A plant cultivated in fields, in many parts of England, for the use of the dyers, and commonly met with in places near those where it was sown, as if a wild plant; but it is not properly a native of our country. It is a tall, erect, and handsome plant; the stalk is round, thick, firm, upright, and four feet high; but it is usually so covered with the leaves, that scarce any part of it is to be seen naked. The leaves are long and of a considerable breadth. They are large at the base, where they grow to the stalk, without any foot-stalks; and narrower all the way to the point. They are of a bluish green colour, and the whole plant is covered with them, so the top has a pretty aspect. The flowers are little and yellow; they stand in great numbers about the tops of the stalks, which are divided into a multitude of small branches; and they are succeeded by small seed vessels. The root is long and thick.
Although the dyers are the people who pay most regard to woad, and for whose use it is cultivated, it has virtues that demand for it a great deal of respect in medicine. The top of the stalks, before the flowers appear, contain the greatest virtue, and they are best fresh. They are to be given in infusion, and they are excellent against obstructions of the liver and spleen; they work by urine, and so take effect; the use of this infusion must be continued a considerable time: these are disorders that come on slowly, and are to be slowly removed.