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Mercurialis perennis

Dog's mercury

Euphorbiaceae Europe and southwest Asia

Deer Resistant

SunPart Shade – Full Shade

WaterMedium

HardinessZones 5–9

Overview

Mercurialis perennis is a creeping, rhizomatous woodland perennial in the spurge family that forms extensive carpets 6-16 inches (15-40 cm) tall on shaded forest floors. Unbranched upright stems carry pairs of oval, toothed, dark green leaves 1.5-3 inches (4-8 cm) long, and the whole plant gives off a foul smell when bruised. It is dioecious, with male and female flowers on separate plants: males bear slender erect spikes of tiny green flowers, females a few inconspicuous green flowers on short stalks, both opening in early spring. The plant spreads mainly by branching underground rhizomes into single-sex colonies that can cover wide areas. All parts contain methylamine, saponins, and other compounds and are toxic to people and livestock, causing nausea, vomiting, and worse in quantity. It grows in shaded, base-rich soils of ancient woodland and is used by botanists as an indicator of long-established forest. The dense colonies leave little room for other ground flora.

Native Range

Mercurialis perennis is native to Europe and southwest Asia, from the British Isles east across the continent to the Caucasus and Iran, growing in shaded deciduous woodland and on shaded limestone soils.

Suggested Uses

Rarely planted on purpose, it occurs as a wild groundcover in shaded, base-rich woodland and is valued chiefly as an indicator of ancient forest. It holds bare soil under trees where little else grows in deep shade. All parts are toxic to people and animals if eaten.

How to Identify

Identify dog's mercury by its low colonies of unbranched upright stems carrying paired, oval, toothed dark green leaves with a strong unpleasant smell when crushed. In early spring male plants raise slender erect spikes of tiny green flowers, while female plants bear a few small green flowers on short stalks. The single-sex carpets and the foul scent distinguish it from harmless woodland herbs.

Appearance

Size & Dimensions

Height6" - 1'4"

Width/Spread1' - 2'

Colors

Flower Colors

green

Foliage Colors

green

Fall Foliage Colors

no change

Bloom Information

Bloom Period

~6 weeks

Spring

The small green flowers open in early spring, generally February to April, before the woodland canopy closes. Male and female flowers grow on separate plants and are wind-pollinated rather than insect-pollinated, so they are plain and scentless. Seed forms on female plants by early summer.

Detailed Descriptions

Foliage Description

dark green

Growing Conditions

Sun Requirements

SunPartial Shade SunFull Shade

Tolerates up to 4 hours of direct sunlight daily

• Full Sun: 6+ hours of direct sunlight

• Partial Shade: 3-6 hours of direct sunlight

• Full Shade: Less than 3 hours of direct sunlight

Soil Requirements

pH Range6.5 - 8.0(Alkaline)

357912

Soil Types

loam chalk

Drainagemoist

Water & Climate

Water Needs

Medium

Frost Tolerance

hardy

Time to Maturity

2-3 years

Care & Maintenance

Care Guide

Mercurialis perennis grows in partial to full shade in moist, fertile, base-rich or limestone soils at a neutral to alkaline pH around 6.5-8.0. It is a plant of established woodland rather than gardens and spreads steadily by rhizomes once settled. It tolerates deep shade and dry shade beneath trees but declines in open sun and acidic, waterlogged ground. No watering or feeding is needed in its woodland setting. Where it appears in a garden it can spread into a persistent colony that is hard to remove because of the deep rhizomes. All parts are toxic to people and livestock if eaten.

Pruning

No pruning is part of growing this woodland plant. Where colonies spread into cultivated ground, repeated digging out of the rhizomes is the main way to reduce them, though fragments regrow. The foliage dies back on its own in late summer and fall.