WASHINGTON: Greening of roofs by having plants on them is gaining popularity in the US, where their numbers have increased by 30% from 2006 to 2007.

Benefits include improved storm water management, energy conservation, reduced noise and air pollution, improved biodiversity, and even a better return on investment than traditional roofing.

But a healthy roof requires a plant species that can survive extreme climates and propagate easily to reduce erosion and weed growth.

Kristin L. Getter, Michigan State University (MSU) horticulturist, conducted a study to determine the effect of the soil depth on success of green roofs. The research focussed on Sedum, a variety of succulents known for its drought tolerance.

Plots were constructed using the drainage mats and waterproofing systems typical of green roofs, but the growing material varied in depth -- four cm, seven cm and 10 cm.

Twelve species of Sedum were planted, fertilised, and watered once. The moisture of the growing material was measured at random times each week.

Plants were monitored over four years. Since the average lifespan of the inorganic components of a green roof is about 45 years, the researchers determined that it was important to study the longevity of the plants.

The study found that the shallowest plot had the lowest moisture levels on average and dried the fastest after a rain. At the four-cm depth, four species failed to exhibit significant growth over the four-year period.

Results indicate that, for the surviving and most-abundant species, no benefits accrue in case of depths more than seven cm, but make for lighter roof loads, says an MSU release.

"However, at deeper depths, these plants would likely be healthier, contain greater biomass, and be less susceptible to adverse environmental conditions," says Getter.

The study was published in HortScience.