It's growing everywhere in my garden and it pops up faster than I can pick it.
What species could it be?
I haven't come across this one before and, without a specimen, it isn't easy to identify it. However, I've looked it up in the "Concise British Flora" (Keeble Martin), and it would certainly seem to be a variety of Mercury.
If it has (1) a round, hairy, unbranched stem, with (2) opposite leaves which smell faintly unpleasant when rubbed, and (3) long-stemmed flowers, it is most probably Dog's Mercury (Mercurialis perennis - see illustration). If, on the other hand (1) its stem is quadrangular, branched from the base, and only slightly hairy and (2) it has up to ten leaves, growing in spiky clusters, it is likely to be Annual Mercury (Mercurialis annua - see illustration 2). The first flowers in March-April, and the second, from July to September. There are some good images of Dog's Mercury here.
I hope this helps.