Naturalistic herbaceous vegetation for. urban landscapes

James Hitchmough

Introduction

Naturalistic herbaceous vegetation differs from conventional herbaceous vegetation in that it mimics the spatial and structural form of semi-natural vegetation. Individual species are generally not planted in clearly defined groups or blocks, and where they are aggregated, ‘outliers’ of the same species will generally occur elsewhere in the planting. Aggregations of individual species will be largest and most prevalent with clone-forming forbs or grasses of moist fertile soils. As these species spread, they often eliminate their neighbours. On less fertile, drier sites, species will generally repeat across the planting, often many times over, creating distinctive rhythmical patterns, especially when plants are in flower. There will sometimes be several distinct canopy layers; shade tolerant vernal species near the ground, above these the main canopy species, punctuated, often at wide spacings, by tall emergent species. These spatial arrangements allow a larger number of species to be located within an area of planting. This facilitates a lengthy display season, with many dramatic changes of character, with fewer negative visual effects.

In conventional herbaceous planting, the use of spring flowering species is problematic, as post flowering these often look untidy or may even disappear completely in summer, leaving unattractive gaps. In naturalistic herbaceous vegetation, the decline of early species is effectively masked by the growth of adjacent later flowering species. The tall emergent species that form the uppermost canopy of the vegetation are normally late summer or autumn flowering. An additional virtue of naturalistic herbaceous vegetation is its capacity to act as a unifying element with disparate surroundings. With conventional planting the eye reads individual groups or blocks of plants, and unless these are repeated many times the vegetation has a clear grain or direction, often looking more comfortable from one side than another. This is problematic when the surroundings are complex and cluttered, as is the case in many urban settings. By comparison, naturalistic herbaceous vegetation reads as a continuous sheet, from which different species emerge to flower as the sheet grows taller, but without obvious directional grain.

The spatial arrangement, and multi-layered character of naturalistic herbaceous vegetation, also confer functional benefits to people and other organisms. Conventional herbaceous planting is based on either ground cover-like blocks, or widely spaced clumps surrounded by unplanted space to accommodate summer growth, with minimal competition with neighbouring clumps. The groundcover model is highly functional, providing robust long-lived species are chosen, although if a taxon fails a large gap is

inevitable. In naturalistic plantings the system can accommodate the failure of species without obvious gaps, as all plants are surrounded by 3-4 neighbours of a different species which expand into the space vacated. The loss of one species is an opportunity for other desirable species, as much as it is for invading weeds. In the widely spaced clump model, plants are heavily fertilised to maximise their size and luxuriance, however the diameter of the gaps are such that closure is not achieved until midsummer. In naturalistic herbaceous plantings, plants are present at much higher densities than in either the ground cover or clump and gap model, typically at least 10 plants/m2 and in sown vegetation over 100 plants/m2. At these densities, closure of spaces between plants has generally been achieved by May. The intense competition for water and light significantly reduces the vigour of many previously established weed species, and inhibits weed invasion from outside the planting. Weed-control requirements, the main impediment to the use of conventional herbaceous vegetation in urban landscapes, are reduced. As the individual plant is not the focus of attention in naturalistic planting, the traditional costs associated with ‘titivating’ are also avoided, as are division and replacement costs; the goal being for regeneration to occur from within the planting. Where management is required, it is generally compressed into critical phases of the lifecycle, often spring, and is undertaken as much as possible using crude unselective management techniques borrowed from nature conservation, for example burning or cutting.

Through typically being taxonomically and structurally diverse, naturalistic herbaceous vegetation is potentially of high habitat value. The degree of openness of naturalistic herbaceous vegetation will depend upon soil moisture and fertility, and also the species chosen. The drier and less fertile the site, the more open vegetation will become, and vice versa. The vegetation selected must change with the site, there is no standard version to be achieved. This illustrates one of the central tenets of naturalistic planting design, plants must be chosen to fit the site, and to be compatible with the other plants chosen if communities are to be created that are robust and sustainable. This requires the application of an ecological understanding to plant selection and management. Naturalistic herbaceous vegetation does not need to be confined to species native to a particular geographical region, although it is of course understandable that practitioners should make this connection. Providing attention is given to assessing the environmental and social context of the site, and the ecological needs and attributes of the species in question, naturalistic plantings may consist entirely of non-native species, native and non-native species mixed together, or native species alone.

Whilst naturalistic herbaceous vegetation aims to be more resource sustainable than conventional plantings, its key role is to create meaning and pleasure for human beings, and in urban contexts non-native species have a very important role to play in this process.

Although naturalistic herbaceous vegetation is not a new idea (see Chapter 2), in terms of scientific research it is still relatively poorly understood. Its creation and management fits less comfortably within the client-landscape architect-landscape contractor model than does traditional herbaceous planting. In addition, it represents something of a paradigm shift in visual terms; it often has no clear directional grain or apparent order, ‘focal points’ are absent and the individual plants are neither distinct nor cherished. This is a very different aesthetic for the public and landscape professionals to embrace and understand. To traditionalists in the horticultural world it represents poor cultivation practice, to the nature conservation movement it represents an unholy mix of species that should not be pretending to be naturalistic. Anna Jorgensen addresses some of these perceptions in Chapter 11. Some landscape professionals inspired by naturalistic form have rather naively perceived this style to be a ‘magic bullet’ that somehow arrests the normal ecological processes of colonisation and change in designed plant communities. This has led to disillusionment in the past, as it becomes apparent that this is not so, The author makes no such claims; ecologicallyinformed naturalistic herbaceous vegetation is less intensive to manage but is not management free. It is imperative that practitioners recognise that, whether native or exotic, these types of vegetation rely on informed management for the persistence of desired species. Where management is not informed, the vegetation will simply change into a less desired plant community.