Plant growth responses to different growing media for green roofs

•Growing media with higher water holding capacities had a beneficial effect on plant growth.•Limiting growth of drought tolerant species allows other species to compete more effectively creating greater species diversity.•Organic matter in growing media could be increased when inorganic substrates limit plant growth.

The vegetation on green roofs contributes to water retention, thus reducing peak flow and flooding in urban areas. Green roof vegetation can also have a positive effect on the health and well being of urban dwellers, contributes to reducing the urban heat island effect and is important for improving the biodiversity value of a green roof system. To deliver these benefits, it is essential that vegetation growth and development is supported by the growing medium. The response of sedum and forb shoot biomass to variations in growing media composition was tested outside in green roof simulation decks (decks) over a two year period from seeding. Decks were filled to a depth of 150 mm with growing media made from crushed brick, fine crushed tile or Lytag®, amended with 20% v/v or 30% v/v composted green waste and planted with flowering meadow plants. The relationship between shoot biomass and water holding capacity demonstrated the benefits to plant growth and survival of increasing growing media water holding capacity. Use of crushed brick as the inorganic component of the growing media limited the growth of sedums even at the higher rate of composted green waste inclusion. This reduced the competition to growth of the less drought tolerant forbs. Using growing media components to limit the growth of drought tolerant plants may therefore increase green roof species diversity. It may be possible to increase organic matter content when the inorganic substrate limits plant growth. This would reduce growing media bulk density and therefore increase the number of buildings that could be retrofitted with a green roof system.