Sunday 10 June 2018

Are nettles really taking over our roadside verges?

And what we can do about it?

Umbellifers in nitrogen rich ground
A scientist has proposed that excess plant nutrient nitrogen is released by car exhausts and its build up in roadside verges encourages coarse weeds such nettles, brambles and hogweed. The popular press has mangled this story and it is now received knowledge that this is a national crisis. Nettles have had a sudden image transformation from the darlings that ‘every garden should have them’ to being coarse monsters that outgrow the wild flowers.
I suggest that we should concentrate on car exhausts because they might be killing us all and not to worry too much about the nettles.

Delightful coarse growing vegetation
I am sorry to return so soon to the confusion between weeds and wildflowers and that frequently they are one and the same. Only in my last post I marvelled at how dandelions and buttercups had returned to roadside verges since we desisted from herbicide spraying. 
The new opinion is that this transformation might be not be so desirable and invading coarse weeds such as dandelions are a bad thing. Or have I got it wrong and dandelions and buttercups are good and only nettles and hogweed are bad?
Dare I suggest that any so called weed is alright as long it is in the right place.

Pain at first. I have sprayed a three metre length of verge to kill brambles, nettles, ragwort and couch. I will encourage fine grass, buttercups and dandelions to replace them.
No one explains that any coarse weed problem has in truth arisen with the reduction of spraying and even less likely that you will be told that intelligent directed spraying might be the remedy. 
It might surprise you I do not advocate this! 
The dead hand of bureaucracy that indiscriminately sprayed in the first place is incapable of sensitive roadside plant management now. I do not of course reject inspired herbicide management under local control

Lovely, but ragwort is deadly to animals if it seeds into grazed fields 
I became alerted to this perceived problem when half listening to John Humphreys as I invariably do when writing my blog. As so often with scientists (and gardeners) he was not the rottweiller he is with dissembling politicians but was in his pussycat mode.
He can be a mixture of amused confused condescension and fawning acceptance of a scientist’s crackpot ideas (and especially so if nonsense is uttered by some high profile media gardener). 
In this case the news item had already been garbled by the newspapers and the scientist himself when he brought the problem of nitrogen run off from farm fields into the equation.

Confusion between nitrogen pollution and high soil fertility

Here the roadside is probably infertile and this legume thrives because it fixes its own nitrogen
It is fashionable these days to create infertility by removing vegetation to make gardens and landscapes amenable to wild flowers. I have no difficulty with this and admire wonderful wildflower meadows that have been so created. 
It still however goes against the grain for me personally when all my instincts are to create healthy soil as a result of sound soil management.

It would be pie in the sky to expect motorway verges to be like this
I see everywhere gardener’s zealously making a nitrogen rich store of goodness by way of making garden compost and delighting in how it makes plants healthily grow.
In contrast it is now sometimes wrongly perceived as environmental pollution for farmers to create fertile sites ‘that takes years of endeavour to extinguish’
To me it is good for mankind that soil can retain its organic fertility and nutrient store for quite a long time.
It’s not even that fertile soil as such is bad for wildflowers. It's just that the nettles grow so much better and smother them. I believe that there is potential for gardening methods that will grow wildflowers without denuding the soil.
Old gardeners would tell you that overgrown allotments with luxuriant nettles might be hard work at first but have wonderful potential.

Let us not solely ‘blame’ nitrogen for long lasting soil fertility. Indeed the very different problem of nitrogen pollution arises from the high solubility of nitrate when it washes away. The most long lasting soil nutrient is phosphate which in many soils after two hundred years of farming with fertiliser is too high. Major nutrients such as potash have varying soil retention depending for example whether it is sandy or clay.

Do we have a problem on roadside verges?

No problem here with sensitive infrequent mowing
I don’t doubt that car exhausts might be a source of extra nitrogen. I do doubt that it is sufficient to matter in other than areas of very dense heavy traffic. I did notice on my recent trip to Liverpool that they had plenty of examples of nettle rich verges.
Very much less so along the M62 motorway where in places coarse colourful vegetation such as dog daisies, buttercups, campions, umbellifers and a little earlier in the season dandelions, were lovely. Central motorway edges were somewhat devoid of vegetation other than grass.
I would suggest that most of the landscape reflected levels of management - mainly mowing - rather than exhaust pollution.

The trees lining the M62 on that recent Spring day were strong and verdant. Nitrogen is essential for healthy plant growth and motorways are generally constructed with spoil rather than soil. Perhaps a little extra nitrogen has been a good thing?

Do any extra nutrients alongside roads come from agriculture and elsewhere?  

In the countryside I am sure extra nutrients spill in from farm fields
Some surely must where ground is higher and soluble nitrates leach and phosphate rich soil erodes down. Leaves from trees blow and alight and even create organic rich pockets. In terms of plant growth the diverse habitats created might be a good thing.
Salting of roads has certainly favoured salt loving wildflowers to spread from the seaside.

What to do about it?
It was when the mad scientist (I might yet eat these words) suggested ‘solutions’ to his perceived problem. He fell back on the idea of removing fertility on roadsides by mowing and carting vegetation away.
How much would that cost? How much extra carbon dioxide would be created with extra machinery and transport. What on earth might we do with that extra vegetation. Our amateur composts might become even greater rubbish than now.
What about the extra roadside pollution created?

Posts prevent parking (and allows wild flowers to grow)
It’s alright to want delicate wild flowers to grow in favoured roadside habitats. It is wonderful to delight in rare plants alongside country roads and in serendipitous habitats accidentally arisen. 
Let real local native vegetation thrive in the verges and hedgerows and let us do what we can to conserve it.
But don’t expect to achieve tracts of of feeble yet lovely wild vegetation along busy roads.

Roadside fertility might not be such a bad thing
I do not believe that the way forward is to reduce general roadside fertility. We need strong growing stabilising vegetation and all the better that the buttercups are beautiful.

Even 'alien' invaders here in Derbyshire countryside stabilise the verges
I have written extensively about Cathi’s grass verge which borders our country road that carries perhaps 200 lorries a day that fly by. Their trajectory veers when they meet oncoming traffic.
I am grateful to coarse grass, silverweed and buttercups for constantly growing back where the edges erode.
(I hasten to add only about a metre in my case, the rest is fine grass and flowers)

I am grateful that coarse vegetation quickly grows back when lorries cut up the edge
I reviewed the lovely book 'Wild flowers on the edge'
I wrote about corn marigolds on the edge of a farm field

Confusion between weeds and wild flowers


  1. I love to see wild flowers on verges. Each year we have a sea of dog daisies on the verge of the motorway roadabout that looks like snow. I did feel angry enough to write to our council one day after when travelling home from work I noticed that the bluebells that were in full flower along what was signposted as a roadside nature reserve had been mowed down. The council response was that nearby farmers maintained the verges so it was nothing to do with them!

    1. Oh yes it does.. The farmers are on contract to the council - as far as I know.
      Our own local farmers who maintain the verges seem to be quite sensitive - at least they respect what gardeners have planted outside their homes.
      Mowing bluebells in full flower. How dreadful

  2. A number of years ago I noticed a remarkable profusion of Common Spotted orchids growing by the fresh new verges of new by-passes close to Preston, Accrington and Haslingden. No doubt the spores had been lying dormant for many years. Unfortunately they have now disappeared, due to the heavy growth of coarse grasses and the dreaded brambles. It is sad that autumn mowing did not take place. Also, I have been impressed by the wonderful profusion or roadside dandelions in the Spring. They are found very close to the road edges, especially those of busy roads. I assume that they have a natural selection from the salt sprayed on to the verges in the winter months.

  3. Some orchids such as dactylorhiza are wonderful colonisers of the of infertile subsoil exposed and brought in for road construction .
    I would suggest fertility gradually arises as a result of several natural process such as blown soil, leaf litter, animal droppings,as as well as those discussed in the post. The nettles often inevitably follow unless action such as mowing is taken
    On a technical note, orchids are spread by windburn tiny seeds. I am less sure about the movement of mycorrhizal spores that are necessary for survival of newly germinated seed
    As to the salt tolerant dandelions I am sure you are right. Natural selection of tolerance to toxic chemicals can be surprisingly rapid as witnessed around former lead mines in Wales


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