Planting design, or composition with plants, has two basic components: plant selection and plant placement. A discussion of these two activities, as they are most often practiced in traditional landscape design, together with a summary analysis of the visual effects that result from them, is given below.
The primary criteria affecting the selection of plant species in traditional landscape practice are the following.
– Aesthetic characteristics, such as the form, texture, and seasonal colour characteristics,
for example the colour of flowers, fruits or bark.
– Functional capabilities, such as the plant’s usefulness at providing shade, windbreaking,
visual screening or framing, and erosion and sedimentation control.
– Environmental tolerance, including hardiness, sun, shade and wind tolerance, and,
increasingly, the ability to withstand the effects of soil, water and air pollution. Water requirements have typically been given only secondary consideration, until recently, because of the relative ease and low cost of providing supplemental water to plants needing more water than might naturally occur as precipitation in a particular region.
– Commercial availability is a prerequisite for any particular plant species being
incorporated into a designed landscape. Because of its efficiency, the common practice has been for large nurseries to mass-produce hundreds of plants of a limited number of species that are well known (or well-marketed) and ‘reliable’, often because of wide environmental amplitude (‘it’ll grow anywhere’). These abundantly produced plants include a mix of native and exotic species, as well as hybrids and cultivars. It is unusual for growers to differentiate between these plants on the basis of origin, or to provide such information to buyers in catalogues or in nursery sales areas.
As a result of these influences on plant selection, there is little tendency in traditional landscape design to select a native species over a non-native, or exotic species, so long as it meets the criteria established for size, form, colour and function.
Following traditional plant selection practices, ornamental trees and shrubs that have conspicuous flowers, fruits and/or autumn colour, or that are evergreen, are the most likely selections. Furthermore, the same showy species are planted across broad geographic areas. In terms of general landscape treatment in the US, the most ubiquitous species are the ground-cover species. For example, the most often selected ground-covers in the US are a limited number of evergreens; for example English ivy (Hedera helix and its many varieties), periwinkle (Vinca minor) and Japanese spurge (Pachysandra terminalis), all of which were introduced to North America from Europe or Asia over a century ago and which are now being found to be invasive. The most widespread ground – cover plants in the traditionally designed landscape are the lawn grasses, estimated to occupy over 30 million acres in the US. With a few exceptions, these are exotic and/or hybridised species, typically requiring supplementary water (and fertiliser and pesticides) to provide the uniform dark green carpet that has been promoted as the ‘ideal’ cared-for landscape in this country.
Since neither the evergreen ground-covers nor the turf grasses typically provide conspicuous flowers, the element of ‘seasonal colour’ is characteristically supplied through the introduction of spring-flowering bulbs and summer annuals. In commercial and corporate landscapes particularly, these ‘seasonal colour’ plantings are typically dug up and replaced several times a year.