Plant materials serve two primary architectural uses on the residential site by creating space and either screening or enframing views. Plant materials can function as floors, walls, and ceilings to establish the spatial envelope of a residential design, just as the architectural components of a building create indoor rooms. It should be noted that the term architectural refers only to enclosing space and does not mean using plants in straight lines or formal layouts. Plant materials can be used architecturally in any design theme.
Creating Space Plant materials of all sizes and types can be used to define outdoor space. However, it is best to locate trees first when creating outdoor rooms with plant materials because their size and mass establish the overall framework of the spatial composition. Trees should be placed in a design to create vertical walls and overhead ceilings of foliage (Figure 11—27). After the trees have been arranged in the design, smaller plant materials can be located to complement the spatial organization of the trees.
In the vertical plane, trees can define space by two different means. First, the trunks of trees can suggest the edges of space, particularly when they are massed or
lined up (top of Figure 11—28). The tree trunks can act like the columns in a building, subtly separating one room from another. Tree trunks only imply the edge of a space because views are not completely contained within the space. To create complete enclosure, smaller trees or shrubs must be used in association with the tree trunks (bottom of Figure 11-28).
The second way trees create space in the vertical plane is by means of their foliage mass. Two different levels of spatial enclosure are possible (Figure 11-29). Large trees provide walls of foliage that define the upper limits of outdoor spaces, while smaller trees create lower walls for enclosure at eye level. The residential site designer can work with these two planes to make varied degrees of spatial enclosure (Figure 11-30). Large trees are best for outdoor rooms where views below the tree canopy are desired, while smaller trees are appropriate where walls of foliage are needed at eye level. In creating vertical enclosure of outdoor space with trees, the designer should decide whether year – round or seasonal enclosure is desired. Evergreen trees should be used for year-round
enclosure whereas deciduous plants can be used to enclose a space during the late spring, summer, and early autumn months of the year.
Trees can also be used to furnish ceilings over outdoor rooms. As discussed in Chapter 2, a vegetative ceiling can provide a sense of vertical scale in an outdoor space, a feeling of comfort, and shade. These uses are frequently desirable where people spend time sitting and socializing in the outdoors, such as in the outdoor entry foyer, the outdoor living and entertaining space, or other sitting and gathering spaces. The spacing of the trees, density of the canopy, and height of the canopy above the ground are variables that influence the degree of overhead enclosure in outdoor space (Figure 11-31).
In defining space with trees, thought should be given to coordinating their placement with the desired design theme and grading of the ground plane. Trees should reinforce the shapes in the form composition by extending the lines and forms of the ground plane upward into the third dimension. Trees should not be scattered indiscriminately in a design but should be massed together so their trunks and foliage mass reinforce the base plane patterns. Figure 11-32 shows both a bad and a good example of coordinating trees with the underlying ground plane. Trees should be placed in a structured alignment in an axial design theme and in a flowing composition in a curvilinear theme (Figure 11-33).
Outdoor space can be established with other plant materials. Tall shrubs that are 6 feet or more in height enclose space at a lower level than trees. Tall shrubs can be used by themselves to create space or in association with trees (Figure 11-34). The tall shrubs can function like walls below the ceiling of the canopy overhead.
Low shrubs, between 1 foot and 3 feet high, can also be used to indicate the limits of outdoor space much like low walls, which define a space’s edges without limiting views outward to other areas of the landscape (Figure 11—35). This is desirable treatment for outdoor living and entertaining spaces or entry foyers where partial containment with views to other points in the landscape gives a sense of separation without
complete enclosure. Partial containment is often a good balance between complete enclosure (with no views) and no enclosure (with unlimited views in all directions).
Ground cover, spreading plants growing to a maximum height of 1 foot, low annuals, and perennials can likewise imply the edges of space. A bed of ground cover next to an area of lawn or pavement can imply an edge to a space (Figure 11—36). The change in material and the slight height of the ground cover suggest where the outdoor room stops on the ground plane. The shape and edges of ground cover beds should be consistent with the overall design theme of the site (Figure 11—37).
Screening and Directing Views Another architectural use of plant materials on a residential site is to screen and enframe views. In relation to other design elements, plant materials have several advantages and disadvantages for screening and directing views. In comparison to steep slopes or berms, plant materials take up less room and provide more height (Figure 11—38). Therefore, plant materials are usually better than slopes or berms for screening views on a small residential site. However, plants occupy more room than a fence or wall. Plant materials also require some time to reach mature height and may vary in their density with the season if they are deciduous. Plants also require proper conditions for growth. Walls and fences, on the other hand, give instant screening and separation.
On a residential site, plants can screen undesirable off-site views to neighboring driveways, backyards, and storage areas or to unsightly on-site elements such as an air conditioner, vegetable garden, and so on. Plants may also give a sense of privacy by screening views to the neighbor’s outside living and entertaining space or recreational lawn area (Figure 11—39). Evergreen plant materials are usually more desirable for screening views than deciduous plants because they furnish year-round screening,
though a mixture of evergreen and deciduous plants provides the most visual balance and interest.
Both foliage mass and tree trunks can be used to enframe views (Figure 11-40). Again, they should be coordinated with other elements to enhance their ability to direct views to selected areas or points of the landscape.
