As Athenians know all too well, in many of today's cities the color green is as rare as a hailstorm in July. Though weeds may occasionally sprout through the cracks in the concrete walls and sidewalks, and balconies frequently serve as surrogate gardens, nature is conspicuously absent amid the high-rises, cars and crowds of these modern metropolises. It is, of course, to a large degree a matter of space.

But an innovative new trend in urban architectural practice is taking advantage of the unused acres of apartment-building rooftops to plant oases of green that will allow nature to reclaim some of her lost territory. This is the so-called greenroof architecture . Already being applied in urban jungles in Europe and America, the technique is currently in the process of being tried out in Athens' populous western district of Rendis.

Initiated by the district's municipal council, and implemented by the company GreenHellas , the pioneering program involves the planting of a veritable prairie on the roof terrace of one of the area's public day-care centres. Adapted by a Swiss environmental expert for conditions particular to Athens - especially the relatively dry conditions and high CO2 levels - the roof has been sown with 13 species of ground cover and aromatic plants. These include Dimorphetheca , Verbena , the succulent Lampranthus , species of lavender, Santolina and geranium, all sprouting in a soil mix laid over hydrophyllic stonewool, insulation, and a waterproof membrane.

According to specialist Katerina Mavroidi, these materials are light enough for ordinary roofs and inexpensive to install. Planting was done in the fall and the designers are eager to see how their field fares in the summer heat.

What is it good for?

In addition to aiding plant conservation, Rendis' greenroof will absorb CO2 and produce much-needed oxygen, help reduce runoff during heavy downpours, cool the area in summer by reducing the «heat island effect» and reduce the building's energy needs. According to a study performed by researchers in the physics department of Athens University , a greenroof could help lower summer energy use by as much as 30 to 40%.

An illustration of the promise of greenroof techniques can be found on top of Chicago's City Hall in the US. With an area of about 20,300 square feet, the building's lush roof hosts approximately 20,000 plants belonging to 156 species of sedum, forbes, grasses, shrubs, vines and trees. Chicago is a leader of greenroof industry research and will be hosting a conference focussing on green roofs this May.