Dog’s Mercury (Mecurialis perennis) is sometimes referred to as ‘dog’s cole’. It is a perennial and is found in woodlands (particularly beech and oak); and in shady places (like dense hedgerows); it is found widely throughout Britain (except Northern Scotland). Its distribution in Ireland is much more limited.
It can spread by means of its underground rhizomes (stem-like structures that penetrate through the soil) and when it finds suitable conditions, it may form a carpet of plants. There is some evidence that high light intensities inhibit the growth of the rhizomes. The plant can be quite invasive – especially in shady places.
The stem is hairy and unbranched, with more leaves present near the top than the base. The leaves are dark green in colour, hairy, have a toothed (serrated) edge and an overall ‘spear-like’ shape – i.e. much longer than they are wide and they come to a point at the tip. The leaves are arranged opposite to each other. Plants may reach a height of about 15 in / 40 cm.
There are separate male and female plants (i.e. the species is dioecious). The flowers are born at the base of the leaves (not dis-similar to nettles). The flower spikes (about 1 to 1½ inch long, stalked tassels) appear between February and May. The catkin-like male flowers have a yellow colour, due to the many yellow stamens (see photo), and the female flowers have 3 tepals (a term used when petals and sepals are the same or indistinguishable). It is one of the first plants to ‘bloom’ in the Spring, shortly after its shoots push their way above ground. When the fruits form, they are hairy. The seeds are quite large (about 3 – 4 mm in diameter) and 'offer a meal' to small mammals and birds.
The smell or scent of the plant is often described as ‘foetid’. This is probably due to the presence of chemicals such as trimethylamine, a compound that can give an aroma not dissimilar to rotting fish. The plant is toxic / poisonous. At one time, the leaves were used to make dye. Though dog's mercury is a member of the Spurge family (and related to wood & purple spurge), it does not have the usual ‘milky juice’ in the stem.
For a detailed article on the ecology of Dog's Mercury : http://onlinelibrary.wiley.com/doi/10.1111/j.1365-2745.2007.01348.x/full from the J of Ecology 2008.