Horticulturalists provide food to feed the world, beautify neighbourhoods, decorate gardens, and give ambience and wellbeing by combining the energy of the sun with soil, seeds, water, and ingenuity.
Horticulture has many components which include vegetables, flowers, fruits, spices and essential oils, turf grasses, edible fungi, nutraceutical plants, landscape and green spaces.
Plants are the foundation of the outdoor environment. The diversity of plant species and the multitude of cultivars, hybrids, and varieties make for creative and appealing landscape compositions.
There are a number of reasons for choosing plants for the landscape. Attraction to their ornamental appeal or the need to serve a specific function or purpose in the landscape, such as providing a screen, blocking unwanted views, or stabilizing a soil bank, are common reasons.
Others may be selected because of their ability to adapt to poor soils or simply for the ease of subsequent care.
Generally, when horticulturists are asked about plants, responding is not as simple as blurting out a few names. Plant selection is an organized process that examines several factors: function, aesthetics, site adaptability, and management.
The priority placed on each category varies with the individual. The freedom to choose from a wide variety of plants depends on the flexibility or restrictions imposed by the individual, the site, or in some cases the local availability of plants.
Viewing plant selection as a process may at first seem cumbersome, but in time the process can make it easier to make decisions and provide more choices than first thought.
Function or purpose defines the reason for using a plant. Looking good certainly justifies a selection, but the value of a plant may go far beyond aesthetic appeal.
Function guides the selection of a plant type, such as tree, shrub, or perennial for a specific space. Plants are packaged in many ways their aesthetic qualities are as diverse as the species available.
Plant aesthetic qualities include the overall habit or shape of the plant and its foliage, flowers, fruit, and bark. The combinations of plant forms, foliage, flowers, fruit, and bark can result in creative, artistic displays.
Once the potential aesthetic qualities have been identified, the next question is to determine whether or not the plant will truly perform in the soil and environmental conditions of the property.
A helpful adage is “Don’t fight the site.” Testing or challenging Mother Nature by picking a plant that does not match the site conditions creates a good chance of failure.
Factors related to site adaptability such as the plant’s cold hardiness and tolerance for site conditions such as soil type, exposure, and light levels will define whether aesthetic selections will perform to expectations.
The final consideration in plant selection is management. Landscape management or maintenance guides the development of plants in the landscape.
After initial establishment, accent plants start to show off their qualities, plant masses begin to integrate, and border plantings achieve their intended shape.
It is the feasibility and quality of maintenance that ensures the long-term aesthetic appeal of any plant and certainly highlights its contribution to the overall appeal of the landscape.
Horticultural practices such as pruning, fertilization, irrigation, and pest management need to be considered in making final plant choices.
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Practical Guide to Making Appropriate Plant Choices in Agriculture and Landscape Planning
The following is an approach to selecting plants and identifies specific information that will be useful in making appropriate choices.
1. Function in Plant Selection for Landscape Purposes
“Function” refers to the purpose that the plant serves in the landscape. The shade of a tree canopy, the filtered screen from a hedge, or the erosion control of a ground cover addresses the specific objectives of the planting.
Plants serve three major functions in the landscape:
Architectural function, Engineering function, Environmental function
Individually or in concert, plants are the foundation of the landscape and reinforce the intended use of outdoor space, whether the use is active or passive.
The development of the landscape can be likened to the development of rooms in homes – each room is shaped for its intended use and is accessorized accordingly.
i. Architectural Functions of Plants in Horticulture
Plants serve an architectural function by defining the floors, walls, and ceilings of outdoor rooms. Floors direct movement into and around the rooms.
They are defined by colours and textures of turf, ground covers, creeping perennials, and other interesting materials. Several ground cover plants have been promoted for their ability to withstand foot traffic.
These, in combination with more traditional turf, have certainly changed the definition of living carpets. Walls establish boundaries and set the mood in the outdoor space.
They can allow or limit visual and physical access. The characteristics of a wall are defined by height, depth, and density. In combination, these dimensions can provide an open, filtered, or enclosed feeling.
Screens of evergreen trees, clusters of multi-stemmed trees, masses of intermediate shrubs, and low masses of herbaceous perennials all contribute to the characteristics of a wall and its influence on the mood or feeling the room projects.
Ceilings are usually formed by canopy trees and are characterized by their height and density. The branching height of a tree can contribute to the openness or intimacy of a room. Density influences light, whether it is blocked, filtered, dappled, or bright.
The location of the canopy can also contribute to changes in the atmosphere of the room through the day or season. Deciduous trees offer a wide array of seasonal characteristics that contribute to the artistic appeal of ceilings.
Plants can also serve an architectural function by highlighting or masking architectural features of a house or building. Framing with plants can emphasize features or downplay an unwanted view.
Plants also serve a very important role in shaping the rooms in the landscape. Their form provides the structural framework, and their foliage, flowers, and branches provide the wallpaper and decorative appeal.
ii. Engineering Functions of Plants in Site and Landscape Management
Plants serve an engineering function by influencing how people walk through the landscape; blocking objectionable views on or off the property; establishing buffers between divergent activities; and minimizing drainage or erosion issues.
Bordering a sidewalk with small shrubs may help direct people along the walk. Screens between patios and utility areas separate leisure from work space. Ground cover shrubs can hold soil on a slope or prevent excessive erosion during seasonal rains.
The engineering role of plants tends to be more utilitarian in addressing site issues or irregularities but nonetheless contributes significantly to the overall success of the plantings.
iii. Environmental Functions of Plants in Modifying Microclimates
When an environmental role for plants is considered, attention focuses on their influence on microclimates within the landscape. “Microclimate” refers to temperature, wind, and light in a relatively small area.
Plants can modify microclimates in the landscape and thus contribute to human comfort. The positive contribution of plants to energy conservation has been well documented.
Plants can reduce heat loss in the winter and minimize heat gain in the summer. Windbreaks are designed to intercept and deflect prevailing winds and reduce wind speed in the protected area. Plants along a foundation can form an insulating layer between the building and the outside wind.
Plants provide benefits in the summer by intercepting direct and reflected rays of the sun. Canopy trees intercept the direct rays, providing a shade pattern, while shrubs can intercept reflected rays from pavement and the sides of buildings.
2. Aesthetics in Plant Selection and Landscape Appeal
Aesthetics or curb appeal tends to be the most notable quality of a landscape. The success or failure of a landscape is often judged on the visual quality of the plants.
Aesthetics starts with the plant type and the size of the designated planting area. All plants will take up space, some more so than others. To ensure the integrity of the planting, it is important to plan accordingly and identify the amount of space the plants will cover.
The size of the available space influences the type of plant selected. Each plant type sparks an image, some large, some small.
Shade tree” implies large canopy; “evergreen tree” usually suggests dense foliage and a conical shape; an ornamental or flowering tree could be a small, delicate threadleaf Japanese maple or a wide-spreading crabapple.
Shrubs offer a diverse array of sizes from dwarf barberries to massive viburnums. Ornamental grasses and perennials offer seasonal changes in colour and texture as well as in size. “Ground cover” implies a carpet-like growth habit.
Looking first at plant type allows the plant to match with the space and leads to consideration of shape. Shape adds another dimension to the ornamental quality of a plant composition.
It also aids in characterizing the relationship between or among plants in the composition. Selecting for shape will provide plants for overhead, underneath as well as side by side.
Many of the landscape compositions have several layers of plants, each contributing to the aesthetic appeal of its own layer and that above or below.
Flowers, foliage, fruit, and stems or branches or bark add to the aesthetic appeal of the landscape by providing colour and texture. The impact, duration, and seasonality of these characteristics vary.
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Insect and Disease Resistance
1. Importance of Insect and Disease Resistance in Plant Selection
Insect and disease resistance is a critical consideration when selecting woody ornamentals for landscaping. Choosing species that naturally resist common pests and diseases ensures healthier plants, reduces maintenance costs, and minimizes pesticide use.
For example, European birch (Betula pendula), while attractive, is highly susceptible to the bronze birch borer. In contrast, river birch (Betula nigra), especially the Heritage™ cultivar, provides similar ornamental value with superior resistance to this pest, making it a preferred choice in many landscapes.
Plants with No Significant Disease or Insect Problems
a. Small Shade Trees
1. Acer buergerianum – Trident maple
2. Acer campestre – Hedge maple
3. Acer ginnala – Amur maple
4. Acer griseum – Paperbark maple
5. Acer japonicum – Fullmoon maple
6. Acer palmatum – Japanese maple
7. Acer tartaricum – Tartarian maple
8. Acer triflorum – Three-flower maple
9. Acer truncatum – Shantung maple
10. Betula nigra ‘Fox Valley™’ – River birch
11. Carpinus caroliniana – American hornbeam
12. Cotinus obovatus – American smoke tree
13. Ostrya virginiana – Hophornbeam
14. Parrotia persica – Persian parrotia
2. Medium Shade Trees
1. Acer x freemanii – Freeman hybrid maple cultivars
2. Acer rubrum – Red maple cultivars
3. Aesculus carnea – Red horsechestnut
4. Carpinus betulus – European hornbeam
5. Cercidiphyllum japonicum f. pendula – Weeping katsura
6. Corylus colurna – Turkish filbert
7. Sassafras albidum – Sassafras
8. Sorbus alnifolia – Korean mountain ash
9. Ulmus parvifolia ‘Athena’ – Lacebark elm
3. Large Shade Trees
1. Acer saccharum – Sugar maple cultivars
2. Betula nigra ‘Heritage’ – River birch
3. Betula nigra ‘Dura-heat’ – River birch
4. Cercidiphyllum japonicum – Katsura
5. Ginkgo biloba – Ginkgo
6. Liquidambar styraciflua – Sweetgum
7. Liriodendron tulipifera – Tulip tree
8. Nyssa sylvatica – Black gum
9. Platanus x acerifolia – London planetree
10. Quercus bicolor – Swamp white oak
11. Quercus palustris – Pin oak
12. Quercus phellos – Willow oak
13. Quercus robur – English oak
14. Ulmus americana ‘New Harmony’ – American elm
15. Ulmus parvifolia – Lacebark elm cultivars
16. Zelkova serrata – Zelkova
Small Flowering Trees
1. Amelanchier arborea – Juneberry
2. Amelanchier laevis – Allegheny serviceberry
3. Amelanchier x grandiflora – Apple serviceberry
4. Cercis canadensis – Redbud
5. Cornus alternifolia – Pagoda dogwood
6. Cornus florida (anthracnose resistant) – Flowering dogwood
7. Cornus kousa – Kousa dogwood
8. Cornus mas – Cornelian cherry dogwood
9. Magnolia x loebneri ‘Leonard Messel’
10. Magnolia stellata – Star magnolia
11. Malus cultivars – Disease-resistant crabapple
12. Styrax japonica – Japanese snowbell
13. Syringa reticulata – Tree lilac
14. Viburnum sieboldii – Siebold viburnum
Medium Flowering Trees
1. Cladrastis kentuckea – Yellowwood
2. Evodia daniellii – Korean evodia
3. Halesia tetraptera – Silverbells
4. Koelreuteria paniculata – Goldenrain tree
5. Magnolia virginiana – Sweetbay magnolia
6. Oxydendrum arboreum – Sourwood, sorreltree
7. Stewartia monodelpha – Tall stewartia
8. Stewartia pseudocamellia – Japanese stewartia
Large Flowering Trees
1. Magnolia acuminata – Cucumber magnolia
2. Prunus sargentii – Sargent cherry
3. Styphnolobium japonicum (formerly Sophora japonica) – Japanese pagoda tree
Plants Suitable for Special Conditions
Trees/Shrubs Tolerant of Moist/Wet Soils
1. Acer rubrum – Red maple
2. Amelanchier spp. – Shadbush, Juneberry
3. Betula nigra – River birch
4. Calycanthus floridus – Carolina allspice
Plants That Can Tolerate Dry Soils and Short Periods of Drought
1. Acer griseum – Paperbark maple
2. Carpinus caroliniana – American hornbeam
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