The Art and Science of Maintaining Biodiverse Green Roofs
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Kneeling among the prairie crocus, buffalo bean, and blue grama grass on an early spring morning, it is easy to forget that I am on a rooftop. I judiciously prune the stalks and seed heads from last year’s giant hyssop, showy locoweed, and purple prairie clover, all of which were left standing throughout the winter season to provide food and shelter for wildlife; some I will leave standing still, throughout the summer months. I consider whether to prune the branches of the self-seeded Canada Buffaloberry that reaches over the boardwalk. I decided to take out just a few branches. This buffaloberry shrub is in full bloom. To my eyes, its discrete yellow flowers are only noticeable when I get close, but the flowers do not go unnoticed by the early, native solitary bees.
Boardwalk and Buffaloberry on a biodiverse green roof in Red Deer, Alberta. Photo: Cynthia Pohl
I reluctantly remove a few branches, not wanting to take away this floral resource for the early season pollinators, but I know the size will need to be controlled if it is to continue to be a part of this rooftop plant community - a community that reproduces the ecoregion that once existed in the footprint of this building.
Upon inspection of the pruned branches, I see a tight cluster of small, white eggs attached to one branch. I feel regret about removing this life form from its environment. Had I noticed it earlier I could have left this section of the branch in place. Every time I make a discovery such as this it is exciting and deeply rewarding.
Taking time to observe how biodiverse spaces are being used is critical in building an understanding of the relationships within a space. Maintenance practices need to allow time for observation.
Time spent in observation is an integral component of biodiversity maintenance.
Landscapes are living systems that are always changing, evolving, and adapting. Maintenance of the landscape is an integral component of all landscape designs, but maintenance is not always given the focus or respect that it merits.
Geoffrey Jellico, an English architect, town planner, landscape architect, garden designer, and author of The Landscape of Man, once said, “The world is moving into a phase when landscape design may well be recognized as the most comprehensive of the arts.”
These words resonate with more depth with each passing year. Mitigating the effects of a rapidly changing global climate requires respect and cooperation with the natural systems that have supported life on this planet for millennia. Paradise is not lost, but it does require a holistic vision of beauty for it to be maintained.
An understanding of the species that inhabit the local ecoregion in which the project is located is foundational. This understanding will guide both the design and maintenance practices. In order to successfully design a biodiverse green roof, the designers must have good knowledge of local plant characteristics, habitat requirements for wildlife, and knowledge of how the space will change over time, and an acceptance that the design will change as it evolves. This knowledge, along with the project’s vision and goals, also needs to be shared with the maintenance team. A detailed maintenance plan for biodiverse green roofs is essential.
Plants are a major component within most designs and therefore a major component of the maintenance strategy as well. Understanding plant characteristics such as bloom time, rooting characteristics, growth requirements and plant and animal relationships is necessary to maintain a system that supports biodiversity. Plants are the producers within an ecosystem; they provide both food and the shelter for wildlife. To ensure the availability of food for wildlife, nectar, pollen and seeds, plant populations need to be managed to maintain diversity in bloom time and type. This requires regular monitoring of species density and in some cases the removal of intended plants that reproduce too readily or uninvited plants that seek to benefit from the green roof habitat.
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Hand pruning reduces the chance of causing damage to wildlife. Photo: Cynthia Pohl
When maintaining biodiversity, both live plants and the plant material from past seasons is valuable. In a natural system the grazers, such as undulates, regularly reduce the vegetative portion of plants. Without grazers as a part of the system, plant material can develop dense layers of growth which impede new growth. In projects that are not able to accommodate grazers, often the case for most urban green roof projects, we must perform the grazing role. In many landscapes this action is emulated using power tools such as mowers and hedge trimmers. In biodiverse landscapes, however, power tools can cause harm and disruption, such as the unintentional removal of host plants, egg sacs, nests, and even desired species. For this reason, it is often better to hand prune plant material, leaving behind stands of old plant growth sporadically throughout the space. Hand pruning has us move through the space more slowly, giving us time to observe how the space is being used and the opportunity to work around areas being used by wildlife.
It seems that weeds are an inevitable component of a landscape. Over time even the intended plants can become weeds and require control measures. Weed control begins with a clear definition of what a weed is, as this can change from project to project. Regardless of the project, however, the control of undesired plants is most effective when the plant is removed in its seedling stage. For this reason, short, frequent maintenance visits are more effective than long, infrequent ones. Weekly maintenance sessions help ensure weeds are not allowed to flower and set seed. Each time a weed goes to seed, it adds to the weed seed bank within the project, increasing the long-term maintenance requirements of a project.
Rusty Tussock moth eggs on Canada Buffaloberry branch.
Biodiverse landscapes include habitat features that provide shelter and nesting sites for wildlife. Understanding how and who are using these structures is critical in their maintenance. An untimely maintenance practice could damage or even kill the species making use of the structure, such as the removal of a fallen snag that is providing organic matter for lichen, fungi and bryophytes, shelter for beetles, and nesting sites for solitary cavity nesting bees. Inspecting habitat features, such as logs and stones once a season, allows for assessment of their integrity and functionality. Knowledge of local fauna and their habitat requirements is necessary in the assessment and management of habitat features.
Pests within a biodiverse landscape may be present. To determine this an understanding of local fauna, and their life cycles, is required. What may be considered a pest in some projects, may be considered the achievement of an important goal in biodiversity projects. Hence context is essential. When it comes to pests it is necessary, once again, to spend time in observation. Observation leads to understanding and understanding leads to respect.
As we learn to maintain our landscapes with a respect for biodiversity, we live more lightly within the dynamically balanced web of life.
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Cynthia Pohl, Living Lands + Landscape is a green roof maintenance professional specializing in biodiverse green roofs. She has a lecture on The Living Architecture Academy and can be reached at: livlands@telus.net
