The SIUC green roof is on top of a portion of the south wing of the Agriculture building and can hold up to 112 kg/m2 of plants and media. The components of the SIUC green roof (from the bottom portion upwards) include a waterproof membrane layer (thermoplastic polyolefin), a soft fabric protection layer, a root barrier, an ‘egg crate’ style drainage layer, a filter fabric, ~8 cm of green roof mix (light-weight aggregate + organic matter), and plant material.
Several studies conducted on the SIUC green roof have indicated that numerous vegetable crops are suitable for production in this environment, although the greatest lessons learned were the importance of fertility management systems to provide sufficient amounts of mineral nutrients, and maintaining consistent moisture levels in the growing medium to create optimal growing conditions [1
]. However, due to some of the issues discussed previously regarding green roof vegetable culture, especially vegetable crop suitability and fertility systems, a study was conducted on the SIUC green roof in the autumn growing season (September to December) during 2010 and 2011. This study evaluated the effectiveness of four different fertility treatments on the production of several shallow-rooted vegetables [kale, lettuce and radish (Rhaphanus sativus
)] in a green roof environment using a randomized complete block design with 4 replications. The fertilizer treatments evaluated were: (1) Vermicompost tea (from composted coffee grounds); (2) Miracle Grow®
fertilizer (Scotts Miracle-Gro Products Inc., Maryville, OH, USA); (3) Organic Miracle Grow®
fertilizer (Scotts Miracle-Gro Products Inc.); and (4) no fertilizer. Although all treatments were in the same expanse of green roof media, no cross fertilizer contamination among plots occurred due to the center-to-center distance between each plot row (~0.6 m) and considering that all treatment applications were applied directly at the base of the plants. All crop plants evaluated were all relatively shallow-rooted and did not move into the adjacent plot medium. The green roof media had a 6.5 pH, organic matter content of 4.3%, and estimated Nitrogen (N), Phosphorus (P), Potassium (K), Calcium (Ca), and Magnesium (Mg) release of 101, 617, 250, 2798, and 491 kg/ha, respectively, with 70% Ca base saturation. Based on fertilizer analyses, all materials provided similar trace amounts of Ca, Mg, Sodium, Boron, Iron, Copper and Zinc. For specific major macronutrient content, Miracle Grow®
had 1.6% N, 0.04% P, and 0.1% K; Organic Miracle Grow®
had 0.7% N content and less than 0.01% P and K; and, Vermicompost tea provided 1.2% N, with P and K in trace amounts. Fertilizers were applied once per week to plants and irrigated 2 to 3 times per week with drip irrigation, depending upon moisture needs of plants. Over the entire growing season, approximately 18, 8, and 14 kg N/ha were added for Miracle Grow®
, Organic Miracle Grow®
, and Vermicompost tea treatments, respectively. Most of the applied N for Organic Miracle Grow®
and Vermicompost tea treatments were in the organic form and were not readily available for plant uptake.
Data collected at the termination of the experiment included mature leaf chlorophyll content (estimated % N), leaf and root dry biomass (g), lettuce and kale plant height (cm), radish taproot diameter (cm), and % N, P, K, Ca, and Mg in mature leaves of all three crops. Data were analyzed by ANOVA (SAS, Cary, N.C., USA), with no interactions (p > 0.05) detected for any variables between treatments and years; therefore, data were combined over years. Mean comparisons between treatments were made used Fisher’s Protected Least Significant Difference (LSD) Test at (p ≤ 0.05).
4.1. SIUC Green Roof Vegetable Research Results
Results from this study indicated that all crops evaluated could be effectively produced in an extensive green roof system with adequate nutrient inputs (Table 2
, Table 3
and Table 4
). Although, the high nutrient fertilizer (Miracle Grow®
) provided the greatest leaf (kale and lettuce) and root biomass (radish), the Organic Miracle Grow®
treatments that provided lower amounts of nutrients would most likely be sufficient in most instances. The amount of chlorophyll also tended to be at the highest levels in the Miracle Grow®
and Organic Miracle Grow®
treatments for kale, lettuce and radish leaves. The overall amount of plant growth as detected by plant heights also tended to be highest in these two fertilizer treatments. For all crops evaluated, N leaf nutrient content also tended to be the greatest in the Miracle Grow®
treatment, although this was not the case for P, K, Ca and Mg levels. The vermicompost tea applications provided consistent low levels of N across all crops evaluated. Besides N content in kale leaves, most other macronutrients tended to be lower when Miracle Grow®
was used compared to the other fertility treatments, which had similar higher values. In comparison, the Organic Miracle Grow®
treatment had N leaf content similar to Miracle Grow®
, but greater levels of K, Ca and Mg. In lettuce leaves, P content was higher when Miracle Grow®
was used compared to all other treatments, although K, Ca, and Mg levels were relatively low with this fertilizer source. Again, Organic Miracle Grow®
provided similar lettuce leaf N levels compared to Miracle Grow®
. This indicates that Organic Miracle Grow®
may be a better fit for salad green crops (such as kale and lettuce), when grown in a green roof environment, since it provided consistently higher macro-nutrient contents in leaves while delivering less overall leachable NO3−
compared to Miracle Grow®
. For radish leaf macronutrient content, Ca content was low when Miracle Grow®
was used compared to other treatments, although P, K, and Mg were similar to the other treatments evaluated.
4.2. SIUC Green Roof Vegetable Research Conclusions
Nutrient management is a critical component of vegetable production in a green roof environment. Better fertility management systems need to be developed to create optimal growing conditions for vegetable crops on rooftops [1
]. Our study indicated that although lettuce, kale and radish can be produced effectively in an extensive green roof medium, nutrient applications are required to maximize their productivity. The Miracle Grow®
treatments provided the highest applied nutrient levels based on recommended rates, which related to the greatest plant growth response. However, over-fertilization with Miracle Grow®
probably occurred, which most likely resulted in high amounts of nutrient leaching and runoff. The Organic Miracle Grow®
fertilizer was a more sustainable nutrient fertilizer source for a green roof system due to less plant available nutrients applied based on recommended rates, and only resulted in a small decrease in overall productivity for the shallow-rooted vegetables evaluated. Vegetable yields will normally increase with greater fertilizer use, but high rates applied will lead to nutrient leaching and runoff which can become a serious issue [21
]. Moreover, use of products, such Organic Miracle Grow®
, that have sufficient nutrient availability (with little nutrient waste) or use of slow-release fertilizer type materials may be better suited for use in green roof environments compared to applications of high analysis fertilizers to prevent nutrient leaching and runoff.
Green roof mediums and fertility management systems are current challenges that limit productivity of vegetable crops in extensive systems [1
]. Additionally, substrate nutrient and moisture holding capacity of a medium are concerns in green roof agricultural systems, especially when using extensive systems. Managing substrate fertility is often challenging because green roof substrates are engineered with less than 20% organic matter and combined with coarse, heat-expanded materials, such as slate or shale [13
]; and, this tends to provide high material permeability and low cation-exchange capacity. Moreover, our research indicated that high nutrient applications are generally required to overcome these issues with most extensive green roof substrates to maximize vegetable crop yields (Table 2
, Table 3
and Table 4
). Although sufficient substrate fertility and moisture are essential to maximize green roof vegetable crop productivity, both are often difficult to manage effectively in extensive systems due to the mediums utilized.