Trees in parking lots: a guarantee for urban comfort

19 June 2017

Trees in parking lots help decreasing pollutants and also reduce temperature on the ground line. Let’sdiscover its advantages and how to design a shady parking lot.

Has anyone, coming back to a car left under the sun, ever been in discomfort because of excessive heat? Isn’t thattrue, especially during the summer, when drivers always look for a shaded parking spot? Then why parking lotswithout trees get designed and built in our cities all the time?

The urban heat island

It may look a trivial concept. When the weather is too warm, being in the shadow is more comfortable thanbeing in full sun , especially if we are in a town. Most people, in their spare time during summer, pile out inavailable green areas. Only there they can experience a feeling of relief from the oppressive heat that rests onareas with more buildings.

Covering the soil with waterproof structures (like building or roads), together with the pollution created by exhaustgas, creates in fact a pronounced increase of the temperature in urban areas. This is especially clear whencompared to green areas as parks, agricultural soils and gardens. This phenomenon is what we are referringwhen we speak of “Urban heat island”. Leaving the city center the temperature can decrease as much as 5°C(9°F)!

Graph of the urban heat island effect, highlighting the difference in temperature between town and countryside

The benefits of trees on urban microclimate

The shadow generated by the presence of trees leads to a strong reduction of temperature on the ground line,especially when that shadow is cast over paved surfaces. Cars parked in full sun can reach temperature even25°C-30°C (45°F-55°F) warmer when compared with same cars parked in a shaded spot. If the shadows aredue the presence of trees, things get even better. The benefits of trees in terms of pollutants reduction are wellknown especially when it comes to urbanized environments. Anyway, surely is more pleasant to come back to acar parked in a shady spot than to a car left in the sun!

Infrared image filmed with thermal imaging camera: the difference between the vehicles in the shadow of trees (blue-green) and those in fullsun (red-white) is highlighted.
Parking lot with trees elevation drawing

Parking lots without trees: why?

Unfortunately, in Italy, trees in parking lots aren’t the rule yet. It’s very common to find parking lots that are verysunny, without any tree. The choice to avoid tree plantings can be due different reasons:

  • Optimization available space. Tree plantings and relative borders would reduce the space available to vehicles,which would decrease in number.
  • Need to reduce the costs. Funding borders and trees with all the needed accessories (banquettes, topsoil,waterings, eventually bushes to cover the soil) can impact on the overall costs of the parking lot.
  • Incompetence of the designer. Not every designer necessarily knows about the tree-generated benefits in amarkedly artificial environment like a paved parking lot.

Trees in parking lots: design tips

A well designed parking lot will produce beneficial effects in a short time, allowing for a better liveability of urban areas. But to guarantee a good result in terms of environmental benefits, some precautions have to betaken. Some suggestions to get a well done job when designing a parking lot with trees are shown below.

  • Trees need room.Too often the trees are in borders which are too small, borders where they are not allowed togrow properly and where they can’t express their full potential. Because of this, it’s very important to design aborder, for trees, that will be big enough to ensure that at least some soil surface will remain permeable and willallow roots development.
  • Waterproof surfaces should be reduced as much as possible. The paving of large surfaces insulates the soilfrom the air and creates problems of asphyxiation on roots. Designing parking spots which use turf as soil cover,instead of paving, especially when using permeable interlocking pavers, allows the water/gas exchanges,improving physical/chemical characteristics of the soil.
  • Tree collars and roots have to be protected.In parking lots cars often deal damage to tree’s stem and roots.The plants have to be adequately protected, by the means of reasonably large borders, or trough specific obstacles in order to prevent the contact between vehicles and tree stems.
A shaded parking lot thanks to thepresence of trees

A shaded parking lot is possible

Increasing our cities liveability is one of the challenges that awaits us. The benefits generated by green areas have been demonstrated beyond any doubt bythe scientific community. Not just parks and garden, but also a tree lined parkinglot can help in ameliorating urban microclimate . So it’s also fundamental to spread the knowledge of trees importance for liveability in urban areas, in order to make trees in parking lots common in the asphalt jungle that our citieshave become.

Benefits of trees in parking lots is a well known topic in our design experience, asfor example in the masterplan of Marinella di Sarzana