The humble tree of heaven (Ailanthus altissima) is actually a weed that is notoriously hard to eradicate but has the decided advantage of being able to absorb 100 times more carbon dioxide from the atmosphere than the plants it squeezes out of its way, according to a commentary in Kathimerini last Sunday by Takis Kambylis (see commentary extract below). He jokingly wonders whether anyone has suggested the hardy, prolific plant as ideal for covering Athens roof terraces to help fight the greenhouse effect over what is one of the most polluted cities in Europe. «Let us hope not,» added Kambylis. «(The use of) ailanthus would be just one more concession, one more inappropriate response to the concept of ecology.» Last month, Kambylis reported on the efforts of a retired Athenian jeweler, 81-year-old Aristomenis Bolotas, to fight a system that raises bureaucratic obstacles to the spread of such roofs, which have been shown to lower temperatures and raise oxygen levels in the atmosphere, among other benefits. Now Kambylis rightly says that while green roofs may perhaps help improve the city’s atmosphere, they cannot be a substitute for the parks that are slowly disappearing under concrete or being razed by wildfires. However, experience in other Mediterranean climates shows that planting on roofs does make buildings cooler in summer and that the roof itself need not be, nor can it be, «green» all year round. According to some experts, in a Mediterranean climate plants should be chosen purely on the basis of their hardiness and not for their appearance – although allowing the ailanthus to go wild across Athens’s roofs might be taking that too far. They suggest drought-tolerant succulents as the most suitable plants for an exposed, hot surface – rather than more delicate flowering shrubs and grasses or larger plants. According to other sources, plants found in habitats where there are climatic extremes, where they have adapted to conditions like drought and wind exposure, such as those found in semi-desert areas, should be used for roofs in a Mediterranean city. Dwarf or spreading plants are more resistant to prolonged drought, as well as plants growing in thin, free-draining soils. Plants with dense twiggy growth, gray or smaller sized leaves are also adapted to losing less water. As for the effectiveness of green roofs, in a paper by Eleftheria Alexandri and Phil Jones of Cardiff University’s Welsh School of Architecture, presented at the 23rd Conference on Passive and Low Energy Architecture in Geneva in 2006, a comparison was made of various techniques for lowering raised urban temperatures in Athens. A white-coated concrete roof was found to lower surface temperatures to a maximum of 20.9 Celsius with a daytime average of 12.4C, but the green roof produced even greater temperature decreases, said the researchers. «For the surface temperature, the maximum temperature decrease (on a green roof) reaches 26.6C, with a daytime average of 14.4C… Due to the dryness of Athens, the evapo-transpiration from plants is able to lower air temperatures to greater levels than a non-transpiring surface, which absorbs lower amounts of solar radiation.» So while isolated green roofs might not do much to reduce the overall temperature in a city like Athens, there is much to be said for their usefulness in bringing down temperatures within the buildings themselves. And if enough green roofs do appear, who knows, maybe a critical mass can be reached to improve the microclimate in a particular area.