Comparison of irrigation efficiency and plant health of overhead, drip, and sub-irrigation for extensive green roofs

This study was conducted in two phases in a greenhouse in green roof modules. In Phase I, five commercial substrate types or systems were subjected to three irrigation methods (overhead, drip, and sub-irrigation) to determine substrate water distribution and retention. Substrates subjected to overhead irrigation or those with a moisture retention fabric (MRF) retained the greatest amount of water. Sub-irrigation resulted in the least amount of water retention and the most wastewater, except when a MRF was present. Substrate volumetric moisture content exhibited similar results. The MRF was effective in retaining water, but for sub-irrigation a visible water front was not visible as water did not reach the surface via capillary action. Differences can be attributed to the fact that overhead irrigation distributed water over 100% of the area, whereas in many cases the water front radiating from the drip or sub emitters never merged leaving dry areas in between emitters. In Phase II these irrigation methods were assessed to see how they influenced plant growth and health of Sedum album and Sedum floriferum. Repeated measurements were recorded for plant survival, growth index, chlorophyll fluorescence, and substrate volumetric moisture content. Results show that overhead was the most favorable for plant growth and health. Plant dry weights averaged 1.00 g, 0.78 g, 0.40 g, and 0.09 g for Sedum album subjected to overhead, drip, sub- and no irrigation, respectively, when no MRF was used. The inclusion of MRF generally improved results for drip and sub-irrigated plants. Chlorophyll fluorescence values were generally highest for plants subjected to overhead irrigation. Because green roof substrates tend to be coarse to allow adequate drainage, water does not move laterally to a great extent as it would in finer substrates. For this reason, drip and sub-irrigation may not be the most efficient irrigation methods.