How to identify Dogs Mercury (Mercurialis perennis) Euphorbiaceae family

Learn how to confidently identify dogs mercury,and stay safe in the hedgerow.

Anyone who has regularly walked in ancient woodland here in Britain will probably recognise this plant. After reading this short article you will soon be able to confidently identify dogs mercury.

This perennial plant is known as an indicator of ancient woodland here in the UK. Because of this it can point to the presence of other species that also require settled, climax woodland ecologies.

In old woodlands are the best places to learn how to identify the perennial dogs mercury
Old woodlands are the best places to learn how to identify dogs mercury.

This is one of only a few native members of the large spurge family. This species, and the annual mercury (Mercurius annua) are quite distinct from the various succulent and tropical-looking members that exude a photo-toxic white sap. Dogs mercury is a common poisonous plant that all foragers who are just starting out need to know.

Antique botanical illustration of dogs mercury
Antique botanical illustration of dogs mercury.

What’s in a name?

A number of plants have the prefix ‘dog’ given to them. In many cases it inferred a plant with no medicinal use. Dogs mercury is also a fetid, rank smelling  plant.

This plant was reportedly called mercury because of its similarity in form to another plant known as mercury, but more commonly called ‘good king Henry’ or ‘Lincolnshire spinach’ (Chenopodium bonus-henricus).

The generic part of the name Mercurialis, means literally ‘of mercury’. Some online references merely suggest the plant was named in honour of the god mercury. That seems a bit wishy-washy to me.

So I side with the American University of Berkley state, whose botanical department point to it being named in honour of the renaissance Italian physician Geronimo Mercurialis (1530 – 1606).

The specific part of the name, perennis, tells us this herbaceous plant returns each year.

Botanical description

If you find any words that are unfamiliar to you, simply head over to our comprehensive A-Z foragers glossary.

Leaves: The leaves begin to appear in late winter. They are elliptical or ovate, dark green, approximately 3 – 8 cm long, and with a distinctive white mid vein.

Oppositecpairs of dark green leaves with white mid veins are tell tale features when learning to identify dogs mercury.
Opposite pairs of dark green leaves with white mid veins are key features to identify dogs mercury.

The opposite pairs of leaves only appear on stems. Tiny hairs are found on both sides of the leaf.

The leaf margins are quite finely serrate-crenate, with tiny white hydathodes at the tips. Ciliate hairs are present.

Petioles: Small, anywhere from 3 – 15 mm, with thin, triangular-shaped, green stipules. The stipules are similar to those found on the stinging nettle.

Roots: Dogs mercury has rhizome roots that creep and spread to form large, dense carpets. In some places it will be the only plant in many square metres of ground.

Stems: These are pretty much round-shaped, sometimes with two ridges.

Dogs mercury has round stems, approximately 40 cm high, slightly swollen above the nodes, with stipules.
Dogs mercury has round stems, slightly swollen above the nodes, always with stipules.

The stems are swollen above the nodes. Stems are unbranched. They are hairy to mostly hairless.

Dogs mercury stems are always unbranched.

Flowers: Small catkin spikes of tiny, delicate-looking green-yellow flowers.

Dogs mercury has catkin-like spikes of green- yellow flowers
Dogs mercury has catkin-like spikes of green-yellow flowers, without petals.

The flowers are dioecious, meaning that male and female flowers are on seperate plants. The flowers consist only of sepals and no petals. The anthers are yellow tipped.

The flowers open to reveal delicate-looking yellow tipped anthers
The flowers open to reveal delicate-looking yellow-tipped anthers.

Flowering season: March to May.

Habitats: Old woodland, hedgerows and green lanes are all prime sites to spot this plant. It can also be found on our uplands, at altitudes of around 1000 metres. This map from the BSBI shows the extent of its range here in Britain.

Tou can easily find and identify dogs mercury in ancient woodlands and hedgerows.
You can easily find and identify dogs mercury in ancient woodlands and hedgerows.

Parts used: None.

Edible and medicinal Uses: None

Other notes: I couldn’t find any modern records of human poisoning, although many instances of cattle and livestock poisoning have been documented.

It may well be the case that as a species, we have long since learnt not to eat this plant. It is known to contain saponins, as well as bitter and acrid constituents.

So, now you have learnt how to identify dogs mercury, how about learning how to identify hundreds of plants in a day?  This must read, two-part article introduces the most common and important plant families for foragers here in the UK. Both part one and part two will fast track your plant I/D skills, and take your foraging to another level.

Coming up next…foraging for common hogweed.

 

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