Good King Henry is Chenopodium bonus-henricus, a perennial to 30cm tall, with triangular shaped leaves on long dark green stalks. Flowers appear in summer, and seeds ripen in early autumn. Sometimes called Fat Hen, English Mercury, or Smearwort.
It's one of the so-called potherbs, like many of its relatives (including Lamb's Quarters, Fat Hen C. album, even Epazote C. ambrodioides.) Many of them share common names because they're so similar.
Grows in most fairly moist soils and prefers full sun, but will grow in dappled shade. Tolerates a pH in the range 4.5 to 8.3. Tolerates some frost. About 30 plants can produce a good supply of food for 4 people. Usually self-seeds readily. Any plant which self-seeds easily can be considered potentially invasive.
Culinary Uses: Young leaves can be eaten raw or cooked. They wilt quickly after picking so should be used as soon as possible after harvesting. (Older leaves tend to be tougher and more bitter.) Because of their high oxalic acid content, raw leaves should be eaten only in moderation. Cooked like spinach, in several changes of water, they are tastier and have less oxalic acid. The young flowering shoots can be cooked like asparagus. The young flower buds can be cooked and are regarded as a delicacy. The seeds can be ground and mixed with flour to make bread and other baked goods. They should be soaked in water overnight and thoroughly rinsed before being used.
Medicinal Uses: A poultice of the leaves has been used to cleanse and heal chronic sores, boils and abscesses. The seed is a gentle laxative that is suitable for children. The plant can be used for indigestion and as a diuretic. Used in a veterinary cough remedy for sheep. The leaf is a source of iron, vitamins and minerals. Rich in iron and as vitamin C.