- Name also: Dog Mercury
- Family: Spurge Family – Euphorbiaceae
- Growing form: Perennial herb. Rootstock creeping, with runners, turning blue as they dry.
- Height: 25–40 cm (10–16 in.). Stem unbranched, usually base leafless.
- Flower: Plant dioecious (pistillate and staminate flowers on different plants). Perianth regular (actinomorphic), lime green, 2–3 mm (0.08–0.12 in.) broad, fused, 3-lobed. Stamens 8–15. Staminate inflorescence spike-like. Pistil of 2 fused carpels. Pistillate flowers solitary or axillary in pairs, long-stalked.
- Leaves: Opposite, quite long-stalked, stipulate. Blade narrowly elliptic–ovate, fine-haired, with densely toothed margin, dark green.
- Fruit: 7 mm (0.28 in.) long, 2-valved, fine-haired capsule.
- Habitat: Broad-leaved forests, coppices. Calciphile.
- Flowering time: April–May.
- Endangerment: Protected on the Åland Islands.
Dog’s mercury is a demanding broad-leaved forest plant which doesn’t really thrive north of the so-called oak zone. It is a dioecious plant, which means that its pistillate and staminate flowers are on different plants, so it is either male or female. The species is probably mainly wind-pollinated. It flowers early in the summer, when the wind is best able to blow between gaps in the still-leafless trees and hedgerows. The pistillate flower has nectaries within its structure, which points towards insect fertilisation, but they are lacking in the staminate flowers so these are only visited by pollinating insects if they are lost. Dog’s mercury spreads efficiently through its rootstock too and forms large stands in suitable places.
Dog’s mercury’s Finnish name refers to its bluish root. It loses its toxicity if it is dried or boiled, and it has a long history as a medicinal plant that stretches all the way back to Ancient Greece to the famous doctor and botanist Dioscorides. In his medicinal work he demonstrated that a tincture of male flowers increased the possibilities of having a male child, and similarly female flowers increased the likelihood of a girl. Regrettably, however, he didn’t say which herb to use, and he also mixed up the male and female flowers. Nowadays dog’s mercury is used as a natural cosmetic.
Annual mercury is the other species in genus Mercurialis that can be seen in Finland. Like dog’s mercury this species is dioecious but an annual as its species name tells. The difference between annual or perennial cannot be seen so this feature does not help when identifying these species. The flowering time of annual mercury is July–September. The differences in outlook are: the stem is angular and more branching, leaves are glabrous (dog’s mercury’s stem is branchless and round, leaves are short-haired when having a closer look). Also habitats are different: annual mercury is a casual species in harbours and loading areas and sometimes in gardens, dog’s mercury favours broad leaf woodlands.