The Role of Plants in a Sustainable Landscape

of its inhabitants, and reduces if not eliminates harmful impacts on the natural environment.

The high-density condominium complex located in the Netherlands shown in Figure 10.10 contains a garden designed using LID principles. This particular design serves several complementary and important purposes when implementing a water retention strategy. The gardens are primarily planted with seasonal flowers, herbs, and vegetables. The canal in Figure 10.10 adds interest as only a water feature can in a dense urban setting. The gardens are essentially a large basin or vessel that has been designed with a capacity for detaining water during heavy storms. Eventually the water is absorbed in the soil and a portion evaporates. Some of the water stored in the concrete canal is used to supplement irrigation of adjacent planting.

It is their interest in nature and perhaps the opportunity of working with plants that draw people into the profession of landscape architecture. Many of the past’s well-known landscape architects considered themselves plants-men and as such knew a great deal about both the aesthetic possibilities of plants as well as their horticultural requirements. Plants also were appreciated for their functional possibilities as well. Plants can provide shade from direct sun and for their cooling effect on a space. Plants are used to create the physical structure to define and create outdoor spaces. Plants can be used to moderate climate (particularly air temperature) of both the exterior spaces and the interior spaces of buildings, thus reducing energy consumption and utility costs. Plants are also used to screen or buffer views, sound, and wind. Plants can be placed with the goal of screening out or making undesirable views less obtrusive. For instance, blocking the view of your neighbor’s windows into your outdoor entertainment space. Selecting particular plants having a density of leaves and favorable leaf shape can buffer the impact of unwanted noise from a known source such as a highway or diminish the intensity of prevailing winds. There are myriad design opportunities for using plants for aesthetic, functional, and cultural reasons. Plants are a flexible material that, in addition to serving some design purpose, will add layers of sensory-related value with their physical changes throughout the seasons and over the years. They instill a sense of permanence and belonging to the spaces created by landscape architects. The term sense of place comes to mind, meaning the physical and ephemeral qualities of a designed space having similarities and providing visual continuity to their context or endemic surrounding.

Plants as Modifiers of Climate

Vegetation can be an effective means to lower building energy consumption. Selecting the right plant and placing it in the strategically correct location can reduce energy consumption and costs associated with building heating and cooling control. The presence of plants can reduce urban heat island effects if trees are installed to shade

paved areas, or the installation of green roofs, or vegetated structures (such as trellises) to cover non-vegetated surfaces, such as walkways, roofs, or parking lots. With the planting of appropriate species of deciduous trees and shrubs and placing these materials strategically along the south-facing side (or sides exposed to sunlight) of a building, the plants can reduce heat gain inside the building in the summer months and allow heat gain to enter the building during winter months.5

During the height of the summer sunshine (and heat!), it is helpful to discuss ways to reduce the harmful effects of sun glare and heat zones. Too much light—and the wrong kind of sunlight—can create glare, which can affect our ability to see clearly. The glare from sunlight can also be a source of discomfort, in terms of heightened sense of heat as well as its effect on our eyes. Artificial light can be another source of glare, particularly from a high intensity light source. One can combat sunlight glare with sunglasses or by wearing a widebrimmed hat. By planting trees within an urban plaza or any location with extensive paved surfaces, the effects of glare can be reduced. In addition to the planting of trees, a landscape architect might design a trellis or some type of overhead shade structure together with climbing vines where an architectural solution is warranted. Heat islands within urban districts can be the cause of unnecessary discomfort (excessive heat) where a significant surface area in the streets and spaces between buildings are paved with concrete and asphalt. The build-up of heat islands in urban areas can increase summertime peak energy demand, air conditioning costs, air pollution and greenhouse gas emissions, heat-related illness and even mortality. Reducing the urban heat island effect can be achieved through the use of green roofs and the strategic planting of shade trees. With the incorporation of significant areas planted with trees to provide shade, heat gain is reduced, thus reducing the physical discomfort of excessive heat as well as producing a positive psychological effect one experiences walking or resting in the shade under tree cover. Further reductions in heat and glare can be made by the installation of earth-colored paved surfaces in urban areas, which will mediate reflective sunlight and absorb less heat.

Vegetation Provides Valuable Habitat

Habitat is the place where animals normally can be found and the environmental conditions exist to support their survival and life cycles. Plants have specific requirements to successfully take hold and maintain their existence. The primary requirements are soil, climate, moisture, sun and shade, altitude, and longitude. Animals can also be contributory factors to the presence and maintenance of plants. Consider the honeybee and its symbiotic relation with flowering and fruit-bearing plants. A whole host of animals have a semiotic relation to specific plants, from the microscopic fauna in the soil through the animal kingdom phyla to the larger vertebrates and mammals. Likewise, animals have their habitat requirements and preferences for choosing and circulating throughout the seasons. Plant habitat is a source of food, shelter, and procreation for animals. Animals will

migrate to meet requirements that each habitat provides in their lifecycle. Animals, specifically birds and whales, will migrate thousands of miles throughout the year as the seasons progress.

In urban areas animal habitat can be of two types: natural habitat set aside and protected or created habitats reconstructed to simulate the physical characteristics to support specific animals. Habitat conservation or creation can be established as a goal in planning new development or retrofitting older development. In addition to setting aside or creating open space and habitat, green corridors are often necessary for connecting smaller patches of habitat and open space. The establishment of wildlife corridors is an effective strategy to increase the effectiveness of smaller slivers of interconnected habitat in lieu of setting aside large expanses of natural areas. Habitat creation in urban areas can take many forms. Small patches of landscape such as parkway plantings can serve a valuable animal habitat function with an informed selection of plant species. And of course habitat creation to attract specific animal species is increasingly popular as urban dwellers seek to attract birds, butterflies, and other colorful and interesting animal visitors within sight of where they live and work.

Food, water, and shelter are the prime components necessary in creating habitat for animals. The more knowledge the designer has about the animal species they wish to attract and support, the greater the potential success. Landscape architects will often research the habitat preferences of the wildlife they wish to attract. They will create planting plans using selected plant species to attract and support desired wildlife. The designs of specific spaces for butterfly or hummingbird gardens are an example of targeted habitat creation. Where other animals are desired, the landscape architect will create, for example, water features (rain garden or pond) and allow dead trees and large limbs to remain as part of a larger design strategy of creating a viable and robust habitat for wildlife.

Plants Increase Economic Value

Generally, individuals will base their choice of a particular neighborhood to live or a particular house to purchase among several that are similar in style or with comparable spatial features, based on the quality and extent of the plant material present. Well-landscaped neighborhoods and homes tend to sell more quickly and at a higher price than similarly laid out neighborhoods and building architecture but without a well-designed landscape. Of course, amenities such as the presence of parks and access to public services, good schools, and transportation will also influence a purchase decision. The extra dollar of cost in adding plants will have a greater increase in economic value to a project than does the increase in project costs by adding architectural elements. The curb appeal of a well-designed landscape directly increases the market value of real estate.

Plants Fix Things

If used correctly and informed by solid research and experience, plants can be used by landscape architects to perform remedial functions in the landscape. Plants, as you will see in a moment, are not

just used to make things pretty. Vegetation in the landscape performs a variety of functions that include:

1. Improve water quality: Plants associated with specific environments naturally perform a water cleansing function. They do this by filtering out specific chemicals that enter their environment from surface water runoff. Plants have the capacity of creating a chemical bond that either converts the chemicals or sequesters them in their roots, stems, or other physical components. The plants found in wetlands, for instance, efficiently remove heavy metals such as mercury and arsenic from the environment. They also increase dissolved oxygen in the wetlands and other water bodies such as lakes and ponds. Plants are selected to perform a similar water quality improvement function for constructed wetlands or detention ponds. There are examples where municipality public works departments have constructed wetlands as part of their wastewater management infrastructure. Water that has been treated is given a final cleansing after it is discharged into the wetland. In some instances untreated effluent is discharged directly into the created wetlands and processed (removal of undesirable compounds) as it moves downstream eventually considered as acceptably treated water. 2. Plants purify the air: Trees and other vegetation contribute to improving air quality by intercepting airborne particles (dust and soot) and absorbing carbon monoxide, sulfur dioxides, and nitrogen dioxide. Through the process of photosynthesis, plants remove these pollutants through respiration then release oxygen and other elements back into the atmosphere. 3. Reduce soil erosion and slope failure: The soil-holding capability of the root system of many plant species serves to reduce soil erosion in nature. Grasses, willow, and alder are examples of species that reduce erosion as surface water flows down the surface of a slope. Vegetation with a vigorous root system will also reduce slope failure, establishing a strong organic mat that protects the slope from either the mechanical damage of rain or the flow of surface water. 4. Wetlands are habitats for wildlife and reduce harmful impacts from seasonal phenomena: Wetlands occur at the edge of aquatic or terrestrial systems. A variety of wetland types occur throughout the world and include bogs and swamps, tidal marshes, prairie potholes, sea grass beds, and seasonally wet forest wetlands.

They are one of the most diverse ecosystems of flora and fauna.

They perform vital functions, in addition to supporting an abundance of wildlife. They improve water quality, reduce the impact of winter storm surges, and contribute economically to a region in the commercial harvesting of fish and shellfish. The reconstruction of wetlands lost to human activities (such as urban and industrial development) is often an integral part of a larger strategy to reduce the impact from winter storms and impacts from other natural phenomena. Wetlands intercept storm-water runoff to reduce flood damage, and support the re-establishment of wildlife and commercial fisheries as part of biodiversity protection efforts.6