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Urban Horticulture - Purpose And Prospects

First Meeting of West and East German Horticultural Societies, March 1992 © Karen Foley 2019

Urban Horticulture is a relatively new term for a large and complex area of scientific horticulture. In broad terms, urban horticulture is the study of the interactions between plants and municipal surroundings and is concerned with the functional use of plants to maintain and improve the urban environment (Tukey 1983). It is obviously related to, but quite distinct from, production horticulture which is concerned with the growing of fruits, vegetables and other plants for their harvestable crops.

In many ways urban horticulture is just as broad and diverse a subject as commercial horticulture. Plants are often used grown in cities for their intangible benefits - aesthetic, architectural, recreational and psychological - but their physical functions are also valuable; plants create and define outdoor spaces, provide shelter from wind, rain and noise, filter dust, reduce sun glare from glass and metal, reduce unpleasant temperature variations, improve air quality and conserve energy. But with few exceptions, the value of plants for most of these purposes has not been quantified to any extent and little research work has been done on their functional use.

In contrast, much information has been assembled by Government-sponsored and private research on production horticulture during this century. High yields of good quality crops are produced as a result of research done on growing substrates, plant nutrition, irrigation, pest/disease/weed control, plant physiology, mycorrhiza and many other factors. With few exceptions, however, scientists working on the problems of production horticulture have not been aware that their findings are as relevant to plants grown for their functional or aesthetic use in towns and cities as for crops grown for food, fuel or fibre. Consequently, landscape designers, contractors and other personnel directly concerned with the functional use of plants in urban areas have often had to rely on extrapolating, sifting and adapting results from production and related areas.

Emphasis on production research during most of this century is due in large measure to the strength and cohesion of the commercial lobby, the awareness of commercial growers of the contribution that research can make to efficient production and the fact that the benefits of research for commercial horticulture can be relatively easily quantified. In contrast, the amenity side of horticulture is diverse and scattered and with few exceptions, does not appear to have pressed for research support in any concerted way.

The diverse nature of the urban horticultural sector can be seen clearly from the different groups listed by Tukey (1983) as being the constituent audience for urban horticulture. These include landscape maintenance and parks personnel, landscape architects, arboriculturists, highway planters, nursery contractors, members of plant societies and amateur horticulturists.

During the 1970s the International Society for Horticultural Science recognized the need for research specifically on the functional use of plants to support and improve the urban environment. It also recognized the need for better communications between scientists involved in production research and people who utilise plants particularly in urban situations. Accordingly in 1982, at its Hamburg Congress the ISHS established the Commission for Urban Horticulture, indicating that the field of urban horticulture has achieved international validity.

The Commission's goals include the selection of plants adapted for growth in the urban area, better management techniques to ensure their survival and effective fulfilment of their functional purpose, and expanding the creative uses of vegetation in cities to provide a range of amenities (Bassuk 1985).

Since the establishment of the Commission, a data base of people active in the field of urban horticulture has been assembled (Bassuk 1985) and symposia held in the USA , England and Ireland . A symposium on the Breeding and Selection of Woody Ornamental Plants will be held in Angers , France in July 1992. These symposia provide a useful forum through which scientists with widely different backgrounds can meet to discuss mutual problems and benefit from a cross fertilisation of ideas. These symposia have revealed many shortfalls in our present knowledge, both with regard to the value of urban plants and optimum management systems

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Value of Urban Horticulture

There is widespread recognition of the value of plants in the urban environment. Oldham (1991) maintains that a good landscape - the spaces between the buildings - is the most important single factor integrating all aspects of a city and that it is this matrix of spaces that links everything and provides the setting and the essential character of city life. Landscape, of course, is a product of a complex interaction of many phenomena including topography and microclimate as well as vegetation, parks and gardens. Although there is widespread acceptance that the use of plants in cities is not merely cosmetic but is of fundamental importance, the value of such plantings to human well being is not always easy to quantify. The physical effects of plants, such as their ability reduce noise or to purify air, are quantified to some extent, but information is lacking in particular on their psychological effects.

The vibrations of sound waves are absorbed to some extent by leaves, branches, twigs of shrubs and trees (Robinette 1972). Many plants are also effective in scattering and diffusing sound and also in masking offensive noise by more agreeable sounds, such as the rustle of leaves.

The most effective plants for absorbing unwanted noise are evergreens with thick fleshy leaves with thin petioles allowing for a high degree of flexibility and vibration. However the effect of trees and shrubs as acoustic screens is unpredictable for several reasons, such as variability in the density of plant foliage which may be negligible near to ground level. This problem can be overcome to some extent by the use of hedges.

Knudson and Harris (1950) state that a cypress hedge 0.6 metres thick deadens sound by 4 decibels, but much more information is needed on the effect of different types of plants for acoustical control in the urban environment.

The ability of plants to cleanse air through their photosynthetic activity is well known. The role of plants in collecting air borne particles on hairy leaves and stems is also well understood. The particles are subsequently washed from the plants unto the ground by rain. A city street planted with trees may have only 25 % of the atmospheric dust particles (1,000 - 3,000 dust particles/litre, compared with a similar street without trees (Bernatzky 1966).

In his presidential address to the American Society for Horticultural Science, Larson (1989) mentions some attempts to quantify the psychological benefits of landscaping. One of these studies by Ulrich (1984) showed that patients in a hospital with rooms looking over trees and shrubs recovered faster, needed fewer drugs and had fewer post-operative complications than patients whose rooms looked out on a brick wall.

In another study, Langer and Rodin (1976) studied the therapeutic effects of horticulture. In a geriatric hospital, 93 % of elderly hospital patients with pot plants to care for showed improvement while, in a comparison group whose plants were looked after by nursing staff, 71% showed increased debilitation.

Apart from mental and spiritual benefits, landscaping can also bring a financial gain. In a case study in the hotel industry, Evans (1991) reports on a hotel in the USA which spent $1 million on plants and $1.2 million on annual maintenance but increased income by $7 million. This resulted from full occupancy and higher charges for rooms adjacent the garden compared with otherwise identical rooms without a view of the landscaped area.

According to Larsen (1989), the installation of a swimming pool will give a 20 - 50% return on investment, a new patio 40 - 70%, an overhauled kitchen 75 - 125% but landscaping has a recovery return of 100 - 200%.

There are also many gaps in our knowledge on optimum, management systems for plants in cities. We need to know much more about the environment in which our plants are growing, as the urban habitat is very different from that experienced by most crop plants. Most urban plants are growing in close proximity to hard landscapes which can give rise to serious environmental problems such as wind funnelling, shading, restricted root run and excessive surface temperatures (Thoday 198i)

In some situations plants may benefit from the proximity of buildings e.g. winter temperatures may be higher in an enclosed courtyard but, in the majority of cases, plants suffer many environmental stresses as a result of being in an urban situation. Bassuk and Whitlow (1987) suggest that one half of the 1,000 street trees planted in New York during the spring will be dead within 10 years. Such trees often suffer from water shortage because of the way they are planted in small openings in the concrete. Not only is root growth constricted by underground services and by soil compaction, but plants are damaged in a variety of ways (including chemical drought, leaf scorch, adverse soil structure) from the salts (mainly sodium chloride) used to de-ice roads during frosty conditions.

Above ground, street trees can suffer from wind turbulence, atmospheric pollution, vandalism, mechanical injury from passing vehicles and from excessively high temperatures as a result of reradiated heat from buildings, car tops, asphalt and concrete.

Relatively little research has been conducted on these aspects of trees in the urban environment.

In production research much effort has been directed towards finding the optimum spacing and planting arrangements for different species and even for varieties. Little information is available for plants in the urban environment. Bassuk and Whitlow (1987) suggest that street trees planted in groups would provide mutual shade and shelter and would allow the use of a wider range of species that do not have the traditional lollipop shape e.g. Ailanthus altissima, Elaeagnus angustifolia, Morus alba and Maclura pomifera.

Improved methods of planting trees in city streets for better top and root growth also need examination. Soil conditions in urban situations are often disastrous for plant growth as a result of building activities. Soil depth, volume, condition, nutritional status and drainage often need to be ameliorated before urban plants can be satisfactorily established (Thoday 1981). Other factors that can improve plant growth include continuous horizontal soil columns between pits to permit greater lateral root growth and the use of a raised lip around planting beds to act as a barrier to salt water run off (Bassuk and Whitlow 1987).

Although the physical environment of urban plants is generally less favourable than that for commercial crops, pest and disease problems are usually less severe. The wider range of species normally grown in the urban environment helps to confer greater stability on the ecosystem compared with the monocultures used for food production.

Nevertheless where pests and diseases do occur, they can be very troublesome, often causing problems for people as well as damaging plants, e.g. honey dew secreted from sycamore aphides.

Greater emphasis on environmental protection is leading to a demand for a higher level of in-built resistance. The most common approach now is the use of integrated pest management systems with most emphasis on biological factors rather than on chemicals. In some countries it is now impossible to spray street trees in urban areas against pests and diseases leading to a demand for trees that are able to survive without the use of pesticides. Colour infra-red (CIR) aerial photography, which is being used increasingly to monitor the health and pest status of commercial forests, could find a similar role in urban plantings. Healthy vegetation uses incoming red and blue light for photosynthesis and reflects most of the incoming green light. Hence, vegetation appears green to the naked eye. However, as healthy vegetation reflects much more infra-red light than green, a healthy tree will appear bright red when photographed with colour infra-red film, sensitive to both visible and infra-red light. Diseased foliage reflects less light and can easily be spotted by a skilled interpreter in an office. This will prove a valuable help in complimenting health surveys made on the ground.

Apart from disease and insect damage surveys, aerial photography will have other applications in urban horticulture, such as inventories and vegetation mapping.

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Root Growth

While much more information is needed on the above ground portion of urban plants, our knowledge of plant root systems is even more limited. Such information is more important for the horticulturist working in cities because of the many factors, already mentioned, that militate against natural root development of urban plants.

In situations where root areas are unrestricted or where fertilisers can be used liberally, root system characteristics are of lesser importance (Atkinson 1990). For this reason, present interest in the avoidance of environmental pollution by minimising the use of fertilisers will increase the significance of the root system.

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Because they are usually widely spaced, the growth habit of trees in cities is different from that in their natural habitats and their pruning requirements need to be handled carefully.

Research has disproved a number of widespread fallacies regarding tree pruning. Information in older textbooks on methods of removing tree branches is generally misleading .The recommendation in the past to cut branches flush with the trunk has been shown to be wrong. This type of cut removes the branch protection zone which forms at the base of the branch and thus exposes the trunk wood to infection (Shigo 1989a). Research has shown that cuts should be made just outside the branch collar so that the collar is not damaged by the wound. When cuts are made correctly the tree will compartmentalise the wound and wound dressings are unnecessary. On the other hand, there are no data to show that wound dressings stop rot if pruning cuts are made incorrectly.

Much more information is needed on optimum time of pruning. The wide range of species grown in urban areas, the multiplicity of reasons for pruning and the wide range of climatic conditions experienced makes it inevitable that optimum times will vary for particular species under different climatic conditions.

Pruning in the middle of the dormant period is a suitable time in some situations (Shigo 1989b) but not for species and in areas where cold hardiness is an important factor (Haynes and Lindstrom (1991),

Pruning methods will need to take account of the health of the tree. There needs to be greater recognition that pruning affects not just the plant but associated insects and fungi also. Pruning cuts make wounds that may allow the entry of pathogens whereas, in general, healthy branches should not be removed from stressed trees with low energy reserves.

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Reducing maintenance costs

The present high cost of maintenance is a key factor in most sectors of urban horticulture. Weed control in plantings of exotics is a particularly expensive aspect of maintenance and land managers are constantly seeking effective, environmentally friendly methods. Since soil cultivation is laborious and expensive, more interest has developed in alternative methods of reducing maintenance costs such as the adoption of an ecological approach or the use of mulches or weed smothering plants.

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Ecological approach

Ecologists advocate the need to work with nature by adopt techniques which coax and direct natural succession as a form of land management in urban areas (Baines 1987). One such approach in Britain is the use of urban commons, i.e. the incorporation of non-intervention and minimal intervention areas into urban landscapes (Gilbert 1984). Although large areas of urban common can appear dismal in winter, they are robust, cater for a range of informal land uses and have aesthetic appeal as green jungles. As the vegetation is self sown it is well adapted to local conditions. The range of attractive plant species in these commons can be increased by judicious sowing or planting. However, without skilled management a floral meadow tends to give way in time to grassland, open scrub and eventually woodland. Urban commons management and the enrichment of floral meadows with wild flower seed or plants is an area that could benefit much from research support.

Although mulching is an age-old practice, greater understanding of the advantages and disadvantages of the use of mulches in particular situations needs to be determined. Mulches can be very beneficial as means of conserving moisture, suppressing weeds and reducing temperature fluctuations (Robinson 1988). However, many examples can be found where the effect of a mulch has been deleterious rather than beneficial (Borland 1988). Mulches are used in so many different situations and can affect so many different interacting soil factors that it is often difficult to identify the major factors accounting for the good or bad results.

With a reduction in the availability of herbicides for amenity use and with the greater availability of chipped organic material of potential value for mulching, the use of organic mulches is likely to increase in popularity. While the horticultural literature contains results of many trials with mulches especially on annual crops, less is known about the effect of mulches on perennial plants. Little is known, for example, about the long-term effect on trees and shrubs of changes in soil temperature as a result of mulching. Much more research work is needed to put this century old practice on a more scientific basis so that mulches can be applied with greater certainty of successful results.

Herbicides have great potential to make maintenance easier. Many herbicides are unacceptable in urban situations for a variety of reasons, including high mammalian toxicity, long persistence in soil and risk of contaminating ground water. On the other hand there are some herbicides, such as glyphosate, which break down to harmless compounds in the soil and have minimal impact on the environment.

As one of the aims of urban horticulture is more cost effective landscapes, the blanket ban on all herbicides that operates in some parts of Europe seems to be unnecessarily draconian. More information is needed on the long-term effects of herbicides on the environment and also on the incorporation of herbicides into effective, environmentally friendly integrated management systems, incorporating physical, biological and chemical control as appropriate. The judicious use of herbicides can augment the value of an organic mulch.

For example, many species of perennial weed emerging through a mulch can be controlled more effectively with a careful spot-treatment of glyphosate than by digging or hoeing. Physical destruction of the weeds would bring up soil and weed seeds to the surface of the mulch and shorten its effective life.

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Plant selection

Urban design depends heavily on plants; the hard landscape may be the skeleton but the plants are the clothes. Nevertheless, little attention has been given to the selection of plants more suitable for specific situation within the urban environment. Where plants are surrounded by tall buildings, light is often a limiting factor. On the other hand, light spilling from surrounding buildings can increase the photoperiod so that some plants fail to absciss their leaves cleanly in the autumn and have an untidy appearance (Thoday 1981). More could be done too to determine plants that have a weak phototrophic reaction, such as Pyracantha, which can grow close to wall and therefore make good wall plants.

There are many suggestions in the literature that that the criteria for selecting suitable street tree species, with present emphasis on form and seasonal interest, should be broadened to include other desirable properties. Doxon (1991) suggests that selection of plants should also include ecological considerations, such as their ability to fix carbon dioxide, so that more informed ecological decisions could be made. Bassuk and Whitlow (1987) emphasise the importance of tolerance to stress, including drought, salt, heat and other factors. There are possibilities too for collecting seed material from stressful habitats, such as flood plains or sea shores. Biotechnology, in combination with conventional breeding techniques could be useful in selecting more stress tolerant plants for specific situations.

Routine pruning is more stressful to some trees than others. The ability of a tree to resist attack by fungi varies greatly between species and also within species. This has been shown with poplars (Shigo and Marx 1977) and red maples (Shigo 1989b). It may be possible to select strains of trees that compartmentalise tree wounds more rapidly than others of the same species. This could be more easily done in poplars, which can be propagated vegetatively and where clonal material is available than in many other species. However, Lonsdale (1987) suggests that it might be possible, even in trees of unknown genetic constitution, to recognize anatomical or biochemical resistance factors which could be used to aid selection procedures for the development of strains with improved decay resistance. Obviously other desirable attributes such as vigour and form would have to be maintained.

The rapidly advancing science of genetic engineering will inevitably contribute to the development of plants more suitable for the urban environment. Where a genetic engineer puts a new gene into a plant, he has no control over its location on the chromosome. Consequently a range of plants differing widely in morphological characteristics have been obtained in series of experiments. The differences are due entirely to the location of the introduced gene to other genes on the chromosome. By investigating the genes on either side of the inserted gene, much information on regulatory genes and the location of these genes is being accumulated that could have wide application. Once genes are located they can be cloned and used for further transformations. It may be possible in time, for example, to isolate a gene for the fastigiate habit which could provide a relatively simple way to develop a range of new plants of especial value for roadside planting (Douglas 1992).

A major issue in the future will be the sourcing of funds for genetic improvement of urban plants. In a number of countries, public agencies have ceased breeding activities assuming that more work will be done by the private sector. This seems unlikely in the case of woody plants for urban areas in view of the long-time scale involved and unlikelihood of adequate financial return. International co-operation on breeding programmes could help to overcome this problem and should be discussed at the forthcoming symposium on the Selection and Breeding of woody ornamentals at Angers, France in July 1992.

As information is obtained on the suitability of old and new plants for specific situations, this information needs to be transmitted effectively to the final arbiters on the species planted. Many landscape architects and planners have suffered in the past from a lack of understanding of the scientific basis of plant growth. They tend to play safe and to use a limited number of plants that they are familiar with. This has been likened to painting with numbers and in no way exploits the richness of the plant kingdom. This lack of understanding of the basis of plant growth on the part of many urban horticulturists emphases the need for a greater involvement of plant sciences in the training of landscape horticulturists and architects.

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From several viewpoints the future role of urban horticulture is one of increasing importance. With the steady migration of people from rural to urban areas in almost every country, the majority of the world's population will be urban dwellers by the end of the century. Cities are changing, too, becoming larger and using larger quantities of modern building materials giving a smooth, polished, highly reflective finish. This will result in more glare of reflected natural and artificial light from buildings and automobiles. Glare can cause much visual discomfort but can often be softened or blunted by appropriate planting.

There is a new awareness in many countries that urbanisation is associated with many environmental and social problems that will need to be addressed on a broad front. In many countries, the public is showing increased interest in the environment and is developing a greater awareness of the physical and psychological ways that plants can improve urban conditions. It seems inevitable that pressures will increase on municipal authorities for more urban horticulture.

Already in many European countries more people are now engaged in amenity or urban horticulture than are in employed in crop production. While the European Community if faced with the problem of food surpluses and while the total demand for food will not increase significantly, the growing emphasis on leisure activities and the environment suggests that the amenity and functional aspects of plants will continue to increase in importance.

The present trend for home owners to take greater interest in and to spend more on their own gardens will continue. From the time that man first began to cultivate plants some 5000 years ago, the role of the garden has gradually changed. The first gardens had strong religious overtones but this has gradually given way to a more artistic, poetical approach. Gardens will continue to evolve according to the current needs and new possibilities. With increasing leisure time available to large numbers of people, and with more discretional income, gardens today are being designed more for recreation and pleasure. This trend is likely to continue strongly with the garden becoming more elaborate as an outdoor living room. In this the beauty of plants will be enhanced by modern, outside lighting, more sophisticated hard landscaping and other technologies.

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