Here in the West, we’re arriving at the green roof planting party fashionably late by thousands of years—or at least a few decades. Sod roofs and insulating with vegetation have a long history in northwestern Europe and eastern North America where winters are cold and summers wet. Here, with regular periods of drought and a winter growing season, our challenge is finding plants that satisfy our aesthetic desires yet thrive with minimal use of our most precious resource, water.
“We garden in a unique series of climates in the West. When extending the act of cultivation to green roofs, the differences quickly become apparent.”
In Portland, many green roof projects were initially set forth to slow rainwater runoff during winter storms, attenuating its release from vast, impermeable urban surfaces and reducing the likelihood of overwhelming antiquated treatment systems with stormwater and sediment. Some years ago, local designers coined the term eco-roof to lessen expectations that any rooftop planting—unless regularly irrigated—would remain green past spring. Even as people have become accustomed to the “golden season,” a few rooftop prairie fires have induced caution. Today, we have both non-irrigated and heavily watered green roofs but, from a horticultural perspective, the most successful of these air-scapes are those using just enough water to allow a palette of regionally adapted plants to flourish. Purely functional green or brown roofs are fine, but where aesthetics are important there are plenty of low-ebb choices.
For those of us who garden on the Pacific side of Pacific Horticulture, two or more months of drought every summer is our most rigid parameter. The more shallow the planting mix—often less than six inches on a standard extensive roof—and the harsher the dry season, the tougher the plants must be. Woody plants in the winter-rainfall West tend to be drought resilient by having deep roots—an adaptation that could prove unfortunate on a roof. Therefore, succulents, grasses, perennials, and bulbs take the lead in our planting palette; lightly “enhancing” our climate broadens our choices further.
Having a drip or micro spray system to water just a bit after the last rains of spring and before the first in autumn effectively reduces the length of the dry season. Many dry summer plants, especially natives, resent water when temperatures are high. Occasional watering can occur almost any time along the coast but inland should be scheduled during cool mornings in the hot season and as needed during unusually long winter dry spells. In my experience, this practice keeps even a dormant planting healthy and ensures reliable greening in autumn and good coverage.
Limiting the number of winter-dormant plants on roofs in the West reduces bare patches where winter-growing weeds can establish. If slowing stormwater is the goal, why not have the plants as active as possible during the wet season(s) as well? Avoid sedums like S. spurium that go dormant just as autumn rains begin, leaving soil exposed.
Recently, an abundance of insta-systems, including the common “sedum mat,” have become available via a multitude of sources. These are often planted in a one-size-fits-all mix with only some species adapted to our part of the world. Furthermore, what is essentially a succulent sod planted in an organic mix rarely integrates well with the more mineral soils preferred by dry-land or native plants. As an alternative, modular tray systems allow for growing a regionally appropriate plant mix—on or off site—that can then be assembled in a specified pattern without sod-like barriers. Newer tray systems offer removable sides, or sides constructed of organic, biodegradable materials that reduce the amount of plastic on site and allow better air and water movement through the substrate.
These easy-bake-oven solutions are abundant, but I’ve found the most successful method is the more primitive solution of planting a matrix of plugs and cuttings of desired species directly into the growing medium substrate. Where possible, using nitrogen-fixing species, such as plants in the legume or Rhamnus family, reduces any need to fertilize. Substrates can be a mix of reused material such as dairy fiber and compost with larger amounts of mineral components. In Portland, pumice and scoria are preferred though their sustainability could be argued; though not rapidly renewable resources, we know from our volcanic legacy that renewal is assured!
The Plants: Form and Function
The most fun, of course, is plant selection. As with designing any garden, a few parameters help narrow the endless possibilities. The site and the engineering, such as sun exposure and depth of substrate, predetermine some decisions. Assuming low water—and unless you find weeding cathartic—as mentioned earlier, high priority selections should include evergreen or winter-growing plants to prevent weed seeds from germinating on exposed soil. Other important factors include aesthetic color combinations, peak bloom time or season of interest, and consideration of wildlife that might benefit from particular plant selections.
For commercial green roof projects, the bottom line is coverage. Projects that allow us to expand beyond the typical palette offer opportunities to provide broader ecosystem services to benefit creatures around us. For the birds? Yes, but imagine planting populations of plants with specific pollinators in mind. For instance, the ability to grow a relatively isolated patch of a rare lupine might just mean maintaining our own small population of a Checkerspot butterfly while fixing nitrogen for a neighboring plant’s dinner. Stone mulches and wood such as driftwood are aesthetically pleasing while also providing habitat niches.
Succulents are ideal candidates for planting green roofs in the west and the list of western natives, as well as other summer-dry species, is extensive. Sedum spathulifolium, S. oreganum, and S. divergens have worked well in the Northwest; other native sedum species that are slower growing and more suitable for small spaces include S. oregonense, S. laxum and S. obtusatum. Summer-dry sedums such as S. album and S. acre thrive, but invasiveness is a concern. Lewisia, a rock garden favorite, works well if given a more shaded summer position, and the multitude of Dudleya species remain underused. Carpobrotus chilensis, arguably native, and myriad South African ice-plants add great coverage and color for the price of a little additional water. For especially dry regions, smaller agaves—not the huge ones that will cave in the roof—hesperaloe, and nolina species add green under extreme conditions. Hardy sempervivums, called houseleeks in Europe, or colorful echeveria in mild coastal spots, are useful for small spaces or vignettes.
Bulbs provide color throughout the year. Many once-rare natives are now available inexpensively from commercial sources. Calochortus luteus, with large, pleasing yellow flowers in late spring, seems at home and even multiplies in the mineral soils we use. The genera Brodiaea, Triteleia, and Dichelostemma shine when it comes to diversity and length of flowering. D. capitatum can begin the show as early as January with waving purple blue flower clusters while B. elegans and T. laxa extend the color as late as July. The unique reddish flowers of D. ida-maia are an early blooming hummingbird attractor. The West is poorly endowed with native fall flowering bulbs, however, South African nerine species, Mediterranean colchicum, and crocus species fill the seasonal gap. With a more generous soil depth, say 10 inches, Cyclamen hederifolium begins its long flowering period in late summer, and C. coum offers an intense winter show of white, pink, or cerise color popping up through groundcovers such as low growing forms of Ceanothus gloriosus.
A few thus-far successful perennials not mentioned above include Eriophyllum lanatum. One selected for evergreen silver foliage and rather long-lasting, bratty yellow flowers is E. l. ‘Takilma Gold’ named for the Siskiyou town nearby. Lomatium columbianum emerges from a succulent root just after the first rains in fall with tufts of leaves resembling giant blue parsley, then flowers in mauve umbels to at least March. Lupinus albifrons, with silky silver leaves and complementary blue to violet flowers, has succeeded on rooftops where the often heavier garden soils cause the plant to collapse. Grasses include Carex praegracilis and C. pansa for good green coverage, and Muhlenbergia dubia for symmetric beauty. Western Festuca selections add good blue color though they benefit from dividing every few years. Chondrosum syn. Bouteloua species, primarily from the summer monsoon southwest, are very good for cover, though probably best for interior climates where the brown of their winter selves is more familiar.
As we, and hopefully our gardens, get better with age—I’m improving mine with a chain saw at the moment—there is always more to add and experiments to perform. Simply observing what grows on thin or scald soils or a rocky outcrop is good homework. Besides, green roofs needn’t be atop skyscrapers; a shed, garage, or any object deemed stationary, can be treated as an acquisition of new gardening territory.