1 Introduction

Horticulture is a science and technology that focus on the role of some specific economic plant in crop production (Tigchelaar and Foley, 1991). The term horticulture is often defined as the science and art of growing fruits, vegetables, flowers or ornamental plants. It is however important that we look at the other side of horticulture that is the art of horticulture, and the role it plays in human well-being. Considering the traditional definition of horticulture, the above alternatives and the current research into people-plant interaction, we might then consider it as a comprehensive definition. Horticulture as- the art and science of growing flowers, fruits, vegetables, trees and shrubs resulting in the development of the minds and emotions of individuals, the enrichment and health of communities and the integration of the `garden' in the breadth of modern civilization. By this definition, therefore, horticulture encompasses PLANTS, including the multitude of products (food, medicine, O<sub>2</sub>) essential for human survival; and PEOPLE, whose active and passive involvement with `the garden' brings about benefits to them as individuals and to the communities and cultures they comprise (Relf, 1992).

To understand and justify this broadened definition of horticulture, we must look at the research of individuals from a number of different disciplines -- environmental psychology, landscape architecture, social ecology, anthropology, sociology, geography, communications and forestry, as well as horticulture.

Most of the relevant studies have been conducted to understand peoples' responses to and need for natural versus urban or man-made environments. While many people take the terms nature or natural to imply total lack of interference by man, in most developed countries, it is impossible to identify such an environment. Even our wilderness areas have been influenced by forestry activities, man-made fires (or the man-controlled fires), access roads and physical amenities supplied for recreational pursuits. Researchers reporting on the role of nature or nearby nature in influencing people are most often referring to vegetation; however, water and non-domesticated animals also may play a role. The vegetation included in the term `near-by nature' may take the form of a rosebud in a vase, a backyard garden, a street tree planting, a neighbourhood park, a planted atrium or fields and woods (Kaplan, 1992). In discussing the diverse research showing that people overwhelmingly prefer `nature' scenes to urban and built environments, Zadik (1985) explains "people seem to respond to environments as natural if the areas are predominantly vegetation and do not contain human artifacts such as roads or buildings." The fact that an environment under study is a well-maintained, designed landscape does not seem to alter the application of the term `natural' to the scene.

The garden throughout the history and tradition of Western Civilization has served as a pivotal location for change -- the Garden of Eden, the Garden of Gethsemane, the monastic gardens leading into the Renaissance. If indeed, the garden does play a vital role in the `grand scheme of things', then horticulturists, as keepers of the garden, need to begin to try to understand the meaning of the garden to people.

Ornamental and landscape horticulture is the maintenance and design of home grounds, public gardens and parks, private estates, botanical gardens, and recreational areas such as golf courses, football fields, and baseball diamonds (Encarta, 2008).

Environmental horticulture referred to as ornamental horticulture is the study of horticulture for aesthetics, improvement of quality of life and our environment, and for functional uses, such as energy conservation, pollution amelioration, oxygen production and security (Brownson et al., 2001).

In Nigeria, horticulture has not been formally recognized or promoted as a feasible means of improving urban green space, aesthetics or of increasing employment opportunities of urban agriculturalists. The neglect of environmental beautification planning across the nation, lack of public awareness on environmental management which, many of the ornamental plants have an inherent ability to handle has resulted in continuous environmental degradation which could be attributed to population pressure resulting in disharmony between human activities and the environment. This paper therefore seeks to review the use of ornamental plants in environmental aesthetics and management of the landscape.

2 Methodology

The review was a general narrative review type which was based on selected, specific and related studies to the review topic ;Significant role of horticulture in environmental aesthetics and management, and are compared and summarized on the basis of the experience, existing views, existing theories and ideology. Results are based on a qualitative rather than a quantitative level.

2.1 Method of review

The issue review method was used in this studied by investigating the significant role of horticulture in environmental aesthetics and management in terms of its environmental benefits, social advantages and human health and also determining its present status in Nigeria as case study, as there fewer detailed reviews on the untapped benefits of horticulture in Nigeria.

2.2 How it was conducted

The steps and stages involved in the preparation of this review involves the following

The descriptive type of abstract was used in writing the abstract.

The introduction was written based on the subject background i.e. the general topic, issue, area of concern was also given to illustrate the context i.e. the significant roles of horticulture in Nigeria and its potentials in environmental and economic development. The Problems were also highlighted i.e. Trends, new perspectives, gaps, contrary ideas. Then the justification which gave reason for reviewing the literature and the approach were described.

Full preparation of article was done through narrowing of the topic; defining a few research questions like: what is horticulture?, what is aesthetic and environmental management?, what relationship does horticulture has with environment and aesthetic?, and what role does horticulture plays in the environment.

The search for literature sources was done using specific key words, the topic and research questions during the search read.

Evaluation, classification and extraction of related information based on the focus study was made and notes were formed.

A chronological outline structure was developed and headings for the sections in the text body was made find headings for the sections in the text body.

The planning of the content of the paragraphs was made in the sections.

The final drafting of the abstract, introduction, results sections, conclusion, and references were made.

2.3 Keywords used in the search

1. Horticulture

2. Aesthetics

3. Environmental management

4. Urban development

5. Landscape and architecture

6. Benefits of horticulture

7. Environmental horticulture

8. Horticulture in Nigeria (status, implications and potentials)

2.4 Criteria of inclusion

The criteria for inclusion were based on selective, important, and related information on the desired study topic. All cogent reviews, statistics, ideology and definitions related to the search keywords were included with specific references to the advantageous aspect of horticulture to the improvement and management of the environment.

2.5 Analysis

The method of analysis of the information was done through deductive and logical reasoning

Environmental aesthetics is a vital area of aesthetics that focuses on philosophical questions concerning appreciation of the world at large (Carlson, 2003; Dutton, 2005; Smout, 1991; Arriaza et al., 2004) it extends beyond the narrow confines of the arts and beyond the appreciation of works of art to the aesthetic appreciation of human-influenced and human-constructed as well as natural environments. An awareness of the aesthetics of the environment is part of a broader response to environmental problems. Consequently, the term environmental aesthetics has been developed by Carlson (2003) as a means to merge the irrational with the rational, that is, the aesthetics of beauty with the rationality of biodiversity. This theory supports the position that appreciation of the natural environment should be viewed from the perspective of ecological information as regards its constituent parts (Budd, 2002). It is obvious that whereas the general meaning of aesthetics refers to the appreciation of objects which are bounded in space, often by an actual frame or display case, the environment extends infinitely and surrounds us. As one writer points out, “One can step back from a painting. In contrast, in a landscape the viewer is involved, environed, enwrapped, and surrounded. He can go in, and is likely to experience not only the landscape but perhaps also himself in an unusual and vivid way” (Porteous, 1996). Environmental aesthetics provides all humans an opportunity to cover aesthetic investigation of our experience of all sorts of environments, human made as well as natural. The natural environment provides distinct senses of colours, shapes, textures and sounds, and these senses vary as a consequence of the change of seasons, weather, or even time of a day (Miller, 2007). Sense of beauty derived from the natural landscape is associated with each individual. Lots of empirical studies indicate that the natural landscape provides great aesthetic enjoyments to the residents (Wohlwill, 1976; Jim and Chen, 2006; Hettinger, 2005; Mobley et al., 2009). It serves as ornaments or decorations for individual tastes or public enjoyments (Smardon, 1988). Simply through visual contact with nature, individuals can obtain immense pleasure and gratification. Aesthetic enjoyment is not always limited to visual experience. Immersed in some light scent emitted from certain vegetation is also a pleasant experience. Besides, sounds from the rustling leaves and whistling wind in the green space create a sense of peacefulness (Smardon, 1988; Tyrva¨inen et al., 2003).The relatively early developments in environmental aesthetics started from the applied side of the field, which though reflected the public’s awareness and interest in the aesthetic quality of the environment, also encouraged the development of modern design professionals such as landscape architects, horticulturist and, environmental planners, with their focus and design approaches being to enhance the formal qualities of scenic views. Over the last thirty years, the deterioration of physical environments has given rise to a political-social outcry on the urge for legislations aimed at identifying and managing public landscape resources (Galindo et al., 2000), and also raised the public awareness about the value of aesthetic appreciation of one’s environment, be it natural or human-constructed. Environmental aesthetics is one of the very recent fields of aesthetics to emerge in the second half of the twentieth century, with increased significance and scope. Hence, it is no longer confined to the safe precincts of gardens and parks, the boundaries of environmental aesthetics has been redefined to encompass all of nature, city as well as countryside, factory as well as museum, desert wastes as well as glacier-fed fords. Hipple (1957) points out that aesthetics is concerned with the response of the mind to the qualities and relations of objects in nature and art. It is about having a deep and continuous understanding why things in the world had such a strong impact on the mind. Aesthetic appreciation has therefore secured its position as a distinctive feature of modern design in ornamental and landscape horticulture which has no doubt buttress the saying; “beauty is in the eyes of the beholder” (Lothian, 1999). Horticulture offers an increase in aesthetics pleasure that supports psychological well-being.

3.2 Methodological perspectives of environmental aesthetics

Environmental aesthetics has now become a collective term standing for almost any conceivable subject that relates to the human’s appreciation of his physical surroundings (Peciva, 2000). Though still in its early phase of development, researchers' interest in general environmental aesthetics quality has grown and many studies have been generated over the last three decades (Carroll, 2001). Environmental aesthetics will be increasingly recognized by researchers and laypersons alike as a result of ‘mixed’ aesthetics involving interaction between natural and artificial objects, individual preferences and universal tastes, thereby increasing public interest (Peciva, 2000).

3.3 Environmental aesthetics in cognitive form

The contemporary positions in environmental aesthetics have developed from different points of views concerning the aesthetic appreciation of the natural environment. These are frequently divided into two camps, alternatively labeled cognitive and non-cognitive (Godlovitch, 1994; Eaton, 1998; Carlson and Berleant, 2004), conceptual and non-conceptual (Moore, 2008), or narrative and ambient (Foster 1998). Within each camp several distinct positions have emerged. Cognitive accounts hold that appropriate aesthetic appreciation of human environments, like that of natural environments, depends on knowledge of what something is, what it is like, and why it is as it is. Thus, for human-influenced environments such as, for example, the landscapes of agriculture or industry, what is relevant to appropriate appreciation is information about their histories, their functions, and their roles in our lives (Parsons and Carlson, 2008). The same holds for other human and human-influenced environments, both rural and urban , although in all such cases knowledge provided by the social sciences is as relevant to appropriate aesthetic appreciation as that given by the natural sciences (Carlson, 2006).

Some cognitively-oriented accounts also stress, as they do in the case of natural environments, the aesthetic potential of cultural traditions in the aesthetic experience of human environments. Such traditions seem especially relevant to the appreciation of what might be termed cultural landscapes—environments that constitute important place in the cultures and histories of particular groups of people. What is often called a sense of place, together with ideas and images from folklore, mythology, and religion, frequently plays a significant role in individuals' aesthetic experience of their own home landscapes (Firth, 2008). The non-cognitive approaches to environmental aesthetics also provide several channels for exploring the aesthetics of human and human-influenced environments and especially for pursuing the aesthetics of everyday life.

The basic idea of grounding the appropriate aesthetic appreciation of the environment in objective knowledge offers an explanation for the development of the view known as ‘positive aesthetics,’ which suggests that appreciation of environment holds the formal qualities such as order, balance, unity and harmony (Carlson, 2006). It has been demonstrated conclusively that people will, when presented with even the most disembodied of stimuli (colors, lines, shapes, etc.), make aesthetic judgments, and indeed that the task of doing so is apparently meaningful to them (Whitfield, 2005).

3.5 Benefits/significance of environmental aesthetics

It was estimated that by 2008 over half the world’s population will live in an urban environment (WWI, 2007). The urbanization of the world is reducing the connectivity humans have with the natural environment. Despite human disconnect from the natural world, plants continue to provide environmental benefits which are important to our existence and living at every spatial level: global, local and individual. These environmental benefits are: oxygen production, carbon sinks, pollution amelioration, indoor air quality improvements, water management and erosion control, plants in ecological sewage and wastewater treatment systems, wildlife attraction and preservation, windbreaks and noise amelioration, urban shade, green space and location of plants.

Maintaining a sound and healthy environment has always been a challenge to man (Akintola, 1978). Hence, the management of built environment is determinant to the quality of man at any given time. Where this is undermined, there is bound to be poor physical conditions and the consequence is poor human output. Environment has been variously conceptualized to include all the natural resources of air, land and water; visible and invisible elements that affect the development of an organism for its lifetime. Environment refers to all the conditions and influences affecting the development of an organism in its lifetime. Man’s total environment includes all the living and non-living elements in his surroundings which could be natural or built (man-made), etc. in a complex network of systems (Okaba and Obong, 2006; Edu, 2006). As identified by Obong (2007), three major segments of environment include the natural, built and personal environments.

Furthermore, the acquisition of operational land area is usually the very first disruption that visits an oil community. The discovery of oil in an area inevitably means the destruction of the vegetal life of the area to enable the company set up its equipment and embarks on the harvesting of the oil resource. In fact once land is occupied for oil installations or operations, unintended changes in the environment may result namely:- loss of economic and forest trees, loss of farm land and topsoil, loss of flora and fauna, habitat destruction, loss of fishery and crop cultivation etc. Thus in the course of carrying on oil exploration, productions, and refining activities, the environment experiences what can be referred to as ecological disruption.

3.6 Environmental aesthetics and management in Nigeria

As a result of environmental degradation, Nigeria has lost about 84 per cent of its total forest cover and around 90 per cent of its moist forests, the remainder of which exist in small reserves. By 1995, woodland accounted for only about 29 per cent of the country’s total land area (2) of 923,768 km² (356,669 mi²) (Wikipedia Encyclopaedia, 2008). Although Nigeria covers a land surface area of 923,800 square kilometers, 14,800sq km of these (1.60%) is covered by forests and 32357sq km protected to maintain biodiversity (World Bank, 2004).

As a nation, Nigeria has been experiencing an accelerated shift of her population from rural to urban areas engendering several challenges and problems.The Human Development Report (2004) records that 45.9% of the 120.9 million (2002 estimates) strong population of Nigeria resides in urban centers. The increase in urban population has resulted in the Proliferation of slums and informal peri-urban settlements known as shantytowns; (Mabogunje, 2002). These shantytowns are deprived settlements characterised by excessive residential densities, largely uninhabitable housing and absence of sanitation, basic infrastructure and social services, urban congestion and environmental degradation (Aina, 1990). Unfortunately approximately 80% of the Nigerian urban households live in these conditions (Lawanson, 2006).

With Nigeria's population continuing to increase, the pressure on the country's environment appears likely to increase as well, making it increasingly difficult to achieve environmental sustainability. Nigeria is presently losing annually, about 351,000 sq km of its land mass to the desert which is advancing southward at the rate of 0.6 km per year. (FOS, 1997) The intensification of the use of fragile and marginal ecosystems has led to progressive degradation and continued desertification of marginal agricultural lands Flash floods from torrential rains wash away thousands of hectares of farmland. Uncontrolled logging and tree felling accentuated by lack of re-stocking are the order of the day in many parts of the southern states of Nigeria. This carries with it loss of precious biological diversity. Also an estimated 484 plant species in 112 families are threatened with extinction because of habitat destruction and deforestation. Many of our cities are concrete jungles where plants are no longer used for home landscaping. The new Federal Capital Territory at Abuja is a pathetic example of this development where the rich and natural vegetation is being systematically depleted as a result of increasing human pressure. More recently, areas earmarked as green belts and recreational areas are being systematically converted into building sites (Lawanson, 2006).

The use of solid biomass, such as fuel wood, is also prevalent and constitutes a major energy source. During the 1990s, for instance, Nigeria lost nearly 500 square miles of forested land annually, in part due to fuel wood consumption (EIA, 2003). Also the rampant bush burning is threatening the growth of trees and wildlife species and reducing the ecological diversity of the area; gravel mining for construction is aggravating the problem of erosion and surface run-off; while indiscriminate discharge of particulates from construction sites is already leading to pollution and siltation.

Nigeria total primary energy consumption is steadily rising as the country’s population is multiplying. The effect of iindustrialisation on the environment is noteworthy; the increased demand for energy to run air conditioning and electrical appliances is contributing to pollution. Producing the energy required to run modern urban systems often involves burning fossil fuel, which releases such greenhouse gases as carbon monoxide, carbon dioxide and nitrogen oxides, these emissions lead to global warming which can cause destruction of the ozone layer, climate change, rising sea levels, changes in vegetation and severe weather events. Though statistics are currently unavailable, real indicators show that this problem is worsening. Therefore, addressing these pressing challenges will result in the reduction of environmental degradation and the achievement of sustainable urbanization.

Ornamental plants can be strategically located to engineer a more pleasant environment in which to live. They can be placed around pools to provide shade but also to reduce the glare that is reflected up to surrounding buildings; they can be placed in front of houses to prevent lights from vehicles or the street shining into the house; and they can also play a role in reducing noise from highways (Robinette, 1972). Green roofs also reduce noise distractions from the outside world (Grower-Talks, 2006). Plants in the urban environment provide shade to escape the heat of the sun and they also provide green spaces for rest, relaxation and recreation. However, there are various ways in which green spaces can be designed. For example, they can be managed as natural woodland areas or manicured to provide formal settings such as those found in botanical gardens. For some people the “natural” landscape is something that should be kept for wilder places and doesn’t have legitimacy within the urban context due to an association of “natural” with fear of physical danger (Özgüner et al., 2006). Others, however, feel a natural landscape gives them a sense of freedom and is a better place to socialize than the formal landscape. Conversely, it was found that in a formal landscape people had a sense of safety and that they found it a better place to relieve stress, and that it was more peaceful, quiet and calming. Extending the calming effect beyond parklands, Wolf (2006) suggests that forested urban roadways increase the level of safety, both by defining the roadside and focusing the drivers’ attention. They also create a more pleasant, less stressful environment in which to operate. In a separate study, Wolf (2004) provided a comprehensive list of environmental, social and economic benefits that can be achieved from utilizing trees in parking lots. Not only do the trees provide shade for the parked vehicles and, therefore, make returning to a vehicle a more pleasant experience, but the trees were also found to provide many other benefits simultaneously.

Green roofs are an innovative way to incorporate the benefits of plants into the urban landscape. Green roofs produce oxygen and help with energy savings via climate control both inside the building and for the city as a whole. They assist in water management by cleaning and retaining water, and can reduce the noise levels around the city. A study in Canada showed that green roofs reduced greenhouse gases by approximately 2.18 metric tons (MT) based on 6.5 km² of green roof coverage. Additionally, 30 MT of pollutants were removed from the air and summer temperatures were reduced by 1-2^oC. The reduction in temperature also leads to a reduction in energy requirements (Erskine, 2003).

The United States Golf Association also points out that golf courses are green spaces with trees, turf grasses and natural areas that provide many of the benefits illustrated in this section (USGA, 2007). The USGA has been involved with numerous studies evaluating the relationship between golf and the environment and highlighted the benefits of golf courses as follows:

Provides wildlife habitat: More than 70% of most golf courses are rough and non-play areas which include natural grasses, trees and shrubs that, with the open areas of fairways and greens, make golf courses an attractive area for wildlife habitat.

Protects topsoil from water and wind erosion: The dense system of roots and shoots helps turf control erosion by capturing and slowing down water from storms. Studies have shown that during extremely intense rainstorms (three inches per hour), turf holds up to 20 times more soil than traditionally farmed cropland.

Improves community aesthetics: Turf improves community aesthetics in a number of ways including keeping things cooler on a hot day, reducing noise pollution, reducing the glare of sunlight on pavement and buildings.

Absorbs and filters rain: Turf easily absorbs rainwater that might otherwise flow into streams and lakes.

Improves health and reduces stress for people who participate in golfing: Research studies show that participants of golf lowered their cholesterol levels by walking the course when they played.

Improves air quality: The growth process for turf takes carbon dioxide from the air and releases oxygen. Studies have shown that a landscape of turf, trees, and shrubs about 2,000 square feet in size can generate enough oxygen for one person per year.

Captures and cleanses runoff in urban areas: Turf provides a good growing area for many microorganisms and these, in turn, help cleanse water by digesting the pollutants in the trapped water and speeding up their normal breakdown.

Discourages pests, pollen and disease (e.g. ticks and mosquitoes): Turf that is dense and well maintained reduces weeds and the pollen that aggravates allergy sufferers. Furthermore, mowed turf discourages pests such as mosquitoes, ticks and chiggers, which, in turn, reduce the threat of Lyme disease.

Restores damaged land areas (e.g. former landfill or mining sites): Restoring damaged areas such as abandoned quarries, strip mines and landfills with golf courses combines the benefits of turf with the benefits of a landscaped area and results in a reclamation of previously unused land for the community.

Makes substantial contributions to the community’s economy: Golf is growing in popularity, and appealing to a broad range of people. Golf courses can have large impacts on the economy by providing jobs and creating tourism to an area.

The combination of mowed turf, trees and natural areas provides a diverse environment for people and wildlife, and preserving these green spaces improves the environmental quality of the entire community (USGA, 2007).

3.7.3 Wildlife attraction, preservation and biodiversity

Ornamental plants can also provide environments that encourage the presence of wildlife both during production of the plant (e.g. Christmas trees and other nurseries) and at the end use in the garden or parkland. With the production of Christmas trees for example, it takes 7-10 years for the trees to reach heights of two metres, at which point they are ready for sale. During this time, the trees provide habitats for a variety of birds and other animals ((CCTGA, 2004). The research of (Damschen and slatte, 2006) revealed that establishing landscape corridors encourages plant biodiversity. The research showed that habitat patches connected by landscape corridors retained more native species than isolated patches. They also found that corridors do not promote invasion by exotic species. Overall, patches connected by landscape corridors had 20 percent more species of plants than the unconnected corridors (Damschen and slatte, 2006).

There are significant benefits that can be gained from plants with respect to water management. Plants can be used to remove pollution from soils and air. In addition, plants can also remove pollution through filtration and purification processes and can also assist in flood control by retaining water. These functions are also achieved by the turf grasses that cover residential yards, golf courses, parks and road medians (Bolund and Hunhammar, 1999) Plants can be used in conjunction with storm water management ponds to manage water run-off from urban areas (Inglis, 1999). The water flow is slowed by the presence of plants allowing the water time to infiltrate the soil (Brack, 2002). Consequently, this provides time for the process of phytoremediation (mentioned above with regard to soil contamination). As the water seeps through the soil biological organisms (plants and micro-organisms in the soil) reduce or remove pollutants from the water (Brack, 2002). Using plants to reduce run-off and pollutants can occur in many situations. As watersheds increasingly become urban landscapes the amount of penetrable ground is reduced. This can be improved by using “rain gardens”, a small swale or retention ditch, near impenetrable surfaces (e.g. roads, parking lots). This creates areas for water to be cleaned and reduces the speed with which water re-enters the watershed. Plantings located on roof tops, known as green roofs, can also provide this service. The plantings filter and reduce run-off (Erskine, 2003).

The development of phytoremediation techniques has lead to more intensive use of plants in water purification. Some organizations specialized in this area have developed purification systems for municipalities, food processing plants and agricultural operations. They design and install “restorers” which are engineered ecologies on floating rafts. The plants and micro-organisms that live on these rafts clean and purify the water by encouraging natural processes; they introduce oxygen to the stressed system allowing the ecology to re-balance. The system can reduce organic loading, and improve water clarity by reducing the total amount of suspended solids. Organic pollutants such as fats, oils and greases can be broken down and removed. Ammonia, nitrate, and pathogens such as fecal coliform levels are reduced and heavy metals can be removed from the water (OAI, 2007).

3.8.2 Windbreaks and noise amelioration

The planting of hardy trees to build shelterbelts protect the soils from wind erosion, and encourage the protection of a natural resource. The trees planted in shelterbelts are also carbon sinks (Akbari, 2002).      Screens and hedges provide noise reduction, especially in urban areas where noise is easily reflected from hard surfaces like pavements or buildings. Plants are better at absorbing high-frequency sounds, which are most bothersome to human ears, than they are at absorbing low frequency sounds. Noise is measured in decibels (on a logarithm scale) and is best reduced when plants are placed in two or three rows. For instance, two feet of plant width can decrease the decibel level by four, while increasing the plant width with two or three plants can lower the noise level more than seven decibels. Some examples of noise levels include:

People talking, 30-60 decibels

Large trucks, 60-80 decibels

Jet airplanes, more than 120 decibels (Fare and Clatterbuck, 1998).

The fundamental benefit provided by plants is their production of oxygen; thus providing the atmosphere with the element that assist humans to breathe and live on this planet. Oxygen is produced by the plant during the process of photosynthesis. It requires carbon dioxide, water and sunlight. The products of photosynthesis are sugars and oxygen. On average, a tree can produce 118kg of oxygen per year and two mature trees can produce sufficient oxygen for a family of four. Similarly, a natural turf area just 23 square meters releases enough oxygen for a family of four (VSU, 2004).

3.9.2 Indoor air quality improvement

As our population continues to urbanize, the number of people spending 80-90% of their time indoors (Orwell et al., 2004) is also increasing. Many studies have reported that the concentration of Volatile Organic Compounds; VOCs emitted as gases from many solids and liquids e.g. carpeting, furniture, wall coverings and ceiling tiles in homes is health threatening (Dixon, 2006). Some of the chemicals labeled as VOCs may have short and long-term adverse health effects and are higher indoors than outdoors (Sakai et al., 2004). Plants continue to function as atmospheric filters indoors as they do outdoors and enhance the air quality of confined environments (USEPA, 2007). Recent studies show that indoor plants are effective at removing VOCs (Orwell et al., 2004; Liu et al., 2008). Seven plant species/varieties which were tested; Dracaena ‘Janet Craig’, Epipremnum aureum, Dracaena marginata, Schefflera ‘Amate’, Spathiphyllum ‘Petite’, Spathiphyllum ‘Sensation’, and Howea forsteriana, and demonstrated that the rhizosphere (substrate) and the plant are both required to maximize the removal rate of VOC from the air (Orwell et al., 2004).

A similar study conducted by Liu et al. (2008) identified 10 species of ornamental plants (Crassula portulacea, Hydrangea macrophylla, Cymbidium ‘Golden Elf’., Ficus microcarpa var. fuyuensis, Dendranthema morifolium, Citrus medica var. sarcodactylis, Dieffenbachia amoena cv. ‘Tropic Snow’; Spathiphyllum ‘Supreme’; Nephrolepis exaltata cv. Bostoniensis and Dracaena deremensis cv. ‘Variegata’) that are effective at removing benzene from the air and are, therefore, considered effective at removing other gaseous pollutants. The Green Dragon Tree (Dracaena deremensis 'Janet Craig') was found to be the species with the largest capacity to remove benzene from indoor air. As with the outdoor environment, particulate matter such as dust, ash, pollen and smoke are also irritants and pollutants of indoor air. Lohr and Pearson-Mims (1996) found that the levels of particulate matter accumulation in a room were lower when plants were present than when they were not. In addition to finding a reduction in particulate matter it was also found that relative humidity was slightly higher when plants were present. An increase in relative humidity, particularly in heated environments, increased the comfort level. Another means by which indoor plants improve enclosed environments is by removing offensive odours from the air (Oyabu et al., 2003).

Plants have been found to be used as efficient filters of air. Researchers at the University of Guelph developed an air filtration system that has a wall of living plants which cleans the air of a building and incorporates it back in to the air distribution system. In the most complex systems, water to irrigate and fertilize the plants comes from integral waterfalls and vertical drip systems sourced from ponds which support fish, plants and other organisms that cleanse the water while adding nutrients for the plants. The biowall system, known as Naturaire®, is adaptable to a variety of spaces including condominiums, homes, office buildings and public buildings, and may be used during space exploration in the future (Dixon, 2006; Prescod, 2005).

Prescod (2005) investigated the use of plants as air purifiers. The results of the investigation suggested that one potted plant per 100 square feet of indoor space in an average home or office was sufficient to cleanse the air of pollutants. Prescod (2005) provided the list of plants which are effective air purifiers along with the pollutants that they are best at removing.

Orchids are very effective at removing numerous pollutants during the daylight hours. They are also effective at removing carbon dioxide and xylene at night, while at the same time releasing oxygen into the air. This is because orchids and bromeliads have a unique metabolic process whereby their stomata open at night. This is significant because air can be continuously filtered, day and night (Prescod, 2005). Although plants can also contribute to reduced air quality by releasing pollen and spores which can cause discomfort in the form of allergies (Lohr and Pearson-Mims, 1996). It is however, important to acknowledge, that, there is substantial amount of research that indicates having indoor plants has the net effect of improving indoor air quality.

3.9.3 Pollution Amelioration

Trees and plants have been labeled as the “lungs of cities” (McPherson, 2005) because they have the ability to remove contaminants from the air that is breathed. Acting as natural filters and reducing air pollution, it has been shown that plants generate health benefits by reducing the mortality rate and reducing visits to the hospital (Powe and Willis, 2004). A few ways in which plants reduce air pollution are as follows:

Absorption of gaseous pollutants through their leaves, e.g., ozone, nitrogen oxides, and sulphur dioxide.

Further reducing ozone concentrations at ground level by reducing the temperature via evapotranspiration as mentioned above.

Collection of dust, ash, pollen and other particulate matter on their leaves hence reducing its presence in the air breathed.

Releasing of oxygen, as mentioned above, which increases the quality of the air for human use. (McPherson, 2005)

The amount of air-borne pollutants removed increases with leaf surface area. Therefore, trees tend to be better filters than shrubs and grasses. Due to their large surface area and year round coverage, conifers (evergreens) are very good pollution filters. However, conifers tend to be sensitive to phytotoxic air pollutants and deciduous trees are more efficient at absorbing gaseous pollutants. It is, therefore, beneficial to have a mixture of species in order to have the greatest effect in reducing air pollution (Bolund and Hunhammar, 1999). The total amount of air pollution removed by urban trees annually within the United States is estimated to be 711,000 metric tons (Nowak et al., 2006).

Researchers in the United Kingdom quantified the benefits of air pollution absorption in terms of mortality and morbidity. It was estimated that the woodlands in Britain saved 5-7 lives and reduced hospital admissions by approximately 4-6 annually. This had an estimated economic value of at least £900,000 (N225 million). Their argument was that by absorbing pollutants such as particulate matter (PM₁₀) and sulphur dioxide the woodlands provided additional benefits beyond the traditionally conceived market benefits, although the authors did indicate that these particular health benefits were relatively small in comparison to other non-market benefits of forestry (Powe and Willis, 2004).

Plants are also used to remove contaminants from the soil. This process of reduction or removal of pollution is known as phytoremediation (Glick, 2003; Licht and Isebrands, 2005). Phytoremediation includes strategies such as riparian buffer zones, often used within the agricultural sector to act as natural filters of agricultural run-off, consequently increasing local and regional water quality. Phytoremediation is also used within an urban context on brownfields that will only achieve their economic value from development once the soil or water contamination is removed or controlled (Licht and Isebrands, 2005). Gas Technology Inc. has experience in the phytoremediation of soils, focusing on cleanup of manufactured gas plant sites and on soils contaminated with other hydrocarbons (e.g., crude oil). The company has identified several plant species that show promise for remediation of soils contaminated with polycyclic aromatic hydrocarbons (PAHs).

Even plants that are relatively tolerant to environmental extremes struggle to produce normal biomass in the presence of environmental contamination. Through the use of growth-promoting bacteria it has been possible to increase the speed and efficiency of phytoremediation (Glick, 2003). These developments contribute significantly to the ideas of Licht and Isebrands (2005) who illustrated the economic opportunities of linking biomass harvesting with phytoremediation. Trees that are grown on contaminated ground with growth-promoting bacteria can also be harvested for their wood so that the benefits are two-fold; increased phytoremediation efficiency (i.e., increased ability to grow in a difficult environment) and increased harvested product.

4 Discussions

The importance of these plants as providing aesthetic function by creating attractiveness for human activities has resulted in complete neglect of their use for environmental conservation and beautification across the nation. The present environmental situation requires redefining strategies to bring about environmental management that are in conformity with national and international development efforts. Changes in the attitudes and beliefs about the direct effect of ornamental plants on the ecological position of humans.

This increase demands the functional and aesthetic integration of vast amounts of diversified plants (Smith et. al., 2005) which if carefully chosen can play a decisive role in environmental restoration and protection. The presence of plants in an environment makes it beautiful and a beautiful environment is always cool and inviting. The functional and aesthetic interaction of people, building and site, using plants and space as its main tools is very important in solving environmental problems (Acar et al., 2005).

Trees alter the environment in which we live by moderating climate, improving air quality, reducing stormwater runoff, and harbouring wildlife. Local climates are moderated from extreme sun, wind, and rain. Radiant energy from the sun is absorbed or defected by leaves on deciduous trees in the summer and is only filtered by branches of deciduous trees in winter. The larger the tree, the greater the cooling effect. By using trees in the cities, we can moderate the heat-island effect caused by pavement and buildings in commercial areas (Geiger, 2005).

5 Conclusion

The quantifiable role of horticulture and the values it brings to people, business, the environment and municipalities especially in Nigeria cannot be overemphasized. Trees and shrubs can be used to build physical and visual barriers, providing security and/or privacy and effective landscape design can also add value to municipalities and communities. Ornamental plants play a role in the development of healthy communities by providing an environment that is more comfortable physically in which to live and work. Also significant is their role in providing or building the environment in which these physical activities can occur and the associated benefits can be achieved.

Ornamental plants contribute to the beautification (aesthetics) and the pride people have in their cities, towns or dwellings. Therefore, addressing the role of horticulture in urban development and environmental aesthetics will require the application of a full range of developmental instruments including infrastructure and institution building, technology transfer and development, creation of an enabling environment, technical training programs, research and capacity building. To this end, the complexities involved in studying horticulture and its role in environmental aesthetics and urban development arises not just because the factors affecting the appreciation of surroundings are complex out there in the environments, but humans are in themselves, deeply uncertain about their relation to nature. Hence there is need for more emphasis on horticulture as a viable industry in the ever changing landscapes of the man-made world and the natural environment.

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