We have new retaining wall about 80 to 100 cm high. It is made of red gum sleepers and ‘H’ beams. Behind the wall is 20-30cm wide strip of scoria with the slotted agi pipe at the bottom. We would like put a layer of soil on top of the scoria and plant up to the wall. What is recommended to stop the soil falling into the scoria? I am tossing up between shade cloth and a layer of 14mm gravel. The shade cloth would be quick and easy, but may break down and create a mess of fibers in the soil. The gravel would be lots more work and cost. Is it worth the extra effort and cost of the gravel, or is the shade cloth a good long term solution? Is there a better way? Also how deep would the soil need to be, to be useful in growing small perennials?