INFLUENCE OF IRRIGATION FREQUENCY ON CULINARY HERB GROWTH AND PRODUCTIVITY IN AN EXTENSIVE GREEN ROOF ENVIRONMENT

TITLE: INFLUENCE OF IRRIGATION FREQUENCY ON CULINARY HERB GROWTH AND PRODUCTIVITY IN AN EXTENSIVE GREEN ROOF ENVIRONMENTMAJOR PROFESSOR: Dr. Alan S. Walters Southern Illinois University maintains an extensive green roof that has a growth media depth of 5-15 cm of a kiln-expanded aggregate containing perlite and ~3.5% organic matter. A study was designed to monitor the growth and productivity of culinary herbs in this environment. The first objective was to measure the vigor, growth and overwintering ability of different perennial culinary herbs under various irrigation regimes applying 1 L water/plant once a week, twice a week or once a week every 2 weeks. In this experiment, four culinary herbs were evaluated: Allium tuberosum ‘Garlic chives’, Lanandula angustifola ‘Munstead Dwarf’, Thymus vulgaris ‘Winter Thyme’, Melissa officianalis ‘Lemon Balm’. Additionally, the second objective was to evaluate the vigor, growth, and productivity of basil which is an important annual culinary herb using identical irrigation regimes. The second objective was to determine the effect of these irrigation levels on overwintering potential of the perennial culinary herbs evaluated. The perennial herb study indicated that water applications to plants either once or twice weekly provided greater dry perennial herb biomass than applying water once every 2 weeks. Regardless of the perennial herb evaluated, some weekly watering is required to provide the greatest amount of plant growth and vigor. Additionally, more frequent water applications also improved winter survival. Less frequent water applications will not only reduce plant growth and productivity but lead to perennial herb plant loss from one season to the next. Lavender was highly affected by the lack of water compared to the other perennial herbs evaluated. For most perennial herbs evaluated, the lower water applications provided less winter survival rates but not to the same extent as lavender. A 43% decrease in lavender overwintering survival rates was observed when plants received 1 L water once every two weeks, compared to the weekly irrigation applications. Basil plant growth differed among the irrigation timings evaluated in an extensive green roof environment. Generally, the twice a week application provided greater basil plant growth characters, followed by the once-a-week application, with the one water application every two weeks providing the least. Like the perennial herb experimental results, basil requires weekly water applications to maximize productivity in drought and heat stressful extensive green roof environments. This study indicates that basil is the most suitable annual for an extensive green roof environment. Our results indicated that infrequent watering of perennial herbs in an extensive green roof environment will most likely result in lower plant growth, vigor, and productivity, as well as reduce the overwintering potential of specific herbs. Thus, water management is critical to maximize productivity for herbs grown in extensive green roof environments. Although rooftops can be used to create spaces to produce edible crops, water management should be considered as an important factor to maximize their productivity