In the face of climate change, Britain's lawned parks should be replaced with urban forests to help control city temperatures and keep green spaces green during hot summers, writes Phineas Harper.

Successive heatwaves have turned England's formerly green and pleasant land into an arid patchwork of yellow and brown. As the UK government declares droughts following the driest months since records began in 1836, it's clear the tradition of lush mown lawns and bucolic grassy parks is no longer viable in the face of global heating.

To reduce ground temperatures and keep urban green spaces habitable and verdant even in 40-degree summers, a radical shift in landscape design is needed.

The tradition of lush mown lawns and bucolic grassy parks is no longer viable

Many British urban parks are modelled on the garden designs of past aristocrats. Picturesque lawns dotted with trees, architectural follies and the odd lake were popular among the 18th- and 19th-century super-rich, who employed influential landscape architects to create manicured imitations of the agrarian countryside.

As social reformers gradually founded urban parks, they created municipal versions of the wealthy's grand gardens, adding public facilities like drinking fountains and flat playing fields for sport. Though its design was rooted in genteel tastes, the grassy British public park with its wide open lawns thrived in the UK's mild climate, damp enough to sustain thick mown grass year-round.

Writing in 1935, Danish architect Steen Eiler Rasmussen praised Britain's parks, which he felt had become unique among European cities. For Rasmussen, while the UK's urban squares were functional, they were never the inclusive mixing pot of the continental esplanade or piazza, but in parks he found "an emancipated outdoor life, to which all classes have access". "No one is nowadays too good to play with balls or to kick a football in one of the numerous parks," he observed.

Yet today, Europe's worst drought for 500 years and successive heatwaves have baked green spaces across much of England to barren wildernesses with bone dry playing fields and crispy dead grass, making bleak landscapes for recreation or relaxing. The mild weather that once sustained the expansive green lawns of landscape garden-inspired public parks is gone and local authorities, landscape professionals and park goers must face the fact that urban green space will only remain green in the future by ending the British obsession with mown grass.

A typical mown lawn needs 10 litres of water per square metre per week in dry weather to stay healthy. At the scale of parks or other large urban green spaces such as golf courses this exorbitant water consumption is enormous, putting huge pressure on limited water supplies. Activists in France have even poured cement into golf course holes to protest their water use.

Urban green space will only remain green in the future by ending the British obsession with mown grass

Mown grass is not only extremely thirsty, it is often terrible for biodiversity. According to Adam Hunt of landscape architecture studio Urquhart & Hunt, "Many large expanses of grassland, especially public parks and cultivated fields, are effectively green deserts with very limited biodiversity." Research by conservation charity Plantlife confirms that even a well-watered mown lawn supports 10 times fewer bees and other pollinating insects than less aggressively managed green spaces of equivalent size.

Yet even as our emaciated grassy parks have turned yellow, most of their urban trees have remained green, leafy and lush. Unlike mown grass which has shallow roots, mature trees are able to tap moisture deep underground. Trees can also soak up vast amounts of liquid in wet seasons and hold it through months of dry weather. A single healthy tree can hold thousands of gallons of water.

Not only do trees stay green in dry weather, research published last year shows that trees can bring down urban temperatures by between 8 and 12 degrees Celsius, providing shade and reducing local evaporation. Which is why the only green blades of grass left in otherwise singed parks tend to be under tree canopies.

Despite the advantages of plentiful trees, most UK parks have been planted with far fewer than there is space for. London's Royal Parks such Hyde Park and Regent's Park, for example, contain on average only 84 trees per hectare (about 8 trees every 1000 square metres) while many less well-funded green spaces have fewer still. This low ratio of trees-to-open-space means that with increasingly dry summers, large swathes of ground will be baked hard as they are exposed to the full force of the sun's rays for longer.

British parks could retain plenty of space for frisbee, football and picnics while supporting 75 million new trees

Rather than continuing to model British parks on the lawn-heavy landscape gardens of long-dead aristocrats, a far better strategy would be to massively increase the number of trees in urban green spaces – turning big open lawns to small urban forests. There are roughly 150,000 hectares of urban green spaces in Britain. If we generously assume that these have roughly as many trees per hectare as the Royal Parks, there is easily capacity for at least a further 227 million urban trees to be planted across Britain's towns and cities.

Hunt proposes that a simple formula of one-third trees, one-third scrubland and one-third uncultivated pasture would produce far more sustainable and biodiverse green spaces than acres of mown grass. If this strategy were adopted, British parks could retain plenty of space for frisbee, football and picnics while supporting 75 million new trees – a combined forest capable of sequestering nearly two million tonnes of carbon dioxide a year as well as cooling our urban centres.

Reforesting would not only keep green spaces greener in extreme heat but could fend off floods too. Robert Thompson at the University of Reading has demonstrated that dry ground hardly absorbs water, meaning heavy rains after droughts can cause severe flooding. Tree roots, however, improve water infiltration by 157 per cent, helping soak up downpours quickly and alleviating flood risks. Even in wet seasons, urban trees help prevent flooding by reducing surface water runoff by 80 per cent compared to hard surfaces like asphalt.

For decades British municipal parks have been modelled on centuries-old aristocratic tastes. Innovators of the past successfully reinvented the English landscape garden tradition, turning rolling lawns from spaces of power and status into municipal facilities for all. To ensure British parks continue to provide hospitable green space for a comfortable and convivial outdoor life in the face of a changing climate, they must now be reinvented again. It's time to turn our scorched parks into urban forests.

Phineas Harper is director of Open City and formerly deputy director of the Architecture Foundation. He is author of the Architecture Sketchbook (2015) and People's History of Woodcraft Folk (2016).

The photography is by Tom Ravenscroft.