Every Wednesday, I hope to find a new ‘weed’ to investigate. My only criterion will be that I will not have deliberately planted the subject of our inquiry. Who knows what we will find…..

What a diffident plant Annual Mercury is, growing away on disturbed land without any flamboyant flowers or bright foliage to catch the eye. And yet, in the south of England at least, it pops up everywhere. It is a member of the spurge family  (and so related to Sun Spurge), and has a similar milky-white sap if the stems are broken. I found this patch growing just opposite the tube station, and it seems to be doing very well. The plant is dioecious, which means that it has male and female flowers on different plants. Most of the ones on my patch were male, with the flowers held within tall spikes.

And here are the female flowers, along with the burr-like fruits.

I remember first meeting this plant when I was about nine years old. We had recently acquired an allotment in Manor Park, East London, with a great view of the Victorian gas holders at the nearby gas works. When we first got our plot, it was waist-deep in brambles and stinging nettles, and it was a good while before it was cleared and we could get down to planting. Annual Mercury was such a doddle to keep under control compared with some of the other weeds – it pulled up easily, and didn’t draw blood, or raise blisters. But it was a stealthy little devil, popping up as soon as we turned our backs. What I didn’t realise is that the seeds are often carried away by ants as a food store for when times get hard. Some of the seeds will then germinate in the ants’ nest, in perfect conditions. No wonder it can become so prolific.

Annual Mercury has been used as an emollient ointment, and historically has been used as a purgative, diuretic and anti-syphilitic. There is some debate about how it got its name – for Pliny, it was because the plant was discovered by Mercury, the messenger of the gods. On the Poison Garden website, the author suggests that

“Perhaps, as an attractive, athletic man travelling widely to deliver the gods’ messages, Mercury had need of its properties.”

The plant is also eaten, boiled, in Germany – it is said to taste a little like spinach, but as it is poisonous (though not as poisonous as its lookalike, Dog’s Mercury (Mercurialis perennis)), I would be inclined to give it a miss. I would hate to think of my readers dropping like flies. Also, in France the plant is known as Mercuriale ou la Foirolle, with la Foire meaning diarrhoea. You have been warned.

In spite of its toxicity, the plant has been fed to pigs in France (though maybe it’s cooked first), and the seeds are eaten by Bullfinches. I would be very happy to have a garden full of Annual Mercury if I was visited by Bullfinches.

Annual Mercury is also known as ‘Girl’s Mercury’ and ‘Boy’s Mercury’, which intrigues me. According to Pliny, the plant could be used by a pregnant mother to select the sex of her child. The author of the Poison Garden website wonders if Pliny is referring to the two different species of Mercury (Annual and Dog’s), and I wonder if there was an understanding that Annual Mercury could be either male or female, and if the mother-to-be would choose her ‘poison’ accordingly. At any rate, this undemonstrative ‘weed’ has a long history of interactions with us, and with the animals in its habitat. I will never look at it in the same way again.