the agronomy of the crop and also modern methods of hot-water extraction of the dye, which are expected to be more efficient and less time-consuming than the old methods. The case for renewing interest in the production of dyes from natural sources rests at least partially on the fact that current methods of dye production can cause pollution; similar interest is also being shown in the re-examination of other formerly used dye-plants such as madder (Rubia tinctorum), weld (Reseda luteola) and golden rod (Solidago spp).