About twenty years ago I fancied trying my hand at dyeing ..... and where better to start than with indigo extracted from woad? The woad seeds germinated well, the plants (which are biennial) flourished but somewhere along the line I lost enthusiasm for the enterprise - perhaps when I realised how much woad I'd need to grow to produce a worthwhile amount of dye and what a messy business it would be. You can find a description of the whole process here.
So the plants ran to seed, after producing a spectacular display of small flowers that were very popular with hoverflies. Even today, two decades later, occasional seedlings from those original plants appear in the garden so my redundant woad crop must have left a persistent seed bank.
Woad has been used as a source of blue dye since ancient times (for a detailed account click here) but was superseded by true indigo Indigofera tinctoria which in turn was replaced by modern synthetic indigo dyes.