But what are the criteria for growing media in such applications?
The multiple benefits of green roofs also bring special challenges to growing on them: intense summer heat (which the garden moderates for the building interior), drying winds and unpredictable water supply from rain (with only limited opportunity for water storage). So, when choosing media for rooftop situations, the effects of rainfall and average seasonal (high) temperatures are more crucial than for other (traditional) applications.
According to Premier Tech Horticulture, “during the past several years the performance of growing media on roofs has been evaluated in Europe … and it was determined that the most crucial physical property the medium should have is good drainage.”1 In other words, it needs good drainage and simultaneously good water retention. It also needs to be light enough not to damage the roof, yet heavy enough not to blow away and to provide good plant anchorage and stability. And it needs durability – replacing it several stories above street level is costly.
So what are the options?
Soil. While this is great for conventional (ground-based) growing systems, its not ideal for green roof applications: “it’s bulky and heavy – the targeted saturated weight of roof top growing media should be 48-65 lb/ft3 – soil can be up as high as 120 lb/ft3. Soil also has potential for chemical and pathogen contaminants, and rocks or other debris may make the product inconsistent.”1 And good quality soil is becoming increasingly difficult to find.
Peat-based medium. It’s better. It typically has a high air-filled porosity and saturated weights of only 55 lb/ft3.1 But it dries out quickly, is very hard to re-wet (remember the hysteresis curve from your soils class?) and is best if amended (to limit shrinkage).
Chunk coir-peat mix. “Offers good aeration, compacts less over time and doesn’t dry out as fast as peat-based media. Easy to handle and can be amended with clay aggregates for optimum durability.”1
Other artificial media (such as rockwool or perlite). Many such products are too light for rooftop applications, and since they were often developed for hydroponic growing, they may need more frequent irrigation than many rooftop gardens can provide.
Compost. “Blending compost into peat-based media provides good water retention on hot days, but the fine particles in compost can block the airspaces.”1 Also, remember that compost will almost invariably be heavier than anything but soil-based growing media, so it also adds weight that needs to be accounted for in the roof engineering.
Apart from major physical criteria, a man-made (engineered) growing medium should be mindful of the biological requirements. Adding beneficial micro-organisms has been shown to maximize plant nutrient uptake, increase plant growth, confer resistance to abiotic stress and suppress disease.
“Sustainable” practices in horticulture are increasing demand for microbial enrichment in horticultural soil and soil-less mixes and engineered green roof growing media.2 There are a number of companies (for example Earthfort Environmental, based in White Rock, B.C.) who supply biological amendments to build soil micro-organism populations.
Horticulture is a growth sector contributing more than $2.3 billion to the Canadian economy annually and the 2012 Landscape Canada Survey indicated that industry growth will be through environmentally responsive initiatives.2
Healthy soil biology, including in green roof media, will be part of the solution to these issues.
- 1 Premier Tech: 1, Avenue Premier, Rivière-du-Loup (Québec) www.pthorticulture.com
- 2 Institute for Sustainable Horticulture, (ISH), Kwantlen Polytechnic University.