The relationship between percentage of organic matter in substrate and plant growth in extensive green roofs

Choosing an appropriate substrate is one of the most important factors for green roofs because of their harsh growing environment. However, there have been few studies on the amount of organic matter necessary for sustainable plant growth in extensive green roofs. This study examined the relationship between percentage of organic matter in substrate and plant growth in extensive green roofs, particularly at establishment stage. Different percentages of organic matter (green waste compost: 0%, 10%, 25%, 50% by volume) were added and mixed well with a commercial green roof substrate (crushed brick base, containing less than 4% organic matter). Four species of plants (Allium schoenoprasum. Limonium latifolium, Melica ciliata and Nepeta x faassenii) were grown in a greenhouse in trays containing substrates with these different percentages of organic matter. Two watering regimes were imposed: wet and dry (watering every 5 days or every 15 days). It was concluded that addition of 10% organic matter was optimal for these four species and the substrate used in this study because the plants showed stable growth regardless of the watering regime. In the wet regime, increased organic matter resulted in lush growth with may result in damage during drought. In the dry regime, increased organic matter did not result in increased growth. The response to higher organic matter content was different for different species, and species from a nitrogen-rich habitat tended to be encouraged by a high content. (C) 2011 Elsevier B.V. All rights reserved.