SELECTING PLANTS FOR LANDSCAPES

Many influences guide the choice of plants for use in landscapes. Too often, personal preference, sentimentality, or an unfamiliarity with plants prevail over more logical reasons for selection. The best reasons for selecting one species of plant over another are that

• the plant fills the role assigned it in the design

• it will survive the growing conditions of the site

• it is affordable by the client

If several species meet these specifications, then personal prefer­ence and sentimentality can be applied unabashedly.

Plants are selected for certain roles based on architectural, engineer­ing, and aesthetic considerations. For example, when plants are used to shape the outdoor room, frame a view, shade a patio, or soften a brick wall, their function is architectural. When they solve a problem such as directing traffic, reducing wind velocity, or absorbing dust and noise, they are engineering elements. If valued essentially for their appeal to the senses, through fragrance, sound, color, or other visual attributes, they are aesthetic contributors.

When comparing plant species, landscape designers look at charac­teristics that include the following:

• hardiness (ability to survive the winter)

• mature size (height and width)

• flowering qualities (blossom color and fragrance)

• fruiting qualities (color, edibility, or toxicity)

• foliage and bark color

• foliage and bark texture

• rooting system (effective or ineffective in erosion control, moisture tolerance, drought resistance)

• foliage silhouette (Figures 9-15 and 9-16)

• deciduous or evergreen foliage

• presence or absence of thorns

• brittleness of the wood

• rate of growth and length of life

• ability to attract wildlife

• soil preferences (nutrients, composition, and pH)

• susceptibility to pests

• frequency of pruning required

• availability

• cost

The relationship between rate of plant growth and brittleness of the wood is worth noting. In general, rapidly growing trees have more brittle wood. That is why silver maples, cottonwoods, and willows drop branches all over the yard after a windstorm. They grow quickly and break up easily. It also explains the greater strength of slower growing trees like the oaks.

Silhouette and Examples

Characteristics

Possible Landscape Uses

Silhouette and Examples

Characteristics

Possible Landscape Uses

wide-oval

Flowering crabapple Silk tree

Cockspur hawthorn Flowering dogwood

• spreads to be much wider than it is tall

• often a small tree

• horizontal branch;ing pattern

• branches low to the ground

• focal point plant

• works well to frame and screen

• can be grouped with spreading shrubs beneath

round

Shinyleaf magnolia Cornelian cherry dogwood

American yellow wood Norway maple

• width and height are nearly equal at maturity

• usually dense foliage

• if the tree is large, a heavy shade is cast

• lawn trees

• mass well to create grove effect

• larger growing species may be used for street plantings

• smaller growing species can be pruned and used for patio trees

vase-shaped

• American elm

• high, wide – spreading branches

• majestic appearance

• usually gives excellent shade

• an uncommon tree shape

• excellent street trees

• allows human activities underneath

• frames structures

• use above large shrubs or small trees

• note: the American elm is easily killed by Dutch elm disease; this limits its use

columnar

1

Columnar Norway maple

Columnar Chinese juniper

Fastigiate European birch

• somewhat rigid in appearance

• much taller than wide

• branching strongly vertical

• useful in formal settings

• accent plant

• group with less formal shrubs to soften its appearance

• frames views and structures

pyramidal

і

Pines Fir Spruce Hemlock Filbert Sweetgum Pin oak Sprenger

magnolia

• pyramidal evergreen trees are geometric in early years

• pyramidal deciduous trees are less geometric

• pyramidal shape is less noticeable as the trees mature

• accent plant

• large, high – branching trees allow human activity beneath

• older trees may be valued for their irregular shapes

• note: avoid planting large trees near small buildings

weeping

Weeping willow Weeping hemlock Weeping cherry Weeping beech

• very graceful appearance

• branching to the ground

• easily attracts the eye

• grass or other plants cannot be grown beneath them

• focal point plant

• screens

• attractive lawn trees

• note: avoid grouping with other plants

figure 9-15. Typical tree silhouettes, characteristics, and landscaping uses. (Delmar/Cengage Learning)

Shrub Silhouette and Examples

Characteristics

Recommended Landscape Uses

Shrub Silhouette and Examples

Characteristics

Recommended Landscape Uses

globular

d

Brown’s yew Globe arborvitae Burford holly Globosa red cedar

• as wide as it is tall

• geometric shape

• attracts attention

• does not mass very well

• accent plant

• use several with a single pyramidal shrub for strong eye attraction

• avoid overuse

pyramidal

1

Upright yew Pyramidal junipers False cypress Arborvitae

• taller than it is wide

• rigid and stiff

• attracts attention

• geometric shape

• usually evergreen

• accent plant

• focal point

• use to mark entries and at incurves

• group with less formal spreading shrubs

low and creeping

• low growing

• much wider than it is tall

• masses well

• irregular shape

• loose, informal shape

• use to edge walks

• cascades over walls

• controls erosion on banks

• grown in front of taller shrubs

Andorra juniper Bar-Harbor juniper Cranberry cotoneaster Prostrate holly

upright and loose

й

• taller than it is wide

• loose, informal shape

• usually requires pruning to prevent leggy growth

• closely spaced for privacy

• use to soften building corners and lines

• useful for screening and framing views

spreading

Hetz junipers Pfitzer junipers Spreading yew Mugo pine

• wider than it is tall

• medium to large shrub

• masses well

• usually dense foliage

• use at outcurve

• place at corners of buildings

• useful for screening, privacy, and traffic control

Lilac

Smoke bush Rose of Sharon Rhododendron

columnar

Hicks yew Italian cypress Arizona cypress

• width is about half the height

• geometric, flat topped, and dense

• accent plant

• foundation plantings

• closely spaced for hedges

• mass closely when a solid wall is desired

arching

Forsythia Beautybush Vanhoutte spirea Large cotoneaster

• wider than it is tall

• prevents the growth of other plants beneath itself

• graceful silhouette

• usually requires yearly thinning

• provides screening and dense enclosure

• softens building corners and lines

• background for flowers, statuary, fountains

figure 9-16. Shrub silhouettes (Delmar/Cengage Learning)

Different plants possess different combinations of the selection factors, and plants can only be judged superior or inferior within the role requirements established by the landscape designer. An immediate need for shade might make fast-growing species preferable to slower – growing ones. Both might be planted, one for immediate effect and the other for long-term benefit. Location may determine the quality of a species. Certain weed trees of the Northeastern and Central United States are the premier lawn trees of the Southwest, where they surpass less adaptive native Eastern trees. The plant lists given in Tables 9-2 to 9-6 (on the following pages) represent many such lists given in count­less texts and journals worldwide. In the lists, plants are described using some of the selection factors mentioned earlier. Other specialized lists (for example, plants for seashores, plants for containers, plants for street trees) are often available from the local Cooperative Extension Service.