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Green roof substrate composition: The importance of being ‘Earnest’

by Matt Downs

Dr Tom Young explains why, in his opinion, the substrate / growing media “is the single most important component of a green roof”, and looks at the key considerations to make to ensure it performs as it should.

Green roof substrate, also known as growing media, is – in my opinion – the single most important component of a green roof. Why? Well, it is because without it you would not have any plants, and without plants a green roof is, well, just a roof. Now, we ask a lot from the green roof substrate – according to Ampin et al. 2010, Friedrich 2008 it needs to be;

• Lightweight to help reduce loading on the roof

• Free draining and aerated to prevent excess water build up

• Able to retain sufficient water to support long-term plant growth

• Contain sufficient nutrition to support long-term plant growth, but not excessive in order to prevent weed growth or unnecessary plant growth

• Stable to prevent breakdown in the long term. 

Purpose blended green roof substrates
A number of companies produce purpose blended green roof substrate. It is important to point out that in 99% of cases, a purpose made green roof substrate should be used (see GRO’s website to see GRO approved substrate suppliers). Spoil from site workings, or conventional topsoil is not appropriate for a green roof and its use is strongly discouraged. Generally, green roof substrate is made from a combination of lightweight aggregate (crushed waste virgin brick, pumice, heat expanded clay or Lytag), coarse sand, green waste compost and sometimes other organic sources such as coir or wood chippings/fibre. As a substrate, moves from extensive – shallow between 80-150 mm, more drought tolerant green roof – to an intensive system – deeper between 150-300+ mm – organic content increases to reflect the changing requirements of the plants (Ampin et al. 2010).

It should be pointed out, however, that not all green roof substrates are the same, and should be carefully chosen for each specific project depending on plant section, substrate depth, irrigation provision, aspect, location and climate. Substrate composition has a huge impact upon plant establishment and growth. For example:

• Increasing substrate organic content from 10 to 25% by volume increased shoot biomass of various grasses and forbs by around 50% under well-watered conditions (Nagase and Dunnett 2011). 

• Increasing organic content from 10% to 50% resulted in a 115% increase in shoot weight (Nagase and Dunnett 2011).

• The addition of artificial fertiliser to Sedum species caused excess growth which led to poorer survival during drought conditions (Rowe, Monterussio and Rugh 2006).

• Reducing water holding capacity from 26% to 17% by increasing substrate porosity decreased shoot growth of grasses by 17%, did not have an impact upon root growth, but did increase root:shoot ratio by 15% (Young et al. 2014). 

• The above results were expanded upon and drought tolerance examined. Reducing initial water holding capacity actually improved plant drought tolerance by increasing root:shoot ratio and preventing excess shoot growth during well-watered periods (Young et al. 2015)

• Different species of plant have different optimum pH requirements, with five different Sedum species growing best in different substrate pH ranging from 5.71 to 6.43
(Zheng and Clark 2013).

Depth matters
And please don’t forget the importance of depth! There have been numerous studies showing that even a small increase in depth can have huge impacts upon long-term plant development and survival (Rowe et al. 2006). These include improved drought tolerance (Thuring, Berghage and Beattie 2006, Olszewski and Young 2011), improved frost tolerance (Boivin et al. 2011) and improved species diversity with heterogenous substrate depth across a roof (Bates et al. 2015, Heim and Lundholm 2014).

So, in a nutshell, substrate composition (and depth) is extremely important for the success of a green roof. There is no one size fits all, and substrate should be tailored to each individual roof. It is always good practice to speak to a substrate/green roof expert and ensure that the project specification is correct. Always ask for up-to-date specification sheets from substrate manufacturers to assess if the substrate is the correct one for the job. There is now a British Standard for Green Roof Substrate Test Methods (BS 8616:2019) which ensures that substrates are all tested with the same methodology, regardless of laboratory. This makes comparing results much easier for the client or specifier. Please get in touch with any green roof substrate related enquires.

E: tomyoung@tep.uk.com

Tweet: @DrGreenRoof

About the author:

Tom works for The Environment Partnership (TEP) as a Blue-Green Infrastructure Specialist. TEP are a multiple disciplinary environmental consultancy with specialisms in Landscape Design, management, Ecology and Arboriculture. TEP have recently become Associate members of GRO and are the first Landscape Architecture company to do so.

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