How unsealing your paved garden can help reduce city heat

It's a summer night. After a sweltering day, temperatures have barely dropped. Heat radiates from the pavement. Sleeping is difficult because it's too warm. But just 15 kilometers away in the leafy countryside, everyone is sleeping soundly.

Urban areas can be up to 7 degrees Fahrenheit (3.9 Celsius) warmer than nearby countryside, according to the US Environmental Protection Agency. That's because of the "urban heat island effect."

The phenomenon is down to the ways cities are built. Glass, steel and concrete buildings and paved-over ground surfaces trap heat and radiate it back into the air. The result is tropical nights.

Heat can be dangerous. One study in scientific journal Nature found more than 60,000 people died in Europe from heat-related illnesses in summer 2022. The IPCC, the UN's climate science institution, said deadly heat could affect more than half of the global population by 2100. Cities, which are home to 56% of the world's population and rising, will be exposed to much greater heat compared to rural areas.

Some cities like Copenhagen in Denmark are tackling heat island effects by rolling back impermeable surfaces and creating green spaces. But individuals are also trying to do their bit by greening their sealed over gardens and driveways. 

How do I unseal surfaces at home?

"You can use a garden hoe or a shovel to get started," Fatma Özkan told DW. She works at a consumer advice agency in the western German state of North Rhine-Westphalia and helps to guide people through the unsealing process.

Özkan unsealed her driveway a few years ago because she wanted a more environmentally friendly property. Over the course of a few days, she removed the pavement, replacing it mainly with grass.

"If you are dealing with a bound surface like concrete or asphalt, you usually need a better tool such as a drill," explained Özkan, adding that it's best to consult professionals if you want to unseal such a surface.

Unbound surfaces like sand, wood chippings or paving stones without concrete or cement mortars, can be ripped out more easily.

Unsealing surfaces helps with water management

Rainwater flows on impermeable surfaces like asphalt and can't seep into the soil. Excess water drains into the sewage system but when heavy rain hits, sewers can become overwhelmed and cause severe flooding. In some cases, people can lose their homes, belongings or their lives.

Unsealing surfaces allows soil to store water that can then be used by plants in drier periods. Replacing concrete and sealed surfaces with grass and vegetation also creates "natural air-conditioning," said landscape architect Wolfgang Heidenreich. That's because some of the stored water evaporates when it gets hot, cooling the surrounding area.

"It is like stepping out of the shower. Once the water evaporates, it gets cold on your skin," said Heidenreich, who works for Green City, a German environmental organization offering advice on creating eco-friendly urban environments.

It's crucial to pick a surface that lets water seep through in the space you've unsealed. Stone gardens are trendy but "they don't make any sense. They tend to retain heat," said Heidenreich, who is based in southern Germany. Unlike soil they cannot store water.

Heidenreich advocates for the use of drought-resistant grass and herbs which are suitable for cold winters and hot summers.

"You should check which seeds grow well in your region, so they are fit to survive in the regional climate," Heidenreich added. This also helps vital pollinators like wild bees flourish and promotes biodiversity.

But what if I need that space and can't just turn it into a garden?

For those looking to unseal a driveway or garden without losing space to walk or park their car, Fatma Özkan suggests opting for "wood chips, wooden grates, or grass pavers with open stones in the middle."

In her own driveway, Özkan chose a few stones that provide a good amount of space in between for grass to grow. This way, water can seep into the ground, but she can still drive up to her house.

Flat roofs, especially gravel-covered ones on garages, for instance, can be unsealed as well.

"You just remove the gravel and open up the possibility to plant herbs on your roof," Heidenreich said.

To help the herbs grow you can use a "roof garden substrate," which is" a special soil mix to support plant growth."

People don't need to wait for governments to start big unsealing projects "everyone can start with their own property," Heidenreich said.

Edited by: Jennifer Collins and Sarah Steffen

Sources: 

Federal Environment Agency (UBA), "Better usage of unsealing methods for the restoration of soil functions and climate adaptation," 2021 (in German):  
https://www.umweltbundesamt.de/publikationen/bessere-nutzung-von-entsiegelungspotenzialen-zur 

Consumer Advice Center North Rhine-Westphalia "From Sealing to Unsealing," May 2023 (in German): https://www.abwasser-beratung.nrw/wissen/verbraucherzentrale/von-der-versiegelung-zur-entsiegelung-50358