Searching to make my thin lightweight green roofs plants flourish with less watering, I have found a great solution. The quantity of the water that the growing media can store, influences the sustainment of the plants. With high temperatures in Sydney wilting point represents that level of soil after which plants are not able to extract water from the soil. "The main limiting factor and the one which will determine the way in which vegetation develops will be the water supply." (FLL Guidlines 2002).
The additional of organic matter or clay to garden or farm situation improves water holding capacity. However neither of these materials is suitable for green roofs as the first would block drainage systems and the second will not last the test of time.
Solution
I want to maximise plant health on my roofs, so I added Flox to the growing media at different percentages. Flox is a unique mineral wool product produced by Knauf. Flox has an abundance of fine particles that are hydrophilic, it is lightweight, safe to install, holds water until plants extract it, has no additives and is stable for the long term.
http://www.green-urbanscape.com/en/solutions/green-roof-purple-roof-concept
I found that with the addition to 5-15% Flox in the mixture I was able to increase water holding capacity for a number of days at higher rates. Soil without Flox after 48hrs held around 10% water volume, with around 7% being permanent wilting point. With 10% Flox I increased this by 100%.
Taking the water holding capacity measurements in differnet mixes of Flox
It is worth noting that adding 20mm layer of Flox in it's Green Roll form at the base of the growing media will help even more. So for our hot climate we can now make thin and light weight roofs that successfully grow plants with less stress.
Let me know if you need more information at rgriffi8@gmail.com
Robert Griffith