How often is it that you see natural selection happening before your very eyes? Well, it’s been happening at the Royal Horticultural Society’s Garden at Wisley, just south of London, where a weed has evolved which is less likely to be pulled out by the gardeners!

It’s like the moths in the old industrial north of England. For centuries, the white form of the peppered moth had been the most common, camouflaged against the pale bark of birch trees upon which the moths rested. Then, as industrial pollution grew, the bark became covered in soot and the white moths became highly visible and were snapped up by birds. (Click the image, left, to see white and black forms on birch bark.) The less common black form then became much more common. More recently, as the air became cleaner again, the white form has enjoyed a resurgence.

At Wisley, annual meadow grass (Poa annua) is a persistent weed but the gardeners are diligent in rooting it out and consigning it to the compost  heap. But over the years forms with darker stems and leaves have developed – colours which show up less well against the dark soil and which are less likely to be noticed and pulled or hoed off by gardeners. The result has been the development of a distinct brownish purple leaved form known not seen anywhere else. Known as Poa annua f. purpurea, it was first formally described in 2003. Click on the image, left, to see how well camouflaged the purple form is.

On the Rock Garden and on the Portsmouth Field (the trials field) at Wisley this new “invisible” form has become a particular problem, especially now that less chemical weed control is used and control depends more on the gardeners’ sharp eyes. I only noticed the plant in the picture when I bent down to pick up my glasses!