Green Roof Living Is Stressful For Plants: Here's Why That's a Good Thing

By Johnny Moore

April 25, 2022

What do driving wind, scorching sun, sub zero nights, and shallow soil have in common? They are all found atop a Colorado green roof. Plants that live here find no relief. They are tested by the full force of every extreme the environment can hurl. If you were a plant, there is a good chance this divergent landscape locale might not be for you.

Fortunately, stress is something plants in our environment have come to understand. Those plants that don’t handle stress well typically don’t survive long here without a lot of intervention.

You can easily note this by observing the ubiquitous, tragic planting beds between a sidewalk and a street, or parking lot landscapes. Dead trees, weedy cobblestones, junipers, and the occasional lone ‘Karl Foerster’ grass are typical residents of these unloved, socially outcast landscape spaces. While these can be particularly challenging environments, often lacking supplemental water or care, these vignettes tell the story of what it takes to survive if you’re a plant on your own along the Front Range.

Our modern human response to environmental stress in a landscape is often misguided. We assume because garden conditions are “harsh” (and no doubt the plants we have planted there are dead), we should try and alleviate the harshness of those conditions. We amend the soil and replant things that have died, hoping for a better outcome.

If the conditions are weedy, we determine we should sterilize the environment and free the soil of noxious invaders so our special-blooming varieties of plants will grow without competition—and look nice. Hopefully.

While this sounds like an epic tale, it always plays out the same way: man’s noble attempt to “fix” nature, horribly miscalculated and underestimated, devolves into an attempt at control and domination of nature, rather than stewardship. Nature, with whom we’ve picked a fight, turns out to be an incalculably vast, indefatigable system, too complex, diverse, and mysterious for any one species to dominate for very long.

We have failed to understand the system. We were taught that if there is a problem, we should find the solution. Our paradigm is mechanical. But the real world we come from is not. If we want our gardens to thrive, we must change our approach. Green roofs teach us this.

When we began working on this green roof design, a previous design firm had planned to use low growing Sedum species to cover the roof. Sedums are popular green roof plants due to their great ability to handle dramatic climatic conditions–drought, flood, sun, shade, heat, cold, etc. However, a major issue with using creeping Sedums on this roof was that they would never be seen. The roof is surrounded by a low parapet on three sides and does not have any windows that look out onto it. The primary view of the roof is actually from the top of the driveway due to a rising hill.

When approaching the driveway from the street, one sees the flat roofed garage and behind it an expansive view of the Boulder Valley. The Flatirons, jutting out of the plains below, cry out in exultation in the sweeping vista beyond. Looking at the residence under construction, I couldn’t help but imagine waving grasses and billowing plants atop the monolithic concrete structure echoing the joyous strains of the surrounding landscape.

We needed to find a way for the rooftop landscape to blend the house with the natural surroundings. Landscapers who deny this sense of place are neglecting the most important part of design. We didn’t want to banish the surrounding landscape, we wanted to invite it into our garden. As it turns out, even something as boring as a flat roof can be a place to do this.

But what, we asked ourselves, could survive the harshness of a green roof? Having studied this for years but only had few opportunities to experiment, we were uncertain what could handle the level of stress inflicted on plants in these exposed conditions.

We turned to the surrounding shortgrass prairies and plants from steppe regions around the world. The natural environment has produced a host of plants (nearly 40,000 species) bred to not just tolerate, but thrive in the difficult conditions of our environment.

Instead of engineering a bulletproof solution to the “problem” of the harsh green roof environment, we looked to nature’s system; we designed with plants who would weave themselves together to create a resilient system in such conditions.

With drip irrigation, these plants grew and filled in more rapidly than their in-ground counterparts, loving the added summer heat of the roof. They held fast through the winter, and have all begun to emerge this spring. Time will tell how well this planting withstands the rooftop garden, but it is clear these plants have thus far enjoyed their time here.

When we look at the world as a whole, we see a web of interconnected species designed to support life in nearly every environment without human inputs. Can we, with a skillful and artful hand, create gardens in nature’s image? Hellstrips, parking lot landscapes, and green roofs are begging us to try.