Hydrogel Utilization and Water Management for Annual and Perennial Herbs in an Extensive Green Roof Environment

Extensive green roofs can be used to provide spaces for local agriculture in urban environments, although extreme moisture and temperature conditions typically found in these systems can often be challenging for crop production. The Southern Illinois University-Carbondale extensive green roof was utilized to determine the effects of a polyacrylamide hydrogel, pine bark mulch, and irrigation frequency on the growth and productivity of ‘Compact’ and ‘Italian Large Leaf’ basil (Ocimum basilicum), and the growth and overwintering ability of two perennial culinary herbs, sage (Salvia officinalis) and thyme (Thymus vulgaris). Results indicated that weekly irrigation increased late-season basil and perennial plant vigor, basil fresh and dry weight, and overwintered perennial plant vigor and height compared to bimonthly watering. Although the use of pine bark mulch improved basil fresh weight and plant vigor compared to no mulch, mulching did not influence (p > 0.05) perennial herb growth or overwintering in an extensive green roof environment. Hydrogel applications improved basil plant height compared to none, although fresh and dry plant biomass were not influenced by hydrogel applications. In comparison, hydrogels as additions to the green roof medium did not influence either early- or late-season perennial plant vigor, although the overwintered plant vigor collected the following spring was greater in the no-hydrogel treatment. For perennial herbs, sage had greater vigor, overwinter survival, and overall suitability for extensive green roof environments compared to thyme. This research indicated the importance of perennial herb selection and consistent water supply for annual and perennial herb growth and the overwintering success of perennial herbs. Thus, supplemental water and other management strategies to provide more constant medium moisture content are important considerations for sustaining culinary herb production on extensive green roofs.