- Name also: Dyer’s Woad, Glastum
- Family: Mustard Family – Brassicaceae (Cruciferae)
- Growing form: Biennial herb.
- Height: 40–100 cm (16–40 in.). Stem upper part abundantly branching, glabrous, bluish green.
- Flower: Corolla regular (actinomorphic), yellow, approx. 0.5 cm (0.2 in.) wide; petals four, 3–4 mm (0.12–0.16 in.) long. Sepals 4. Stamens 6, of which 4 long and 2 short. Gynoecium fused, a single carpel. Inflorescence a raceme, extending in fruiting stage.
- Leaves: In basal rosette and alternate on stem, lowest long-stalked, stalk winged, upper stem leaves stalkless, amplexicaul. Blade glabrous, bluish green, lowest blades lanceolate, shallow-toothed, uppermost narrowly elongated, with sagittate base, usually with entire margins.
- Fruit: 1-seeded, indehiscent, lanceolate–elliptic–oblanceolate, flat, with winged margins, 12–18 mm (0.48–0.72 in.) long, pendent silicula. Stalk thin, approx. 7 mm (0.28 in.) long.
- Habitat: Stony, sandy and gravelly sea-shores, kelp banks, sometimes roadsides, waste ground.
- Flowering time: June–July.
Woad has an eye-catching large golden inflorescence, but it is still noticeable after it has withered, standing rigid in the face of driving sea winds throughout the winter and using it to spread its seeds. Woad’s seeds are sensitive to frost so it is limited to the mild maritime climate around the coast. It is most common in the south of the archipelago, but it grows rarely as far north as the Kvarken Archipelago. It grows like a true native on stony, gravelly and sandy shores in the outer archipelago, and is abundant on kelp banks, and it could well be native from a time when Finland had a more continental climate, as it seems to have adapted to the dry, sunny early summer and cold, windy winters of the outer archipelago. It has been suggested that woad is native to the coastal steppe around the Black Sea, from where it travelled to the Baltic with people. International trade has been going on for thousands of years and the Vikings’ ancient water routes stretched throughout Russia and reached as far as Constantinople.
Woad used to have a fine reputation as a dye, and in cultivation its leaves were harvested two or three times a year. These were soaked in water to yield an indigo-white through fermentation. It was then stirred because as it oxidized it turned indigo blue and separated as a blue sediment. The water was poured out and the sediment was dried and used later as e.g. face and skin paint to frighten enemies in battle, and in peacetime to colour wool and textiles. It was replaced in the 16th century by cheaper and more efficient “true indigo” which grew in the East. Woad cultivation was briefly resurrected in the 19th century when Napoleon’s soldiers focused on international trade, but cheap imports from the East squeezed it out of the market. Nowadays most textiles are coloured with synthetic dyes and the use of natural dyes has declined greatly.
“Once introduced, Dyer’s Woad can spread rapidly. A single infestation in Montana increased from 0.8 to 40.5 ha in two years. Annual losses in crop yield and range production of $2 million are attributed to the plant in northern Utah.”
Alberta Invasive Plants Council