1 Introduction
Combining the aesthetic appeal of landscaping with the environmental benefits of vertical gardens help improve urban environments. When properly designed and maintained, these vertical landscapes can transform the visual appearance of buildings, improve urban aesthetics, enhance biodiversity, and contribute to oxygen production and fresh food growth in the surrounding environment
[1]~[5]. Existing research has shown that plant coverage on vertical surfaces effectively absorbs particulates and carbon dioxide
[6], which can significantly improve overall air quality in urban areas
[7].
Vertical gardens allow humans to recreate a life system very similar to natural environments, as adding nature back to places where it has been removed
[8]. As a form of green walls, vertical gardens are categorized into green facades and living walls
[9]. Green facades use annual climbing plants that creep up or hang down the walls. Living walls utilize vertical structures to support plant growth and display diverse aesthetics on the wall by selecting different plants, varied design patterns, colors, textures, shapes, and leaf densities. The idea of living wall is easily compatible with the concept of hydroponic systems by providing continuous irrigation and adequate nutrition.
Plants in green walls have architectural, aesthetic, engineering, and climate control functions
[10]. Examining plant growth, color, flowering, leaf structure, and root growth is very important for the stability of the wall
[11]. However, vertical growth systems often face several challenges, including stability, high initial capital, the need for automated irrigation equipment, intensive maintenance, and limited vegetation types because of uneven lighting
[12] [13]. Particularly, plant establishment, survival, and persistence in green walls pose difficulties, especially in environments with climate and growth constraints and stressful conditions such as high temperatures, moisture deficiency, and nutrient shortages. Studies indicate that the application of plant growth-promoting bacteria (PGPB) can significantly enhance plant survival and increase growth and overall performance in green walls
[14] [15]. Luana Alves de Andrade et al. demonstrated that specific bacterial strains such as
Pseudomonas spp. and
Bacillus sp. have the ability to promote root growth and nutrient absorption, and improve plant performance
[16]. Properly integrating these bacteria into the design and maintenance of green walls can improve their flexibility, durability, and ecological performance.
Considering stressful conditions, the type of irrigation water also plays a vital role in the success of green walls
[17]. In addition, the limitation of fresh water for irrigation, particularly in countries located in arid regions, is a global challenge. To address this situation, the reuse of treated wastewater can be beneficial for the environment and have favorable psychological effects on society, contributing to sustainable urban development
[17] [18]. Here, wastewater refers to the water resulting from human activities, e.g., domestic sewage, industrial sewage, and potable water discharged from sanitation systems
[19]. In water-scarce regions, these resources can be used to supplement or replace fresh water for applications that do not require drinking water quality, particularly agricultural irrigation
[20]. This approach offers an important opportunity to bridge the gap between water demand and supply, with the potential to meet over 13% of total water needs if properly treated and recycled
[21].
Graywater, a sort of recycled water, accounts for approximately 54% to 86% of the wastewater generated by a household
[22]. Its composition varies depending on the source, including those from kitchen, bathroom, handwash basin, and laundry. Using recycled water for irrigation not only helps conserve water resources
[23], but also enhances plant health by substituting chemical fertilizers
[24] [25]. Meanwhile, applying metal-resistant PGPB has been proven to be an efficient way to improve biomass accumulation and plant tolerance to heavy metals in recycled water
[26]. However, despite all these advantages, improper management may also pose public health risks from pathogens, heavy metal accumulation, and soil salinization, thus causing public resistance. These issues require careful planning that considers agricultural and horticultural factors (e.g., PGPB), design factors, and plant factors in soil filtration
[27] for pollutants removal.
Although much research has focused on the environmental and functional benefits of green walls, few studies have investigated how factors such as bacterial treatments and recycled water irrigation impact plant performance and aesthetic quality, especially their combined effects in green walls, which remain largely unexplored. To address this research gap, this study specifically investigates the impact of PGPB and recycled water irrigation on plant aesthetics, including leaf health, color, density, and vibrancy of vegetation, aiming to provide new insights into how to apply sustainable practices of green walls.
2 Materials and Methods
2.1 Experiment Site
The research was carried out in Mashhad, the second-largest and most populous city situated in northeastern Iran. Mashhad experiences a semi-arid climate characterized by cold winters and hot, dry summers. The average annual precipitation hovers around 255 mm. The annual temperatures range from 4℃ to 22℃, with a reported relative humidity of approximately 40%
[28]. Specifically, the experiment site was on a 15-meter wall in the outdoor area of the Pirsha Greenhouse located north of Mashhad.
2.2 Green Wall System Design
In this experiment, vertical planting panels constructed with steel wire mesh were installed on a wall facing east-west without any shading. The vertical planting system utilized in this project is the Elmich system, an original Malaysian design
[29]. The wall design adopted polypropylene plastic pots, measuring 19 cm in width, 19 cm in length, and 21 cm in height. Taking into account the types of ornamental plants and the specified treatments of PGPB and irrigation, this experiment set 3 replications (R1, R2, and R3). Each replication includes 3 walls, each of which comprises 2 vertical panels. Within each panel, 2 columns and 4 rows of experiment units were arranged. Each experiment unit consisted of 1 pot with 2 plants of the same species. This experiment, in total, encompassed 144 experimental units (Fig.1).
This research was conducted as 4 separate experiments for selected ornamental species, in the form of split plots based on a completely randomized block design in 2022 (Fig.2). The culture medium (substrate) used in all experimental units was the same, consisting of 25% cocopeat, 5% vermicompost, 55% perlite, 10% vermiculite, and 5% zeolite. The percentage of organic matter in the culture medium was 20%. This amount aligns with the recommendation by
Green Roof Guidelines from FLL
①[30].
① The Green Roof Guidelines was published by Landscape Development and Landscaping Research Society e. V. (FLL), Germany, providing internationally recognized guidance for green roof planning, construction, and maintenance.
2.2.1 Irrigation Water Treatments
Considering the quality of irrigation water the main factor, three levels of recycled water treatments were applied: graywater (I1) collected from rainwater and a special dual sink for washing fruits and vegetables; wastewater (I2) sourced from the sewage effluent from domestic sewage, industrial wastewater, and water discharged from sanitation systems that originated and discharged into the Kashfroud river in Mashhad; and urban water (I3, control). The waters utilized were gathered in three tanks and administered through drip irrigation system. The irrigation rate was set at 70% of the field capacity of soil, adjusted according to the discharge rate of the drippers.
2.2.2 Plant Growth-Promoting Bacterial Treatments
This research included 4 biological strains of bacteria: Mix B1 (
Psedoumonas flucrecens, Azosporillum liposferum, Thiobacillus thioparus, and Aztobactor chorococcum), Mix B2 (
Paenibacillus polymyxa, Pseudomonas fildensis, Bacillus subtilis, Achromobacter xylosoxidans, and Bacillus licheniform), Mix B3 (
Pseudomonas putida, Acidithiobacillus ferrooxidans, Bacillus velezensis, Bacillus subtilis, Bacillus methylotrophicus, Mcrobacterium testaceum), and non-inoculated controls (B0). The bacteria were strategically inoculated around the plant root perimeter within the substrate. Each plant received 20 ml of the biofertilizer. The bacteria utilized in this experiment were procured from the Soil Biology and Biotechnology Laboratory at the Golestan Agricultural and Natural Resources Research Center LBSG, specifically identified by isolate number 041011 in the laboratory bank. The bacteria were extracted from the rhizosphere of agronomy plants like soybean and wheat
[31]. Except for the non-inoculated controls, the substrates around each plant specimen were inoculated with 20 ml of bacterial suspension in the irrigation water to obtain a bacterial concentration of 108 CFU/ml. The biofertilizer was injected evenly onto the substrate surface around each plant's root zone using a syringe, two weeks after planting. The plants were cultivated in March 2022.
To prevent the leaching of the inoculated bacteria from the planting beds, the experimental units were irrigated up to 80% of the field capacity every 7 days in spring, every 3 days in summer, and every 10 days in winter (maintaining 300 ml for each plant). To ensure maintaining the bacteria within the soil system, irrigation was controlled to prevent drainage flow.
2.2.3 Selected Ornamental Hyperaccumulator Plants
The thoughtful selection of plant diversity, as evidenced in the study of Nektarios et al.
[32], facilitates not only survival and growth but also flowering performance. There are documented cases where crassulacean acid metabolism (CAM) plants such as succulents, exhibit resilience in water-limited conditions. In this study, two plants from the CAM species and two grass species were specifically chosen to achieve a judicious plant diversity, catering to both aesthetic considerations and overall ecological performance (Fig.3, Fig.4). Additionally, for the first time, an attempt was made to challenge the cold-season grasses and their sustainability during the warm season, alongside CAM plants. The plants selected in this research are particularly effective for phytoremediation of pollution, and may have a stabilizing phytoremediation effect for areas contaminated by metals.
1)
Festuca ovina (G1): It is a native evergreen grass consisting of a loosely tufted, soft yet firm clump of needle-like leaves 10 ~ 25 cm long, forming a singular mass
[33].
2)
Ophiopogon japonicus (G2): It is an evergreen perennial plant, native to China, Vietnam, India, and Japan, and hardy and semi-hardy to cold, tolerating temperatures down to −5℃
[34].
3)
Aptenia cordifolia (G3): It is a succulent, perennial plant abundantly used in green spaces in southern Iran. This plant is resistant to heat and drought, with light pink, white, and red flowers that bloom throughout the year. Full sun exposure is needed
[35].
4)
Carpobrotus edulis (G4): It is a succulent, creeping perennial plant, native to South Africa, and thrives in full sun exposure and extremely hot, dry conditions. It adapts to various soils but prefers well-drained conditions
[36].
2.3 Measurements
The aesthetic quality of the plants was evaluated once per month by four trained evaluators (two female and two male, who were familiar with horticulture and landscape) with the evaluation criteria proposed by Monterusso et al.
[37]. Accordingly, the aesthetic quality of plants in each experimental unit was scored from 1 to 5, where 1 represents severely stressed and completely dry; 2 represents stressed, less than 50% of leaves contain green pigment; 3 represents mildly stressed, 50% of leaves have green pigment; 4 represents low stress, more than 50% of leaves appear healthy; and 5 represents no stress, all leaves appear healthy. At the end of the experiment, plants that were completely dry were reported as dead (Tab.1).
It is worth mentioning that each month the aesthetic quality of both grasses were assessed by the same trained evaluators according to the National Turfgrass Evaluation Program (NTEP) guidelines
[38]. It applied the visual grading system with scores ranging from 1 to 9. The aesthetic quality of the grass was evaluated with a personal estimation of visual attributes including establishment, density, color, texture, softness, and thatch (Tab.2).
Except for the aforementioned indicators, additional measurements were necessary to quantify plant distribution and spatial growth patterns, providing a comprehensive assessment of plant performance in the green wall system. Shape, representing the overall quality of the plant, was graded by the evaluators based on factors such as visual appearance, uniformity, and plant vigor. Plant coverage, meaning the horizontal and vertical coverage of plants for each experimental unit, was measured using 20 cm × 20 cm quadrats. Each quadrat was divided into 100 compartments (2 cm × 2 cm). Survival percentage represents the percentage of plant survival days to the total days of the experiment
[39], which was measured at the end of the experiment. Weeds evaluated in the experimental units were monitored monthly with manual counting
[40]. The substrate settlement, the reduction in substrate volume over time, was measured using a straight line from the surface of the substrate to the edge of the pot.
For statistical analysis, JMP 8 software was employed. Data analysis was conducted using analysis of variance (ANOVA) and mean comparison with Tukey's test at a significance level of at least 5%.
3 Results
3.1 Mean Comparisons of Irrigation Water Types and Bacteria Strains on Plants
The ANOVA results (Tab.3) for Festuca ovina revealed that all traits, except those related to texture, and the number of weeds, exhibited statistically significant differences. Regarding substrate settlement, only the simple effect of irrigation water showed a significant difference. In traits of softness, color, overall quality, and thatch, in addition to the simple effects of bacteria, the interaction effects of irrigation water and bacteria treatments were statistically significant.
According to ANOVA results (Tab.4), the qualitative data of Ophiopogon japonicus suggested that the simple effects of irrigation water and bacteria were significantly different in the overall quality. Meanwhile, for establishment, density, and color, only the effects of various bacterial strains were found to be significant. In terms of plant coverage, softness, and substrate settlement, solely the simple effects of irrigation water were statistically significant. However, when considering softness, thatch, and number of weeds, not only were the simple effects of irrigation water and bacteria significant, but their interaction effects also proved to be statistically significant. It is noteworthy that for survival percentage, the simple effects of irrigation water and bacteria did not exhibit statistically significant differences, as all tested plants demonstrated a 100% survival rate.
The ANOVA results (Tab.5) for Aptenia cordifolia showed that the simple effects of irrigation water have a significant difference in leaf rolling. The simple effects of irrigation water and bacteria also demonstrated significant differences in plant coverage and substrate settlement. Additionally, for the indicators of shape, flowering, and landscape performance, both the simple effects of bacteria and the interaction effects of irrigation water and bacteria were statistically significant.
For ANOVA results of Carpobrotus edulis (Tab.6), significant differences were evident in the simple effects of irrigation water and bacteria in substrate settlement. Moreover, for leaf rolling, not only were the simple effects of bacteria significant, but their interaction effects were also proved to be statistically significant. Furthermore, for traits related to shape and landscape performance, only the simple effects of bacteria demonstrated a significant difference. However, in terms of plant coverage, both the simple effects of bacteria and the interaction effects of irrigation water and bacteria showed a statistically significant difference.
3.2 Qualitative Evaluation
3.2.1 The Effect of Irrigation Water Types and Bacterial Strains on Festuca ovina
In Festuca ovina, irrigation with wastewater and graywater led to higher establishment and density compared with urban water (Fig.5). Additionally, the control treatment (without bacterial application) performed better in increasing density and establishment for urban water with the presence of Mix B2 and Mix B3 (Fig.5, Fig.5). Fig.5 ~ Fig.5 show that the plant coverage, color, overall quality, and thatch are associated with the treatment of PGPB; in addition, irrigation with all three different water types inoculated with bacterial strains did not result in a significant difference. It is noteworthy that the lowest quality in terms of thatch amount was observed in urban water without bacterial presence. Regarding substrate settlement, graywater irrigation resulted in the highest amount of substrate reduction compared to the other two sources (Fig.5). It seems that irrigation with wastewater in Festuca ovina performed similarly to urban water, which did not have significant effect on substrate settlement.
3.2.2 The Effect of Irrigation Water Types and Different Strains of Bacteria on Ophiopogon japonicus
Results from the analysis of the mean square effects of bacterial strains on Ophiopogon japonicus showed that Mix B1 has been effective in increasing establishment and density (Fig.6, Fig.6). On average, the Mix B1 increased 57.1% of the establishment and 37.1% of the density, compared with B0, suggesting that the presence of PGPB benefits density and establishment of Ophiopogon japonicus. Regarding plant coverage (Fig.6), softness (Fig.6), texture (Fig.6), and overall quality (Fig.6, Fig.6), irrigation with wastewater and graywater has created better performance compared with urban water. Inoculation of the substrate with Mix B1 increased softness, color, and overall quality compared with B0 (Fig.6, Fig.6, and Fig.6). It is worth noting that the control treatment (urban water and B0) could not effectively enhance the qualitative traits of the plant (Fig.6 ~ Fig.6).
Fig.6 illustrates that the highest amount of thatch is related to B0 and urban water irrigation. However, statistically, the inoculated substrate with all bacterial strains with wastewater and graywater performed similarly, producing less thatch compared with B0. Additionally, the smallest number of weeds was observed in B0 with urban water irrigation (Fig.6). Irrigation with wastewater created the least substrate settlement, while graywater performed similarly to urban water without creating a statistically significant difference (Fig.6).
3.2.3 The Effect of Irrigation Water Types and Bacterial Strains on Aptenia cordifolia
The results suggested that bacterial application has induced a statistically significant difference in the shape of Aptenia cordifolia and its landscape performance compared with B0 (Fig.7, Fig.7), resulted in a more aesthetically pleasing and appealing shape. Statistically, treatments involving Mix B3 and wastewater were found to be more effective in flowering (Fig.7). The use of Mix B1 and urban water resulted in the smallest number of flowers and a lower statistical rank than B0 in terms of flower formation. Regarding leaf rolling, wastewater led to more twisted yet healthy leaves (Fig.7). As depicted in Fig.7 ~ Fig.7, the treatment involving Mix B3 and wastewater produced a larger coverage area and minimized substrate settlement.
3.2.4 The Effect of Irrigation Water Types and Bacterial Strains on Carpobrotus edulis
Fig.8 shows that for Carpobrotus edulis, inoculation of the substrate with Mix B2 and B3 led to the creation of a more suitable plant shape, while the Mix B1 was not very successful in shaping, with a similar effect to B0. Plants irrigated with urban water and B0 treatment showed more leaf curling (Fig.8). As showed in Fig.8, the application of PGPB created more visual appeal and a more attractive landscape view than B0. It appears that the inoculation of bacterial strains into the substrate, coupled with all three water types, proved successful in augmenting the plant coverage area in Carpobrotus edulis compared with B0 (Fig.8). Notably, irrigation with wastewater, and application of Mix B1, yielded the least soil settlement, while B0 and urban water exhibited the highest substrate settlement (Fig.8, Fig.8).
4 Discussion
In this study, a combination of bacterial strains and the utilization of wastewater were employed to fulfill the nutritional requirements of plants, offering a viable alternative to chemical fertilizers. The flowering of various plant species on green wall was introduced as a decorative element. The inclusion of Mix B3 in the substrate and irrigation with wastewater resulted in increased flowering in
Ophiopogon japonicus and
Aptenia cordifolia, while
Festuca ovina exhibited enhanced flowering in Mix B3 substrate coupled with graywater irrigation. The advantages of using ornamental plants with aesthetic features are evident, as their leaves, flowers, flower patterns, fruits, and fragrance visually please people, adding scenic value to it
[41]. Utilizing ornamental species in wastewater treatment systems provides an efficient means of removing pollutants
[42]. Furthermore, this can significantly enhance the visual quality of green wall landscape, which is often undervalued but with positive psychological and social implications in daily life
[43]. In this study, the plants with the lowest visual appeal determined by statistical ranking were cultivated in a substrate without bacteria and irrigated with urban water. Remarkably, the introduction of bacterial strains resulted in a significant enhancement in the coloration of the plants. The most aesthetically pleasing experimental units, featuring succulent plants, were cultivated in a medium enriched with Mix B2 and Mix B3. Conversely, for slender grassy plants during colder seasons, the highest visual quality was observed in the medium containing bacteria B1 and graywater irrigation. It appears that the use of bacterial strains and recycled waters significantly enhances the aesthetic value of plants. In this experiment, the presence of bacteria in the growing medium seems to have resulted in increased nutrient availability for the plants, thereby substantially improving growth characteristics.
Moreover, it was able to enhance visual and qualitative characteristics such as leaf color, flowering, scenic quality, and coverage. The use of PGPB in cultivating plants on outdoor green walls increases their growth and health. Martinez-Garcia et al. demonstrated the positive impact of PGPB on nutrient absorption and overall plant performance
[44]. These beneficial bacteria establish symbiotic relationships with plants, promoting root growth, nutrient absorption, and resistance to diseases.
One of the most important physiological traits of bacteria that facilitate plant growth is the presence of the enzyme 1-Aminocyclopropane-1-Carboxylate (ACC) deaminase
[45]. This enzyme supports plant growth under stress conditions. Research has shown that PGPB possessing ACC deaminase can break down ACC, the precursor of plant ethylene, into ammonia and α-ketobutyrate, thereby reducing the level of ethylene production in stressed plants. In other words, these PGPB act as a sink for ACC
[46]. It appears that the bacterial strains in this study, in addition to being selected for their ability to absorb metals and pollutants in plants, also contributed to plant growth and stability under the stressful conditions of vertical green systems and visual appeal enhancement by generating more biomass.
Regarding the selection of plant cover, attention to factors such as root depth and plant productivity is crucial
[47]; and it is a primary role of plant cover to create a habitat for microorganisms
[48]. Plant cover transports a limited amount of oxygen to the root zone, allowing aerobic bacteria to establish in the root area and decompose organic matters
[49]. Nutrients and organic matters are absorbed and decomposed by densely populated microbial communities
[50]. The presence of PGPB enhances the resilience and adaptability of plants, enabling them to thrive even in challenging urban environments.
5 Conclusions
The research on the aesthetic quality of outdoor green walls explored the impact of PGPB and recycled water types. Key findings include the enhancement of plant health and appearance through PGPB application and the effectiveness of certain recycled water types in supporting plant growth without compromising aesthetics. The results of this study demonstrate that substrates without bacteria alone cannot establish optimal plant growth. However, in combination with bacteria and the use of wastewater and graywater, these substrates can enhance the survival and growth of plants on green walls. Substrates containing mixed strains of bacteria significantly outperformed the control substrate, with the highest coverage area (the most critical factor for green walls) obtained from the bacterial-inoculated substrates with Mix B2 and Mix B3. Among the studied water types, wastewater showed the best performance. A nutrient-rich substrate can be employed to create high-quality green walls with aesthetic traits such as good coverage, establishment, and optimal growth. The outcomes of this research underscore the potential of recycled waters as a sustainable and enduring option, contributing to the conservation of water resources and diminishing reliance on fresh water sources. Effective treatment and management of recycled water emerge as crucial factors in maintaining plant health and fostering the long-term sustainability of outdoor green walls. The findings of this study suggest that planting Aptenia cordifolia instead of less resilient plants, with the of substrate inoculated with Mix B3 and wastewater irrigation, can achieve better aesthetic appeal in outdoor green walls, particularly in dry and semi-arid climates. Outdoor green walls, with their aesthetic appeal, offer a promising approach to purifying graywater and wastewater in urban landscapes. However, the widespread acceptance of green walls for recycled water treatment hinges on their efficacy in removing emerging pollutants from recycled water sources. This study highlights the innovative combination of PGPB and recycled waters, offering a sustainable, eco-friendly approach to creating urban green walls. Overall, the research contributes to urban gardening by demonstrating how biological treatments and water recycling can improve both the sustainability and visual quality of green infrastructure.
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