This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
In spite of its reputed use by the early Britons no trace of this plant has been found even in Neolithic or Roman deposits. It is found throughout Europe and in N. Asia. The ancient Britons are usually said to have used it to stain their bodies with a blue dye they prepared from it, and the Saxons imported it. Watson regards it as an alien and sporadic, except in the two localities where it is said to be wild.
Woad is regarded as wild on the cliffs at Tewkesbury on the banks of the Severn. It has become naturalized at Guildford, and is cultivated at Wisbech, where it is used to fix indigo. Its more or less restricted range in Britain does not give one an adequate idea of its former occurrence. If the Britons used it there should be some traces of it in Roman deposits, but, as we have seen, there are none.
Woad is a tall, erect plant, with a single main stem, dividing at the top into numerous branches devoid of leaves. The radical leaves are stalked, and oblong or tongue-shaped, those on the stem are arrow-shaped, with long, pointed ear-like lobes. The stem is stout and strong, and the plant is a vigorous grower, bluish-green and smooth.
There are numerous, crowded yellow flowers in close panicles, with equal sepals and petals, the flower-stalks being slender and bent back in fruit. The pods are three times as long as broad, wedge-like, narrow below, broad in the middle or spoon-shaped, and blunt in front, with a narrow margin, and hang down when ripe, giving the plant a drooping appearance. When ripe they turn brown.
The stem grows to a height of 4 ft. when well developed. July and August is the time to see it in flower, though it is occasionally in bloom in May. It is biennial.
The flowers are fairly large and conspicuous, and are numerous; but Woad is not largely visited by insects, and is as a rule self-pollinated. The fruit is dispersed by its own mechanism. The pods do not open, but fall, partly aided by the wind, at some distance from the parent plant, being pendulous when ripe. Woad is a sand-loving plant, and requires a sand soil, growing in districts where sandstone rocks contribute to form a sandy soil at the surface.
No fungus infests it, and Aphis brassicae is the only insect that lives on it.
By Pliny, Woad was called Glastum, hence the name Glastonbury (Welsh glas, blue; Gaelic glas, grey, green). Woad is akin to Vitrum, the Latin name for it. Dioscorides gave the name Isatis, and tinc-toria refers to the dyeing properties. Woad is called Ash of Jerusalem, Dyer's Weed, Goud, Ode, Woad, Wad.
When the leaves are used as a dye they are covered with boiling water, steeped for an hour, and weighted down. The water is then poured off, and the leaves are treated with caustic potash and then with hydrochloric acid, yielding- an indigo-blue. Woad mills are still worked at Wisbech, but the use of indigo has superseded it, and it is only used to fix indigo.
Photo. T. R. Goddard - Woad (Isatis tinctoria, L.)
The plant was said to remove inequalities from the skin. When cultivated it is sown on freshly-ploughed land, well prepared, well pulverized, to produce large and good leaves, the plants being well thinned out, and the soil stirred between them.
Essential Specific Characters: 37. Isatis tinctoria, L.- Stem erect, tall, radical leaves oblong, crenate, stalked, stem-leaves sessile, sagittate, flowers yellow, on slender pedicels, deflexed in fruit, in a panicle, pouch obovate, 1-seeded.