Quick facts

Common names: dog’s mercury, false mercury, boggard posy, dog’s cole

Scientific name: Mercurialis perennis

Family: Euphorbiaceae

Origin: native

Flowering season: February to April

Habitat: ancient woodland, broadleaf woodland, hedgerows

What does dog's mercury look like?

A perennial, dog’s mercury is a medium-height woodland plant.

Leaves: large and long pointed oval-shaped leaves with toothed edges. They are bright green, hairy and crowded towards the top of the stem.

Flowers: are small and green. They flower in the spring.

Look out for: it covering the ground in ancient woodland.

Not to be confused with: species of Chenopodium. Dog’s mercury is unrelated, but similar to good-king Henry (Chenopodium bonus-henricus). The common name for this species is mercury, which gave dog’s mercury its name (the ‘dog’ meaning bad or not as good in reference to its poisonous properties).

Did you know?

The plant has an unpleasant decaying smell.

Where to find dog's mercury

Dog’s mercury is not a showy plant and is easily overlooked, but it’s very common. It flourishes in woodland with a preference for living under oak, beech, ash and elm. Although it’s mainly a plant of ancient woods, it can colonise new deciduous woodland very quickly. It favours shady areas and can also be found in hedgerows.

duke of burgundy butterfly on dog's mercury

Credit: Tim Smith / WTML

Value to wildlife

Some species of ground-nesting bird, such as woodcock, seem drawn to areas colonised with dog’s mercury. Speckled bush cricket nymphs feed on the leaves of dog’s mercury, along with other species including beetles, weevils, springtails and molluscs.

dog's mercury leaves and small flowers

Credit: Alex Hyde / naturepl.com

Mythology and symbolism

The name dog’s mercury has nothing to do with the pet – the plant is toxic to many animals, including dogs. It actually refers to it looking similar to, but ‘lesser than’ plants in the mercury family.

dog's mercury covering woodland floor in spring

Credit: David Chapman / Alamy Stock Photo

Uses of dog's mercury

Dog’s mercury is an ancient woodland indicator. This means it can be used to determine if a wood is long-established, even in areas which no longer have tree cover. All parts of dog’s mercury are poisonous. It can induce jaundice, diarrhoea, vomiting and even death.

Threats and conservation

Ancient woodland is one of our rarest woodland habitats. Dog’s mercury is just one species that contributes to the biodiversity of ancient woods. Loss of these habitats threatens some of our rarest wildlife species.