Nettle Monoculture

Nettle control: A few days ago-a group of a dozen or so volunteers tried their hands at nettle scything on the river banks near All Saints Church in Huntington, in preparation for the start next year of our attempt to introduce some biodiversity into areas dominated by a monoculture of nettles. Nettles are a bit of a conservation conundrum. On the one hand, there are lots of insects that feed on them making them good for local biodiversity, on the other hand, they can form big patches, crowding out most less dominant plants.

The stinging nettle (to give its full name Urtica dioica) is a native plant and one of our most successful species.  It is a lot more abundant than it used to be, rather than more widespread – it has always been found almost everywhere in the lowlands – but it has become much more vigorous where it does occur, often in much bigger patches. Nettles are one of a group of plants called nitrophiles which simply means they love nitrogen.  Nitrogen is, as every gardener knows, one of the essential nutrients for plants.  The nitrogen that plants can use has to be in what is called a fixed form, typically as nitrate or ammonia; the air is four-fifths nitrogen gas, but that is no use to plants.

So the air is full of fixed nitrogen and when it rains, down it comes. Human activities have significantly changed our atmosphere in complex ways over many years with the result that every square metre of our land receives a continual dressing of nitrogen from the air in this way. The amounts are big too, equivalent to the amount of fertilizer farmers typically used to add to their fields 100 years ago.  All over Britain, natural habitats are being damaged by this nitrogen rain, with fast-growing, nitrogen-loving plants taking over from the more salubrious vegetation that used to exist there.

It’s even worse by rivers: the water in a river like the Foss is very rich in fixed nitrogen, partly because of run-off from agricultural fields upstream and partly because, in summer, a large part of the flow in the lower Foss is effluent from Strensall sewage treatment works.  It’s not surprising then that nettles do really well along the Foss and if we want to improve the biodiversity of the Foss banks, we need to look at ways of managing them so that they become just another species in the vegetation, and not the entire vegetation.