The successful combination of different plant species is one of the main functions of planting design and landscape management. In traditional, horticultural-based planting design, aesthetic and functional considerations predominate: how do the different component species work together visually and how do they perform the tasks (such as dividing or filling spaces) for which they have been designed? Biological questions relating to how plants interact with each other and their surrounding environment as a community or unit of vegetation receive little or no consideration. This is mainly because the planting environment is generally modified to suit the requirements of standard landscape plants, whether this be through modification and importation of soils, fertilisation or irrigation, or through pruning and other maintenance operations, all of which entail an energy labour and financial cost.
An ecological approach to landscape vegetation can be radically different. Aesthetic and functional considerations can be equally applicable, but questions of ecological compatibility and long-term dynamics are also a central concern. Rather than specifically arranging plants in their final desired positions, and subsequently ensuring that that is where they remain, ecologically-informed planting can be more akin to starting and managing a successional process. However, compared to the vast bulk of ecological literature on the functioning of semi-natural plant communities in the wild, there has been surprisingly little application of ecological ideas in terms of the way plant communities function in landscape or ornamental planting: indeed, the vast majority of mainstream ecologists would probably not recognise this as a valid subject of study. Because, as discussed in Chapter 2, many so-called ecological approaches to landscape planting tend to emphasise the visual connection with naturalistic vegetation rather than the underlying processes going on in that vegetation, there is a real need to develop ecological models that address questions relevant to the way that vegetation may develop as part of human designed landscapes. At the most immediate level, these questions relate to factors that enable plants to co-exist under the wide range of potential environmental and site conditions, and to the characteristics of plants that enable them to be compatible with other plants growing in their immediate vicinity. In other words, factors that promote greater diversity and species richness in vegetation.