Green Roof Maintenance
A green roof, just like any other garden or landscape, requires some maintenance to keep it healthy and beautiful. At the Minnesota Landscape Arboretum, our methods and considerations for maintaining our green roof are as follows:
Green roofs are meant to be sustainable, and, ideally, a simple roof such as ours should be able to survive on rain water alone. That said, during the establishment period watering was critical to the survival of the plants. Our roof has a permanent irrigation system in place that was used during the first season to supplement rain and ensure that the plants received one inch of water each week. In the second season we had adequate moisture and the roof thrived without any supplemental watering. If we encounter drought, however, the irrigation system will be used as necessary to keep the roof in good condition.
Green roofs are not immune to the mundane garden task of weeding -- ours is weeded about once a month during the growing season -- but they do require special considerations for this task. First is the challenge of safety. To this end our green roof has steel cable along the ridge to attach safety lines to when people climb up on the roof to weed. Second is the question of which plants are actually weeds. Green roofs are dynamic places, with the different species in constant competition for space and water. Added to the plants that were installed purposely are a multitude of native volunteers that may or may not have promise as green roof plants themselves. Some green roofs may benefit from allowing some of these volunteers to stay and take part in the natural competition. On our green roof, however, we are limiting the selection to the originally planted species. Most of the weeds on our roof are normal garden weeds such as purslane and prostrate spurge. Of especial nuisance are tree seedlings which spread from local stands of cottonwood and other species.
Fertilizing and Soil Management
Fertilizing a green roof is a delicate balance between keeping the plants well supplied with nutrients and avoiding fertilizer run-off. The Arboretum has currently been fertilizing with slow release fertilizer applied by broadcasting twice per season.
Soil management is an issue that requires some consideration for the longevity of the green roof. In order to minimize weight and maximize water-holding capacity and drainage, the growing media on the roof is made of about 95% lava rock and 5% organic matter. This small amount of organic matter will be quickly depleted without some form of renewal. One way to keep the level sufficient is to include species that produce large amounts of biomass, such as grasses.