Planting Design Guidelines

There are a number of guidelines that the residential site designer should consider when designing with plant materials in preliminary design. Some of these were covered

in the section on design principles in Chapter 9. The reader is urged to review the var­ious techniques for establishing unity, including mass collection, dominance, repeti­tion, interconnection, and unity of three.

Use Plants in Masses Perhaps the most fundamental guideline of planting design is that plants should form a continuous mass when they reach their mature size (Fig­ures 9—10 and 11—55). Plants that are grouped together are more visually unified (the principle of mass collection) than those that are scattered about as individual elements. Massed plants are often healthier, too, because they protect one another from sun and wind.

Furthermore, it is desirable to mass plants so they form groups of multiple plants of the same species. Masses of plants, whether woody, perennial, or annual, should not be composed of numerous singular plants as depicted on the left side of Figure 11—56. Rather, plant masses should be composed of subgroups of numerous plants of the same species, as seen on the right side of Figure 11—56 (also see Fig­ure 9—11). Single plants of a species can be used, but should be reserved for carefully placed accents rather than being located throughout a plant mass.

Organize Plants in Layers Another concept for planting design is to think about and use plants in both vertical and horizontal layers. Studying plants in layers helps the designer to create outdoor rooms, to provide visual depth, and to establish engag­ing complexity in the landscape.

Vertically, plants typically occupy three primary layers: ground, middle, and canopy, as seen in Figure 11—57. These equate to the ground plane, vertical plane, and overhead plane of outdoor rooms described in Chapter 2. Most designs organize plants in each of these layers to provide the maximum visual interest in the vertical cone of vision and to create the strongest feeling of outdoor space. In some instances, one or more layers are intentionally omitted to create a desired spatial effect.

The ground layer is the floor or foundation of a planting design. It is usually de­fined with mulch, ground cover, and/or other low plants to establish a low “carpet” or “planting bed” where the majority of plants in the other two layers are located. The de­sign of the ground layer occurs early in the planting design process, concurrently with form composition, because planting beds occupy one of the largest areas of the ground plane of a site. The size and shape of the ground layer are critical because the planting bed’s configuration directly influences the location of plants in the two layers above.

The middle layer is the vertical plane of planting design and is established by both shrubs and tree trunks. This is the most important layer in terms of creating en­closure around outdoor rooms, as well as controlling views into, through, and from outdoor spaces. Shrubs and most trees are located within the planting beds, so there must be a coordinated study of the ground and middle layer of plants. It is common to work back and forth between these two layers, adjusting one to the other. Particular thought should be given to the height and density of plants in this layer as a means of affecting enclosure and views.

The canopy layer is the overhead plane or ceiling of outdoor space and is created by tree canopies. It is the layer that provides shade, controls the amount of light, and provides a sense of refuge in the landscape. The canopy layer is most needed over areas that are frequently used, such as those for entry, sitting, and social gatherings. A canopy layer may also be desired to extend the height of a planting area and to provide shade and protection over plants located below. Attention should be given to this layer’s density and height above the ground.

It is easy to forget the presence of all three vertical plant layers when looking at a plan view of a design as in Figure 11—58. A trained designer should be able to inter­pret such a drawing and “see” the separate layers as they are portrayed in the bottom portion of Figure 11—58. All three layers need to be considered in concert with each other so they function as a coordinated composition. However, this does not mean that each layer has to exactly repeat the layout of the other two. Many successful

Figure 11-58

The three vertical layers of plants should be "seen" and considered when studying a design in plan view.

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designs allow each vertical layer to be designed with a different configuration as long as it still fits the other two, as seen in Figure 11—59.

Horizontal layering of plants is also recommended within individual planting beds to furnish a feeling of depth and to create a tapestry of plant forms, colors, and textures displayed one against another. Where the size of a planting bed allows, fore­ground, middle ground, and background layers can be formed with different heights of plants, as illustrated in Figure 11—60. The foreground is frequently established with ground cover and/or annuals. The middle ground can be formed with low shrubs and perennials. The background is often composed of tall perennials, tall shrubs, and/or trees. It is frequently desirable to establish contrasts of color and texture among these horizontal layers so that each layer “reads” against the others. Too much similarity among the horizontal layers makes a plant composition that is one undistinguished mass of plants with little excitement.

The depth and composition of the different horizontal plant layers can remain similar in controlled spaces, but should vary from one area of a planting bed to an­other to create visual attraction, as indicated in Figure 11—61. Likewise, it is not desir­able or even necessary to always have a foreground, middle ground, and background. Some planting areas may benefit from only one or two layers to create special visual effects, as in Figure 11—62.

Coordinate Plants with Overall Design Character The organization and selection of plants should conform to the overall style of the landscape design. Plant materials should not be arranged based on the designer’s personal preferences for appearance but rather on a conscious effort to continue the design theme established early on in the design process. In doing this, the designer should be aware of three broad ways of organizing plants: (1) in rows and geometric blocks, (2) in drifts, and (3) in a combi­nation of blocks and drifts.

Plants can be located in straight lines to form rows or placed in a series of paral­lel rows to form geometric blocks, as seen in Figure 11—63. Often the rows and blocks are linear and rectangular in nature. This method of organizing plants is a stylization of the agrarian practices of lining plants in rows for cultivation and irrigation. Rows and blocks of plants should be used in formal or axial designs where rectangular geometry prevails, as in Figure 11—64. This character of plants is also suitable for or­thogonal modern designs or contemporary designs where rows of plants form a com­plex pattern of overlapping lines and forms, as shown in Figure 11—65.

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The presence of the three horizontal layers of plants may vary to create different spatial effects.

Figure 11-63

Example of rows and geometric blocks of plants.

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The second general technique for arranging plants is in drifts. Drifts are amor­phous, naturalistic masses of plants that are curved and/or irregular in their overall shape, as in Figure 11—66. Drifts of plants are an attempt to replicate the organiza­tion of plants in natural habitats and should be used to complement designs that are curvilinear, naturalistic, or cottage style, as in Figure 11—67. Typically, one drift of plants is layered on others to form a complex pattern of heights, colors, and textures within the planting area. Drifts of plants should not be used in formal and modern design styles.

The third way of arranging plants is a combination of blocks and drifts. With this technique, plants are typically placed in rows or blocks to form a frame around the outside of the planting area, as in Figure 11—68. Then plants, often perennials and/or annuals, are located in drifts within this architectural outline. The rows of plants give structure and order to the planting while the interior provides a sense of randomness. The two opposite styles are often attractive complements to each other. This approach to planting design is most appropriate to geometric design layouts.

Design for Different Seasons Another consideration for planting design is to plan for different seasons of the year. That is, one should study how plants will look by themselves and with surrounding plants throughout the year. Almost every geographic region has distinct seasons of one type or another. Spring, summer,

autumn, and winter are the most notable seasons. In addition, some regions experi­ence recurring cycles in precipitation, humidity, and/or wind that establish seasonal variations.

It is important to consider these seasonal variations because plants are not static elements. Many plants respond to and often change with annual climatic cycles, even in regions that have a seemingly uniform climate. The appearance and health of plants is linked to seasonal fluctuations.

Thinking in terms of seasons has a number of design benefits. One is that it cre­ates a planting design that is attractive throughout the year. A frequent mistake is to design exclusively for summer, a problem that is reinforced by countless plant pictures in books and magazines taken during the summer. In some northern regions, summer is, in fact, a relatively short three months or so out of the entire year. What about the other nine months? Does the landscape disappear then? A successful planting design is one that considers all seasons and creates a landscape that is equally as attractive in December as it is in June.

Another consequence of studying a planting design in multiple seasons is that it should help the designer select plants that more dramatically express the notable char­acteristics of each season. For example, plants that emphasize flowers in spring, attrac­tive foliage texture in summer, vibrant color in autumn, and interesting trunk and branch structure in winter highlight important qualities of each time of year. In some

regions, it is important to use plants that are attractive during an annual dry period and other plants that respond to rain after the dry period has ended.

There are several techniques for studying seasonal shifts. One is to examine a planting design’s appearance in different seasons by doing a quick color study of the same plan for each season. Another is to use a chart that shows how the majority of plants will look and change during the course of a year, as seen in Figure 11—69. This allows the designer to see what each plant looks like in relation to other plants for any given time of year. It also helps the designer to determine if some times of the year have too much or too little plant interest. Ideally foliage, flowers, and fruit should alter in a continuing sequence throughout the year, not all at once in one season. However one approaches season, it is one of the most challenging aspects of residen­tial landscape design. It is not easy to think in terms of a dynamic, changing land­scape. It is in some ways easier to design building interiors where the elements remain relatively fixed over a period of time.

Use a Variety of Plant Types As a general rule of thumb, it is advisable to use a variety of plant types in any planting scheme. Annual, perennial, and woody plants, including a mix of deciduous, coniferous evergreens, and broad-leaved evergreens, should all be used to establish visual interest in a landscape. A range of plant types is more able to establish layers and to express seasonal change as discussed in the previous sections. Furthermore, a variety of plant types tends to replicate conditions similar to natural plant habitats where an interrelated combination of plants typi­cally exists.

The extent of plant variation incorporated in any given residential site is a matter of judgment. While some variety is desirable for visual stimulus, too much diversification can be chaotic. Unfortunately, there is no rule of thumb for defin­ing when too few or too many different types of plant materials have been used in a design. There should be some balance between using some plants for repetition throughout a site (Figure 9—22) and other plants primarily to extend the plant palette (Figure 11—70). The decision regarding plant variety should also take into account the desired character of the landscape design and the house’s architectural style. Formal and modern design styles typically require more control and simpli­fication of a plant palette, and an informal or cottage style accommodates a broader range of plants. The character of the regional setting should likewise have an effect on plant diversity. A region that itself has a limited range of plants

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because of various ecological conditions might suggest a similar response in a resi­dential landscape located there.

In general, variety can be established in a planting design with a combination of woody, perennial, and annual plants. Woody plant materials are those with perma­nent woody structure that exist in the landscape throughout the year. They should be used in the landscape to create the primary structure of the planting design because they are typically the largest and the most enduring plants. Woody plants often form the architectural edges around the perimeter of outdoor spaces and provide the back­ground in plant beds. Woody plants are also the most prominent and sometimes the only plants in a winter landscape in northern climates. Consequently, woody plants should be the first plants organized in terms of process.

Perennials and annuals give the landscape seasonal interest. Perennials are plants that die back at the end of the growing season but reemerge the following spring to grow to maturity again. Perennials exist in many forms and provide the designer with a great number of design options. However, most perennials are used for their flower color and/or their foliage texture. Perennials are best used for visual accent near house entrances, at the end of a planting bed peninsula, at the end of an axis, or in a perennial border that is composed of a range of perennial plants (Figure 11—71). Perennials sometimes are used creatively in pots placed around outdoor living areas or house entrances. Perennials are most commonly placed as foregrounds to woody plants, though they can also be used as background in their own right. Annuals are plants that last for only one growing season. They are used primarily for their flower color and are located in the landscape to provide visual interest and accent. They are best used as foreground plants in the same locations defined for perennials.

Among woody plant materials, a balanced mix between deciduous, coniferous evergreens, and broad-leaved evergreens should be the goal. This is especially needed in those climatic zones where deciduous plants lose their foliage in the winter. A win­ter landscape with too many deciduous plants will lack weight and be too transparent because of the lack of evergreens (top of Figure 11—72). On the other hand, a winter landscape with too many evergreen plants (middle of Figure 11—72) will look too dark and gloomy—not a pleasant sight when the general climate conditions are simi­lar. Ideally a proper balance among the various plant types will make the winter land­scape a visual success (bottom of Figure 11—72).

Deciduous plants should be used in a planting for other reasons as well. One is that deciduous plants accentuate seasonal change. Deciduous plants typically estab­lish a dynamic landscape with four distinct seasons. Each season has its own character

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and interest, and collectively they express a sense that the landscape is indeed alive. By contrast, a landscape dominated by coniferous or broad-leaved evergreens frequently tends to appear static and fixed, as if composed of inanimate objects. Deciduous plants can also be used in planting for their flowers. Many ornamental deciduous shrubs and trees have attractive spring flowers that merit strategic placement in a de­sign. Another use of deciduous plants is as foreground or middle-ground plants in a planting bed, especially when the background is evergreen or structural in nature. Spring flowers, autumn fruit, and winter branch structure are often better seen when placed against a dark background, as in Figure 11—73. Likewise, deciduous trees can be used to express a sense of buoyancy and airiness when they occupy the canopy layer in a planting composition, as portrayed in Figure 11—74.

By contrast, coniferous evergreen plants can be used in a planting to establish a sense of permanence. Although coniferous evergreen plants evolve with growth, they do not change dramatically from one season to the next. Their relatively fixed appearance

provides stability and contrast to the more shifting quality of deciduous plants. Coniferous evergreen plants can also provide visual weight in a planting composition. As a group, coniferous evergreen plants have some of the darkest foliage of all plants, though exceptions do exist. Consequently, coniferous evergreens are good plants to place as backgrounds to deciduous plants or accents (Figure 11—73), or near the ground as visual anchors in a design (Figure 11—74). As previously stated, deciduous and coniferous evergreen plants often work best with each other because each tends to bring out the best qualities of the other.

Broad-leaved evergreen plants have deciduous-like leaves that are held on the plant throughout the year. Rhododendron, azaleas, mountain laurel, and magnolia are examples. Most broad-leaved evergreen plants require acidic soil and shade or partial shade conditions for proper growth. As a group, broad-leaved evergreen plants are noted for their showy spring flowers and are often placed in the landscape for this quality. As with all plants, caution should be exercised in selecting plants for a quality that only lasts several weeks. Fortunately, many broad-leaved evergreen plants also have attractive foliage texture, which makes them appealing even when not in flower.

Compose Plants by Texture In addition to all the other considerations in organiz­ing and selecting plants in preliminary design, it is also advisable to think about plant texture. Plant texture is the visual, tactile quality of plants and is primarily the result of foliage size. The foliage shape, a plant’s overall growth habit, and the distance from which a plant is viewed also affect plant texture. In general, large leaves generate coarse texture and small leaves produce fine texture. Pointed leaves establish a sharp texture, round leaves produce a neutral texture, and small, needle-like foliage forms fine textures.

It is usually advisable to organize plants to create contrasts in foliage texture from one species of plants to the next. Foliage texture is more lasting in plants than in flowers or fruit, so it has greater visual impact over time. Contrasting foliage texture establishes visual interest and helps one plant type read against another, as suggested in Figure 11—75. Without contrast in foliage texture, there is apt to be a dull sameness in a plant composition.

Coarse-textured plants are best used as visual accents because they readily stand out among other plants. Coarse-textured plants can also visually move toward a viewer, thus making distance across a space feel shorter, as indicated in Figure 11—76. Fine-textured plants have an opposite effect and tend to recede away from a viewer.

Locate Plants in Proper Ecological Habitat Plants should be organized and se­lected based on the habitat where they are located. Such factors as sun exposure, wind exposure, soil moisture, soil composition, and soil pH all affect the growing conditions of plants. Likewise, these conditions tend to vary throughout a residen­tial site.

Figure 11-75

Contrasting plant textures based on foliage size and shape can provide visual appeal in a planting composition.

One of the most critical ecological factors is sun exposure. Typically, ideal growing conditions for plants can be classified into three categories of sun exposure: full sun, partial sun/partial shade, and shade. The amount of sun exposure at any given location on a residential site is primarily affected by the house and both exist­ing and proposed trees. It is advisable to develop a sun exposure map of a site, as in Figure 11—77, to aid in determining where to place plants with different require­ments for sun and shade.

As discussed in Chapter 3, a house creates four general microclimates around its exterior based on sun and wind exposure (see Figure 3—1). In terms of planting, these microclimates can be interpreted in the following way:

South side of house:

• Plants located here must tolerate full sun exposure because this area receives sun from mid-morning through late afternoon during the summer.

• Irrigation or other means of watering may be necessary because plants dry out here more quickly because of continuous sun exposure.

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• Some plants that normally grow in a more southerly hardiness zone may be located here because this area has the warmest temperatures in early spring and late autumn, thus extending the growing season.

• Care should be given to coniferous evergreen and broad-leaved evergreen plants situated here in northern climates because of the potential damage that occurs during sunny winter days when plants transpire without being able to draw moisture from frozen ground.

East side of house:

• This is a good location for plants that require partial sun/partial shade condi­tions, especially those that benefit from gentle morning sun and generally cool temperatures.

• This is an ideal location for native woodland edge plants and most broad­leaved evergreens that require a protected transitional zone.

North side of house:

• This is an ideal place for plants that require full shade and cool, damp soil conditions; however, this is a surprisingly narrow zone during midsummer because of the relatively high sun angle.

• Caution should be exercised for plants that cannot tolerate cold wind expo­sure if the house is located in an unprotected spot in relation to north and northwest wind.

• Spring flowering bulbs and shrubs located here will bloom later than those planted on the other sides of the house because there is less sun here.

West side of house:

• Plants grown here must benefit from the intense, hot, afternoon sun.

• Plants located here must be drought and heat tolerant because this is the hottest side of the house; in many ways, this is the harshest microclimate of all for growing plants near a house.

• Plants that require a moderate sun/shade condition should generally not be placed here unless other means of creating shade is provided.

• Irrigation or other means of watering are often necessary because plants dry out quickly here.

• Plants that cannot tolerate the drying effect of summer wind exposure or the damaging consequences of cold winter should be avoided in this location.

Similarly, there are different areas of sun and shade exposure around and below trees. Though the exact configuration and amount of shade in these areas varies with the shape, size, and density of a tree, the general characteristics and suitability for planting are similar to the distinct microclimates that exist around a house. The sun/shade zones for existing trees are relatively easy to determine through actual observation or calculation from sun charts based on the size and height of the trees. The determination of shade from proposed trees is more difficult because of the initial tree size and future growth rate. The amount of shade from newly installed trees is commonly rather minimal, so plants located nearby should be sun tolerant. With a number of years of growth, the initial areas of sun nearest to the tree will have converted to shade. Thus, the landscape designer must think of both the short-term and long-term microclimatic consequences of proposed trees.

There are several other things that should be kept in mind about the planting habitat around trees:

• The sun/shade zones that exist on the east, south, and west sides of a shade tree actually extend below the tree canopy because of the sun angle. More sun is received below the tree canopy on the east and west sides in comparison to

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the south side, as illustrated in Figure 11—78. Thus plants that require some sun can actually be planted slightly under a tree on these sides.

• Planting areas directly below the tree canopy are typically drier than planting areas outside the canopy because the tree foliage catches and holds some precipitation. In addition, the root system of the tree takes moisture from the ground and reduces soil moisture available for other plants. Plants under a tree canopy either need to be more drought tolerant or need to be supplemented with irrigation or other means of watering during hot summer months.

• The largest areas of shade occur on the east and west sides, not the north side of the shade tree, as indicated in Figure 11—79. This is because the sun is at a relatively high angle in the sky when shining from the south at midday, thus producing a comparatively shallow area of shade to the north. Shade-loving plants should not be planted too far north of a shade tree, or they will be in the sun during mid-summer.

In addition to considering sun exposure, the landscape designer should also identify drainage patterns and soil moisture on a site. If necessary, a map of these con­ditions might need to be prepared during site inventory and analysis. Such a map can guide the selection of plants to fit the various soil moisture conditions.