Green cities. Stand up for livable cities

What if human cities also became cities of trees?

What if trees and people stood next to each other, with each other and for each other?

This was also the vision of Friedrich Hundertwasser. He wrote: "It should become common knowledge to give nature back on the roof the space that is taken away from it by the floor space of the house."

He describes his vision as follows: "Wherever snow and rain fall, vegetation must grow freely. The roofs must become forests, the streets must become green valleys."

For us humans this would have the following advantages:

• The trees would provide shade in summer and cool the air
• The air would be cleaner
• People would be healthier 
• The green areas could store water during rainfall
• The cities would be more beautiful and lively
• With the trees, other plants and animals also come into the city

Why we need more trees in our cities

In recent years, both the number of hot days and the number of heavy rains have increased. According to Carbon Disclosure Project By 2024, 83% of cities worldwide will report a risk of flooding or heat.

What is important is that trees and green spaces can regulate both: they cool the air when it is hot and store water when it rains.

Trees provide shade and shaded areas heat up less because they are protected from direct sunlight. In addition, trees evaporate considerable amounts of water, thereby creating an additional cooling effect.

Using infrared images, different surface temperatures could be made visible. It was measured that asphalt surfaces such as roads are up to 40 °C warmer in direct sunlight than surfaces covered with plants. Green spaces act like climate oases in urban areas.

In addition, water can seep into green spaces and thus be retained. The retention and storage of rainwater in green spaces can both stabilize the urban water balance in dry periods and reduce the risk of heavy rain events.

In addition to these microclimatic regulations, trees also improve the air in cities, which is particularly important as the heat increases. For example, they reduce the amount of ozone, nitrogen oxides, carbon monoxide and sulfur in the air.

You can find out more about the effect of trees on the urban microclimate in this article. 

Unfortunately, the current reality is that our landscapes are becoming increasingly sealed. According to the Federal Environment Agency, around 54 hectares are newly sealed every day in Germany.

But what would the alternatives be?

Roof gardens and green roofs

The roofs of houses have so far hardly been used for planting. Yet they have incredible potential for greening. Or as Hundertwasser wrote: "Wherever snow and rain fall, vegetation must grow freely. The roofs must become forests."

However, there are some initiatives in Germany. In Stuttgart, for example, private green roofs have been promoted for a long time. As a result, over 60.000 square metres of additional green space have been created on private roofs in recent years, in addition to 130.000 square metres on the roofs of public buildings. And Hamburg is the first German city to launch a comprehensive green roof strategy. Its goal is to green at least 70 percent of both new buildings and flat or gently sloping roofs that are suitable for renovation.

Roof gardens and green roofs offer the following advantages:

• The vegetated ground provides protection from heat in summer and from cold in winter. According to Conrad Amber With a floor depth of around 50 cm, the rooms below are four to five degrees cooler at high temperatures than comparable rooms without such natural protection. The room temperature of the attic would therefore be roughly the same as that of the ground floor. In winter, however, the insulation upwards leads to higher internal temperatures.
• According to Conrad Amber, the initial investment in a roof foundation would be paid off in two to three years due to the reduced heating and cooling costs.
• The vegetated soil also protects the underlying materials against cold and UV rays. According to Conrad Amber, this increases the lifespan of the roof covering by at least double.
• During heavy rain, the vegetated soil acts like a sponge, absorbing water and then releasing it again over a long period of time.
• The plants return the water to the air through evaporation. This process cools the surrounding air.
• Roof gardens would also create valuable habitats for plants and animals.
• A roof garden could also provide additional green space for its human residents. Perhaps a cozy corner or a bed of tasty herbs. Small fruit trees could provide tasty fruit.
• For people without gardening ambitions, the roof gardens can also be planted with easy-care perennial plants.

Here are two more current and prominent examples:

• In Rotterdam there is a project called DakAkker, which includes 1000 square meters of garden plus a café.
• In New York there is a Roof farmwhere lettuce, herbs and vegetables are grown. The plants are pollinated by bees from several hives. 

Green facades

Green facades also make a significant contribution to improving living conditions in cities. They cool the temperature, filter the air and absorb noise. There are basically two options for green facades:

• Climbing plants such as ivy or grapevines can add greenery to house walls.
• Plants can be grown on vertical surfaces on house walls using special modules.

An impressive example of green facades and roof gardens is the building Kö-Bogen 2 in Düsseldorf. The department store was planted with around 35 hornbeams on all floors, which are watered by computer during dry periods. Other well-known greening projects were carried out by the French botanist Patrick Blanc. For example:

• Green Hotel in Paris
• Caixa Forum in Madrid
• One Central Park in Sydneyy

You can find even more interesting projects on the website of Patrick White.

Green gardens, courtyards and tiny forests

With clever design, you can grow many different plants in a small area. A helpful concept for this is permaculture. In a permaculture garden, plants of different heights are planted one below the other and next to each other. The garden is carefully monitored and designed in such a way that wind, sun, shade and space are used optimally.

For example, large trees will be planted on the north side so that they do not shade all the other plants. Berry bushes or vegetable beds will then be planted on the south side.

Through clever design, a microclimate can be created that also allows for more exotic trees such as figs.

You could also create a compost in your garden, which will give you Fertilizer liefert.

Another wonderful way to intensively green small areas is to create highly diverse mini forests. At the suggestion of the Japanese biologist Akira Miyawaki, around 3000 tiny forests have been created worldwide.

Further examples of green cities and greening projects

A global pioneer in the greening of cities is Singapore, .

But there are also some wonderful projects here in Europe:

• Vertical Forest in Milan
• MFO Park in Zurich
• Coffin factory project in Vienna
• Roof gardens on the Hundertwasser houses
• Townhouse in Venlo

Further information

The website of Biotope City offers diverse and qualified knowledge about greening projects from all over the world. Due to the expertise of the editors and the approximately 120 authors, this site enjoys an above-average reputation in the professional world.

We also recommend the Berlin Rainwater AgencyHere you will also find information about green roofs and facades and information about funding opportunities. 

Recommended reading

• Rising up against drought: How nature can help us end the water emergency.
• BTrees on the roofs, forests in the city by Conrad Amber