A specially engineered growing medium for Green Roofs is essential to ensure healthy plant development.
BC Green Roof worked extensively with agriculture Canada to engineer the right growing medium, composed of local materials, to avoid shipping materials that add cost and carbon emissions. This growing medium has been proven to work and meets all standards..
Growing media for Green Roofs have more than one function.
It has to form a base for plants to develop. The medium has to hold water for the plants, however, we also want it to drain fast.
Some suppliers of Green Roof material have an external soil blending company mix the medium. We have seen specifications on these mixes with 60% organic matter - or even more. Typically, we (and internationally accepted codes such as the German FLL) recommend having only 6% to 8% organic matter in a growing medium. This organic matter has a high CEC (capability to bind nutrients) as well as contributes to the water holding capacity.
Too much organic matter, and the growing medium will saturate and will become soggy. It settles over time as the organics decompose and could cause plant roots to break, when the medium dries out. Adding a lot of sand, or pumice is not the solution to a high organic medium.
A educational article has been written on what influence the thickness of the growing media has on plant development, you may find this here: (CLICK)
We regularly see Pumice based growing mediums used as pumice as main component is lightweight weight.
Although there is a benefit to Pumice being light, it also has a drawback. Pumice can absorb lots of water, yet too much water has shown to negatively influence plant development.
Often overlooked too is the settling of the growing medium over time. A layer of growing medium applied at 4 inches after settling only leaves 2 ½" of depth after settling!