Thermal benefits of green life in the urban landscape

Landscaping choices make a difference

As more people move into urban areas and global warming increasingly threatens to affect our lives, can our landscaping choices help? The scientific literature reveals that the answer is a definite yes.

With increased pressure on urban water supplies, is irrigating such landscape elements really a waste of valuable water supplies? The literature shows that living landscape elements are an essential component in an urban environment and the benefits they provide outweigh the perceived disadvantages in maintaining them.

Review and research

A literature review of international research was conducted on the possible role of plants in alleviating high temperatures in our living spaces. The review was finalised in February 2008 by staff from the former Department of Primary Industries and Fisheries for a project between Landscapes Queensland and Horticulture Australia Limited. It identified work that has already been carried out in the area and highlighted the gaps to be filled by experimental research. A pilot study then investigated the thermal properties of six of the most common landscaping materials.

Benefits of greenlife

This project clearly showed that plants can play a significant role in modifying the thermal conditions of urban environments. Tall trees can shade nearby buildings and reduce cooling costs. As well as basic shading, the dispersal of heat via the plant´s natural transpiration stream has long been recognised as an important component of the urban energy balance. Urban temperatures can be up to 7°C higher than nearby rural areas, illustrating the effect of plants on their environment.

These benefits argue against removing plants from landscapes to save water in drought. Similarly, the idea of switching to artificial turf is questionable, as artificial turf still requires watering and can reach temperatures that far exceed the safe range for users.

While vegetation offers evaporative cooling, non-vegetative and impervious surfaces such as concrete do not. Therefore, they can cause greater surface and soil temperatures. In addition, the higher temperatures associated with these impervious surfaces can negatively affect the growth of plants in surrounding areas.

Permeable surfaces, such as mulches, have better insulating properties and can prevent excessive heating of the soil. However, they can also lead to an increase in reflected longwave radiation, causing the leaves of plants to close their water-conducting pores and reducing the beneficial cooling effects of transpiration. The results show that the energy balance of our surroundings is complicated and that all components of a landscape will affect thermal conditions.

Landscaping materials – thermal properties

The temperatures observed in the experimental phase of this study highlight the variability in thermal properties of landscape materials. The temperatures observed over non-vegetative surfaces were well above those considered safe for any form of activity in terms of recommendations from healthcare professionals.

Surfaces tested were:

• concrete
• clay pavers (light terracotta)
• moisture-stressed turf grass (green couch)
• gravel (10-25 mm angular multi-coloured)
• synthetic turf (third generation)
• pine bark mulch (25 mm ´nuggets´).

Temperature readings were averaged according to surface for both 24-hour minimum and maximum temperatures across all replicates and time, for the period between 9 November and 19 December 2007.

Concrete and paving had significantly higher minimum temperatures (22°C and 20°C respectively), suggesting the materials were able to store heat. Meanwhile, moisture-stressed turf grass (18°C), gravel (18°C) and artificial turf (19°C) could not be statistically separated with this parameter, but clearly stored more heat than mulch (17°C), which had significantly lower minimum temperatures.

Concrete (44°C) and paving (45°C), and moisture-stressed turf grass (50°C), had the lowest maximum temperatures. However, moisture-stressed turf grass could not be separated from paving, gravel (52°C) or mulch (53°C).

The experiment was conducted during a drought period. The turf grass was not irrigated and soil moisture levels were low. Cooling effects and lower temperature readings have been recorded in other experiments with actively transpiring irrigated turf grass. Artificial turf had significantly higher average maximum temperatures (61°C) than any other material. The requirement to use water to cool artificial sports surfaces negates the major selling point of such materials.

Implications

This study highlights the need for caution when choosing landscape materials. The time of day when the area is under greatest use and the types of activities to be carried out must be considered when planning any landscaping around residential properties, educational facilities, office buildings or sporting facilities.

A comprehensive database of thermal properties would allow landscape designers, architects and home owners to make informed decisions about the type and proportions of different materials for each landscaping project.

References

• Poulter, RE,  Holborn, S & Dahler, J 2008, ‘Collation, analysis and research of thermal benefits of green life in the urban landscapes’, Horticulture Australia Limited Project Report HG06133, p. 35.
• Steinke, K, Chalmers, DR, Thomas, JC & White, RH 2009, ‘Summer drought effects on warm-season turfgrass canopy temperatures‘. Applied Turfgrass Science doi:10.1094/ATS-2009-0303-01-RS.