Green Roof Substrate Research

Interim results from a Royal Horticultural Society (RHS) research project suggest that the use of green compost has potential for use in green roof substrates.

The research, which is due to be published soon, is specifically looking at the use of green compost (derived from composted food waste and general green waste) and the addition of biochar, a form of charcoal.
According to RHS scientist Dr Paul Alexander, who presented the paper at the World Green Roof Congress in London on 16th September, this combination may provide a useful alternative to the materials currently used.

“Just as the potting compost we use has been developed and improved over many years, substrates for green roofs, which are a relatively recent innovation, need investigation to optimise them for different plant types and climates,” says Dr Alexander. “Green roofs are increasing in popularity and are important for a number of reasons including urban biodiversity, storm water management and helping to improve air quality.”

The RHS research, funded by WRAP (Waste & Resources Action Programme), is looking at the maximum amount of compost, combined with biochar that can be used in green roof substrates. Until now it has been felt that the inclusion of green compost should not exceed 15 per cent, mainly because of concerns around nutrient leaching. The addition of biochar may make it possible to include even more compost as it is suggested that it helps retain nutrients.

The research uses five different substrate mixes combining different proportions of green compost, biochar and crushed brick. The performance of a range of plants common to chalk and limestone areas in these media in simulated green roofs was studied.

“There is an urgent need to combine horticulture knowledge of plant growth media with the emerging discipline of green roof design and today’s conference is an important aspect of this,” says Dr Alexander. “Hopefully, designers will want to work more closely with organisation such as the RHS so that between us we can improve the environment for everyone living in cities.”

Article posted on 21 Sep 2010.

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