The leaves have a very strong flavour that is somewhat bitter, sharp and tart. Consequently, it is used in very small quantities — which is probably wise anyway (see Nutrition below.)
In practice, Yarrow is not really used at all for cooking anymore, though you will see very occasional references to using the very new, young leaves in salads and soups. Most of the few recipes that you will still see calling for it are wines, beers or tisanes.
Yarrow needs to be kept out of pasture land where cows are grazing as eating it can make their milk taste bitter.
The flowers have historically been used to make dye from.
Nutrition
Unlike many herbs whose medicinal quantities are very faint, Yarrow actually contains active compounds — it’s just that they aren’t necessarily good ones. Many people are sensitive to the herb and just handling it will give some people a rash. Consuming too much of it can give some people skin photosensitivity.
Yarrow cannot be used commercially in the US except in beverages. Producers must ensure by law that the finished product is free of thujone, which is one of the active compounds in Yarrow. Thujone, one of the dangerous compound in the liqueur, Absinthe, can cause brain damage in large quantities.
History Notes
Yarrow is native to Europe, but has very successfully naturalized itself in North America.
Language Notes
The word “Yarrow” comes from the Anglo-Saxon name, “gearwe”. “Millefolium”, in the scientific name, means “thousand leaves”, referring to all the little fronds that make up a single “leaf”. The English word, “Milfoil”, came from the French name, “millesfeuilles”.