Assessing the performance of composted green waste as a substrate for green roofs (WRAP contract)

RHS project team
Dr Paul D Alexander
Sheffield University, Forest Research, Vital Earth, Shire Minerals and Bioregional.
Start date
31/01/2009 13:27:03
End date
31/01/2011 13:27:09

Compost, biochar, green roof, WRAP, RHS

The problem

Getter and Rowe (2006) identify a number of requirements specific to green roof substrates: they need to be lightweight, well drained but good at retaining water and nutrients and not readily decomposed over time. These parameters usually mean that otherwise useful organic materials such as compost made from green waste can only be incorporated at up to 15% by volume in green roof substrate mixtures. Increasing inclusion of green compost would provide opportunities for new types of green roof planting and would provide another sustainable application for this useful recycled material. There is therefore an urgent need to combine horticultural knowledge of plant growth media with the emerging discipline of green roof design to establish a robust understanding of the performance of green compost in this environmentally-friendly application.


A two year experiment will be established at Deer’s Farm to examine the use of green compost in green roof substrate mixes. Different rates of green compost (>15%) will be incorporated and comparisons made with and without biochar. The experiments will be undertaken using randomised and replicated large trays. Five different types of calcareous grassland plants will be grown in the substrate mixes to examine their suitability for non-standard green roof substrates. Plant health assessments will include growth measurements and estimated leaf chlorophyll content to infer plant health during the experiment, as well as biomass measurements at the end of the experiment. Leachate will be collected at regular intervals for analysis and substrate structure will also be regularly assessed.


This project aims to investigate the potential for maximising inclusion rates of green compost in green roof substrates by amelioration of some of the potential problems through addition of biochar (also known as charcoal or wood-char).

This investigation will address two hypotheses. Firstly that green compost can be included in green roof substrate mixes without detrimental effects on plant community establishment, leachate quality and substrate structural stability. Secondly, that potential constraints on maximum inclusion rates of green compost in green roof substrates can be addressed by addition of biochar i.e. that this will increase the range of plants that can be grown, improve water holding capacity and nutrient retention (thereby reducing leachate risk), and improve structural integrity of the green roof substrate.

Benefits to gardeners

Green roofs can confer a range of environmental benefits including reducing rainfall run-off, reducing the urban heat island effect, improving air quality, improving building insulation and increasing building lifespan (Dunnett and Kingsbury, 2008) as well as delivering aesthetic benefits and increasing green space within urban areas. Green roofs therefore have a role in both adaption to, and mitigation of, climate change as well as contributing to sustainable urban development.


Dunnett, N. and Kingsbury, N. 2008. Planting Green Roofs and Living Walls. Timber Press, Portland.

Getter, K.L. and D.B. Rowe. 2006. The role of extensive green roofs in sustainable development. HortScience, 41: 1275-1285.