Question of the Week

Do motorways create a microclimate?

Sat, 26th May 2012

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Edward Draper asked:

Dear Chris,


Love your podcast been listening to it driving down the full length of the country - good driving audio - keeps you awake, alert and amused. Thank you.


I do have a question - at this time of year, late March - early April, the verges and central reservations of roads get covered in white, pin-sized blossoms - I've stopped and looked at the plants and they are a scurvy grass


This is normally found on beaches, but have clearly found a successful niche on the side if main roads - I'm sure salting the roads is a major factor in this


My question is: given the ubiquity of this plant - has this plant been spread by the road salting or has it propagated along the sides of the roads following these narrow salty corridors?


It appears to be a single species - or are there species I have yet to find?


Any help in answering this would be gratefully received


Keep up the good work,




Dr Edward Draper


We posed this question to Guy Barter, Chief Horticultural Adviser at the Royal Horticultural Society.....

Guy -   We think the plant in question is a native, despite its name Cochleariamotorway verge danica or Danish scurvy grass.  Itís not actually a grass, but itís related to the cabbage family and known to be a halophyte, thatís a plant that grows in the presence of some salt.  Itís well-known spread along roads, often at a remarkable rate per year where presumably the turbulence of passing vehicles help shift the seeds.  Itís a recent example of the many instances where human activity helps plants spread.  Danish scurvy grass is an annual that thrives on well-drained but not too dry sandy soil.  So run off from roads would suit it well as with the often rubble-like soil beside motorways. It tolerates coastal soils so itís well-adapted to high salt levels beside roads.  The seeds ripen in late summer and disperse that season.  The young leaves are actually edible cooked or raw, but apparently have a pungent flavour.  Itís rich in vitamin C,  lack of which causes scurvy.  However, the dust and the dirt beside roads would make me think twice before picking any.

Hannah -   So, this salt-loving vitamin C rich grass traditionally eaten by scurvy threatened sailors is spreading along the major roads of central England.  Rock salt, carved from underground mines, is sprinkled on the roads by gritters in the winter time and this creates conditions similar to the scurvy grasses normal saline coastal habitat.  I spoke with Cambridge County Council and they go out between 30 and 40 times a year to deliver 200 tons of rock salt, each time, spreading it along 2,200 miles of roads across the county.  That's 90 kilograms of salt per mile or about 10 teaspoons of salt per metre of road.  But in the meanwhile, how is scurvy grass spreading across the length and breadth of the country so quickly?  Well, the air turbulence of passing traffic causes a pocket of low pressure behind the car, sucking in the seeds, and dragging them along for large distances.


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I understand that the Highways Agency have looked into this (partly as they have to keep on top of vegatation to prevent drains blocking up etc). and appears to be a relatively new phenomenon - with the spread really being noticed in the late 1980's.  This may be coupled with improvements in road ice prediction systems (such as the one operated by Vaisala) and more systematic application of grit.

There is little doubt that it has spread along the salty corridors of major roads rather than by spreading of grit contaminated with scurvey seed.

I would suggest that anyone who has been unfortunate enough to stand next to a major highway in the rain will have experienced spray from passing vehicles and how this turbulence could spread seed along roads.  Mazurka, Mon, 30th Apr 2012

Do motorways create a microclimate?

In some ways, yes.


source Lmnre, Mon, 30th Apr 2012

There is also the well documented effect called "Urban Heat Island".  Cities, perhaps due to the large dark colored asphalt area tend to be a couple degrees warmer then the surrounding countryside. CliffordK, Sun, 27th May 2012

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