Green Roof Plants

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Vegetation and Plant Selection

Green roof planting designers must consider multiple criteria when planning a design, including design intent; the client's needs and expected outcomes; budget and maintenance parameters; life expectancy of the green roof; access and safety issues; location; micro and macro-environments; exposure; humidity or dryness; maximum and minimum temperatures; medium weight, depth, and composition; and irrigation. Careful plant selection is essential to the green roof's success, as traditional rules used for ground-level plant selection simply won't work on a roof. By necessity, green roof plants must be tougher and less nutrient-reliant than plants found in most gardens.

The type of planting medium used is also particularly significant. Organic content, pH and nutrient levels, weight, porosity, and water retention capacity all need to be evaluated, just as they would in a regular garden. But because of its location, the green roof medium must be substantially lighter, less rich, and more porous than soil used for a ground-level garden. The Landscape Research, Development and Construction Society (FLL) guidelines for green roofs discuss medium composition at length, and the FLL publication Guideline for the Planning, Execution and Upkeep of Green-Roof Sites (2002) is an excellent planning resource. FLL medium certification is also available through Penn State University (Beattie 2005).

Environment and geography also influence plant selection. Design professionals responsible for plant specification must be knowledgeable about the heat and hardiness zones in which the green roof is planted and what plants normally thrive in those zones. In addition, site-specific considerations for a green roof are far more localized than those that appear on any map. Most high and low temperature hardiness zone maps are based on 25-year average low and high temperatures (White 2005), and they do not account for microclimates that are created due to shade, the urban heat island effect, structures that affect the microclimates, increases or decreases in humidity, and other factors. Plant selection for green roofs must be based on all of these climatic conditions, including exposure to sunlight, wind, shade, and temperature fluctuations.

The effect of elevating gardens to the roof level can lead to particular stresses on plant material, due to the decreased substrate thickness. In addition, sun, heat, and wind are mote damaging to plants on a roof and can lead to plant stress and eventual mortality. While the effects of heat stress are not as immediate as those of freezing temperatures, stunted growth and plant mortality can and do result from overexposure to heat. Regional differences in the intensity of eight hours of exposure to sunlight per day will also influence plant specification. A sunny day in Boston, for example, will generally provide different levels of irradiance and humidity than a sunny day in Phoenix, and plants must be selected accordingly.

In general terms, the most successful green roof plants are low-growing, shallow-rooted perennial plants that are heat, cold, sun, wind, drought, salt, insect, and disease tolerant. At the same time, in most of the green roof market in North America, plants must also be able to tolerate some wet roots, especially on lower sections of steeply sloped roofs and on flat roofs during the winter. Green roof plants should also have a long life expectancy or the ability to self propagate, and they should require minimal nutrients and maintenance (White 2005). Plants that are highly flammable, that develop large root systems and thus a high biomass, or that are excessively "thirsty" should be avoided. Plants that require a high amount of nutrients should also be avoided, because they can exacerbate nutrient loading in the riparian systems, adding to, rather than reducing, runoff-related pollution (Hunt 2005).

Since most green roof medium is fractured, or lacking a continuous column of water that facilitates capillary action, it is difficult for water reservoirs residing at the bottom of the medium to move up into the plant root zone. As a result, green roof plants must be able to withstand periods of dryness and heat, a factor that eliminates most traditional annuals and perennials. In the southwestern and western United States, as well as in Latin America, heat and drought are recurring and enduring problems; this adds even more to the challenge of specifying plants that will flourish in those areas. In drought-prone areas, specifying drought-tolerant plants may nor be enough; irrigation systems may need to be installed to ensure plant survival — not only during establishment, but for the long term. Design professionals planning a green roof must consider local needs and realize that roofs at higher elevations will experience more intense heat and wind and lower medium moisture levels.

On the other hand, excessive moisture can quickly lead to stress and mortality, particularly for common green roof plants such as Sedum and Delosperma. It can also create a beneficial germination climate for weed seeds. Hence, only certain plant species are suitable for green roofs and then only in conjunction with a carefully planned system that ensures adequate drainage, the optimal medium, and, where necessary or desired, an irrigation system to keep the plants alive.

Plant Types for Green Roofs

Plants are usually classified as annuals, biennials, or perennials, based on their yearly and continuing growth cycles. Annuals grow, flower, set seed, and die in one growing season. Biennials grow vegetation the first growing season, and then they flower, set seed, and die the second growing season. Biennials are not generally used on green roofs, as they create gaps in the roofscape after they finish blooming and die. Perennials grow, flower, and set seed in one or more growing seasons, and they do not die after setting seed.

The combined constraints governing plant selection for green roofs eliminate the vast majority of annual and perennial plants associated with traditional gardens, including many native plants. Although some herbaceous perennials and annuals may be used as accent plants or for seasonal interest in carefully selected locations with an appropriately deep medium and proper irrigation, they should not make up the bulk of plants specified for an extensive green roof.

Annuals

Annuals should not be the dominant plant selection for the majority of extensive green roofs because they do not offer the requisite longevity required to make a project cost effective. They can be incorporated into plant specifications as seasonal accents, provided the medium and the green roof system will support them. Where annuals are used extensively, regular rainfall of at least 3 in. (7.5 cm) per month is required, and even then the plants may require a supplemental irrigation system. Drought-tolerant annuals such as Portulaca in the southern US or Phacelia campanularia or Townsendia eximia may be used on extensive green roofs as fillers to provide quick color during the first growing season, or they can be used if they naturally reseed without becoming invasive.

Herbaceous perennials

For aesthetic reasons, herbaceous perennials are the most desired plants for extensive green roofs. They offer the greatest color, texture, and seasonal variability, but they require deeper substrate and moisture than are found on most extensive green roofs. Most herbaceous perennials also have limited drought tolerance or ability to withstand the stresses of the green roof, and the deep roots of many taxa make most herbaceous perennials poor choices for green roofs. Some, however, can work well on extensive green roof installations. Petrorhagia can be used, for example, as can some species of Dianthus, Phlox, Campanula, Teucreum, Allium, Potentilla, Achillea, Prunella, Viola, Origanum, and other low-growing, shallow-rooted perennials. When using taller herbaceous perennials as the primary plant material, however, the medium depth must be increased to greater than 4 in. (10 cm) and adequate or supplementary water sources for the plants must be available. Regional considerations are also important when using perennials — for example, 6 in. (15 cm) of medium in Portland, Oregon, will support more varied vegetation than 6 in. (15 cm) of medium in Houston, Texas, due to the differences in rainfall averages and distribution.

Herbaceous perennials must be chosen carefully for compatibility relative to site-specific considerations. While perennials make it possible to achieve a wider palette of plant material with the requisite accommodations, as they expand in biomass they also increase roof load by 2 to 5 pounds per square foot. In addition, the richer medium required by most herbaceous perennials creates a more hospitable climate for weeds, which may crowd out the specified plants and add to after-installation costs for maintenance. In a city housing authority complex in Portland, Oregon, for example, clover took over most of the green roof because of the relatively high organic content in the 5 in. (12.5 cm) medium and because no action was taken to eradicate the clover once it became established (Beattie 2005). If a deep medium is specified, selected herbicides may be required as part of a maintenance routine to keep weeds in check.

Relatively few herbaceous perennials are evergreen, so if winter interest is a major design consideration for a roof, an alternative must be provided to compensate for the long stretches of brown vegetation visible during the plants' dormant period. In addition, many herbaceous perennials, such as Penstemon, have limited longevity to begin with, and this can be further truncated depending on medium depth, environmental factors, disease or insect susceptibility, and ecological competition. A careful designer must specify perennials that are long lived in addition to drought and heat tolerant. Currently, no data exists to suggest that herbaceous perennials in North America can survive in less than 4 in. (10 cm) of medium — even those choices are limited to low-growing plants such as Petrorhagia, Campanula rotundifolia, and various Phlox and Carex species.