Effect of substrate compost percentage on green roof vegetable production

Use of rooftops to produce locally grown vegetables is increasing. However, due to weight restrictions, shallow substrate depths, and potential harsh environmental conditions, optimizing production can be a challenge. Standard industry practices for ornamental extensive green roofs planted with succulents or other herbaceous perennials and grasses dictate that organic matter should be less than 20% of the original substrate mix. In rooftop agriculture, however, maximizing growth and yields are usually a primary objective and the amount of organic matter incorporated into these substrates are a major factor in this equation. In this study we quantified the optimal percentage of compost in a green roof substrate for optimizing growth and yields for cucumbers (Cucumis sativus) and peppers (Capsicum annuum). The study was conducted on raised green roof platforms over a period of 19 weeks and compared six substrates containing increasing amounts of a commercial compost produced from municipal yard waste (0, 20, 40, 60, 80, and 100%) mixed with a heat-expanded shale and sand base. These treatments were also compared to a typical garden plot at ground level. Plant performance evaluations such as plant growth, chlorophyll fluorescence (Fv/Fm) as an indicator of plant stress, and fruit yields were used as an indicator for optimal substrate mixtures. Generally, the addition of 60 or 80% compost resulted in the greatest plant growth and fruit yields, although compost influenced growth and yield of peppers to a greater degree than cucumbers. In addition, the ground garden plots performed poorly which emphasizes the point that growing vegetables on a rooftop can be advantage because substrates can be engineered to maximize plant health, although the same could be done with raised beds in a garden plot.