Volunteers are halfway to planting a tree outside every school.

Outside St Brannoc’s Primary on Foundry Lane, a row of young lime trees stands in freshly dug pits, each one tied to a wooden stake and fitted with a green watering bag. They were planted on a Saturday morning in October by thirty-odd parents, two teachers and a dozen children who took turns firming the soil with their wellingtons. They are trees number 5,112 to 5,121 in a project that aims to plant 10,000 across Calder by the end of next year.

The scheme, run by a volunteer group called Shade the School Run, has a simple goal: a tree outside the gate of every school in the city and along the streets children walk to reach it. Two years in, it is just over halfway there.

Why schools, and why streets

The idea came from research carried out by environmental scientists at the University of Calder, who measured air temperature and pollution at child height along forty school routes over two summers. They found that pavements with mature tree cover were on average 3.5C cooler on hot afternoons than bare streets nearby, and that levels of fine particulate pollution were measurably lower where trees and hedges stood between the road and the footpath.

“Children breathe faster than adults and their lungs are closer to the exhaust pipes. A line of trees at the kerb is one of the cheapest health measures a city can take,” said Dr Ayesha Brightwater, the atmospheric scientist who led the study and now advises the volunteers on species and placement.

Her work also suggested that the benefits depend heavily on what is planted and where. Dense evergreen hedges at the kerb work best for filtering pollution; tall, broad-canopied trees do more for shade. Planting the wrong species in the wrong spot, she warned, can trap dirty air at street level rather than disperse it.

How the planting works

Each planting begins with a survey. Volunteers walk the streets around a school with a council arborist, mapping underground cables, drains and gas pipes, and checking that pavements are wide enough to keep a clear path for wheelchairs and buggies. Where a pavement is too narrow, the group looks for space in school grounds or on nearby verges instead.

  • Small-leaved lime and field maple for wider pavements and shade
  • Rowan and hawthorn for narrow streets, where a smaller crown is needed
  • Hornbeam and yew hedging along busy roads to screen traffic
  • Wild cherry and crab apple in school grounds, for blossom and fruit

The council pays for the trees, the stakes and the work of cutting pits into the pavement, using a budget of around £400,000 a year set aside from its climate fund. Volunteers do the planting and, crucially, the watering. Each school signs up a team of families who agree to fill the trees’ watering bags once a week through the first three summers, when young trees are most at risk.

That model has kept losses low. The group says that around 6 per cent of the trees planted so far have died, mostly in the dry summer of last year, compared with failure rates of 20 per cent or more in some earlier city schemes that relied on contractors to water.

Measuring what the trees do

Because the trees are young, most of their benefits lie years in the future. A lime planted this autumn will not give meaningful shade for a decade. To track progress, the university has installed small sensors measuring temperature and air quality outside twelve schools, six that have been planted and six that have not, and will compare the data over the next twenty years.

Some critics argue that the money would do more good spent elsewhere, for example on traffic restrictions around school gates during drop-off and pick-up. Brightwater says the two are not in competition. “Closing a street to cars at half past eight helps for an hour,” she said. “A tree helps all day, every day, for a hundred years. We should do both.”

For the children at St Brannoc’s, the science matters less than the names. Each of the ten trees outside the school has been adopted by a class, which has given it a name and will measure its trunk every spring. The lime nearest the gate is called Gerald. Its girth, recorded in a notebook kept in the Year 4 classroom, is currently eleven centimetres.