Green Roof Valuation Durban
A brief evaluation of the Green roof trials conducted in Durban over a period of ten years.
Early in 2009 I became involved in the eThekwini Municipality Green Roof Pilot Project at which time I started the trial of a number of species of local plants at Mount Moreland north of Durban.
As from 1987, prior to becoming involved in the eThekwini Municipality Green Roof Pilot Project I had considerable exposure to green roof construction in Germany. Therefore, it was natural to copy as closely as possible the methods employed in green roof construction in Germany. Using drainage and engineered growing medium consisting of materials such as light weight expanded clay granules, perlite and vermiculite. In addition, we incorporated very little organic material as done in Germany with the percentage not exceeding 5%.
This evaluation has been done after most of my test plants had been grown for ten years with virtually no attention, being left almost completely on their own with no watering, no fertilising, no addition of soils to the trays or mulching with compost.
Ten years later the evaluation of the projects has proven that many of the practices carried out in Europe to not be true for our circumstances.
Firstly, drainage appears to not be essential because the drainage very soon becomes invaded by plant roots which decrease or completely prevents its operation. The reality is that during periods of heavy rain the majority of the water leaves the roof via surface drainage.
Specially engineered soils appear to not be essential other than they are a little lighter a factor that can very easily be mitigated during the design stage of the roof to allow it to carry a little more weight.
As observed in nature many of the plants that we trialled grow in humus rich soils on rocky slabs in nature. In literature found on the internet it is generally recommended not to add more than 5 percent organic matter to the growing medium mix. Experiments have proven that most of the plants that we did trials on grow better with considerably more compost. Recently since the end of the ten-year testing period I have found great benefit in heavily mulching the plants with fresh sheep manure just before the onset of the first rain in the spring time.
Clearly a great range of South African plants are extremely suitable for use on green roofs growing in shallow relatively nutrient poor soils
Many indigenous orchids are ideal candidates for growing on nutrient poor green roofs, especially orchids from the genus Eulophia, in particular the very showy Eulophia speciosa.
All of the local indigenous bulbous plants trialled have been an overwhelming success, one of the main reasons for this appears to be that during the period of maximum growth there is sufficient moisture for good growth from the rain with a dry period during the winter when they are naturally dormant.
A number of new plants have established themselves from seed that was blown in or brought in by birds. Of particular note are Bulbine natalensis, Cyanotis speciosa, Delosperma lineare, Drimiopsis maculata, the orchid Eulophia speciosa and a Plectranthus sp collected at Ndumo.
Below are lists of plants that I have trialled at my home in purpose manufactured planting trays most of which have been in trialled for ten years the minimum period that plants have been grown is 4 years.
Bidens Pilosa established from windblown seed although they are aesthetically unwanted, they complete their life cycles or die from drought before they produce seed. Weeds have not been a threat to the growth or the health of the plants.
Under normal conditions weeds would need to be controlled mainly for aesthetic reasons as well as to keep the plantings purely indigenous.
Establishment of grasses
In addition to unwanted annual agricultural weeds the grass Melinis repens has established in some of the trays during the rainy growing season dying off during the dry winter leaving seed to resprout with a onset of spring rains. Melinis is probably a good and certainly an attractive addition on most extensive green roof plantings.
Conclusion and recommendations
The vast majority of the plants tested did extremely well.
Soil similar to the soils that the plants are naturally found growing in produced the best results however all plants grew satisfactory in a general fairly open well drained growing medium.
Occasional watering and the removal of unwanted weeds would be of great benefit.
An annual mulching with either a well-rotted and leaf mould of bark compost or animal compost such as sheep dung appears to be of great benefit.
I would not recommend the addition of inorganic fertilisers because they would promote weak rapid growth of the plants