Mercurialis perennis, commonly known as dog's mercury, is a poisonous woodland plant found in much of Europe as well as in Algeria, Iran, Turkey, and the Caucasus, but almost absent from Ireland, Orkney and Shetland.
A member of the spurge family (Euphorbiaceae), Mercurialis perennis is a herbaceous, downy perennial with erect stems bearing simple, serrate leaves. The dioecious inflorescences are green, bearing inconspicuous flowers from February to April. It characteristically forms dense, extensive carpets on the floor of woodlands and beneath hedgerows. It has a preference for moderately shady to densely shady habitats. It is able to colonize new deciduous woods on dry, calcareous soils at an annual rate of a meter or more. The plant's common name derives from the plant's resemblance to the unrelated Chenopodium bonus-henricus (Good King Henry, also known as mercury, markry, markery, Lincolnshire spinach). Since Mercurialis perennis is highly poisonous, it was named "dog's" mercury (in the sense of "false" or "bad"). It has also been known as boggard posy.