EVALUATING GROWING MEDIA FOR A SHALLOW-ROOTED VEGETABLE CROP PRODUCTION SYSTEM ON A GREEN ROOF

Green roofs are roof tops planted with vegetation to beautify, modify temperatures, and reduce storm-water runoff to name a few of the reported benefits. Soil or potting mix are commonly used for green roof applications but may be heavy, difficult to contain, and leach mineral nutrients and particulates. The effectiveness of potting soil, petrochemical-based foam sheets and fiberglass sheets was compared on the roof of Seitz Hall on the Virginia Tech campus. In a randomized complete block design, four 4.88 × 0.61 × 0.21 m Plexiglas containers, subdivided into four 0.61 × 0.61 × 0.21 m subplots, were filled with 10 cm potting mix, foam, fiberglass, or blank control over a 6.8 cm drainage layer of gravel. Kale (Brassica oleracea var. acephala) was established from transplants grown from April through May. Runoff, roof temperature, mineral content, media composition, and biomass were recorded for each treatment. A gravity-fed irrigation system supplied water through drip tape as needed. The least runoff came from commercial potting soil and the most from foam. Fiberglass media had the greatest water holding capacity on a dry weight basis. Foam had high moisture holding capacity and good aeration that facilitated excellent root proliferation through the media. Fiberglass had a pH of 8.8 and less root growth. Potting soil produced the greatest kale biomass while foam was second. Foam and fiberglass had lower nutrient content and cation exchange capacity compared to potting soil. The mineral content and particulates in the runoff from fiberglass and foam media was very low. Alternative media, like fiberglass and foam, have potential but need to be refined and tailored to green roof applications.