Introduction
Urban agriculture has received a lot of attention in the last decade. The demographic shift that will result in two-thirds of the world's population living in cities by 2050, as well as concerns about sustainable food supply for this growing urban population, are catalysts for this attention (Goodman and Minner, 2019). In addition to the simple function of food production, urban farming can have effects such as saving the energy involved in food transportation and reducing carbon dioxide emissions and waste by consuming locally produced food (Son et al., 2013), thereby decreasing the environmental burden while increasing climate resilience (Aggarwal et al., 2023). Notably, the continued expansion of urban agriculture has the potential to provide $80–160 billion worth of health and environmental services annually (Clinton et al., 2018). However, the difficulty of securing a spatial base due to land scarcity and rising land prices in urban areas is a substantial obstacle to the revitalization of urban agriculture (Geum, 2013).
In cities where it is difficult to secure green space, the walls and rooftops of buildings are being used as spaces for agriculture (Oh and Choi, 2021). As this method of space utilization can be a practical response to limited urban space and food security issues (Nethmini Sashika et al., 2024), it is presented as an alternative form of urban agriculture that utilizes vertical urban space as green space (Son et al., 2022). Moreover, urban agriculture using surplus space in buildings can contribute to the expansion of the ecological environment, including energy savings and providing habitat for living organisms (Russo and Cirella, 2018). Rooftops and walls have limitations in that it is difficult to ensure the sufficient soil depth that plants prefer, depending on their spatial characteristics, but these can be easily overcome with proper nutrient, moisture, and pest management (Walters and Midden, 2018). In conventional agriculture, farmland and crops are managed primarily with chemical products, including chemical fertilizers, pesticides, and herbicides. However, compared to conventional agriculture practiced on farmland, such urban agriculture needs to consider different approaches and environmental characteristics (Sanyé-Mengual et al., 2019); in particular, as the demand for organic food increases and the use of chemicals is restricted (Appolloni et al., 2021) discussions on new technologies are necessary.
Companion planting is an agricultural technique that involves planting two or more plant species together. It is an environmentally friendly farming practice that controls plant diseases, fixes organic matter and nitrogen in the soil, increases moisture and nutrient retention, and suppresses weeds (Tringovska et al., 2015). In general, companion plants can attract beneficial insects or repel pests (Hicks et al., 2018), provide shade for the propagation of creeping crops, and act as natural supports to aid the growth of co-planted crops (Hong et al., 2022). They can also help create a good soil environment for plant growth by causing physicochemical changes in the soil (Chang et al., 2017).
Since the 1980s, a wide range of research has been conducted that aims to improve the productivity of edible crops, including identifying the effect of companion plants on pests and evaluating the effect of mixed cropping on reducing pest outbreaks (Balmer et al., 2014). Conboy et al. (2019) reported the pest control effect of planting Tagetes erecta as a companion plant to tomatoes. Matsuoka et al. (2020) found that there was a positive effect on growth and soil moisture content when plants of the family Lamiaceae and species of Sedum were planted together in a rooftop environment. Despite the many benefits of planting two different species together, competition between them for light, moisture and nutrients may be inevitable (Tringovska et al., 2015), so selecting the appropriate species and determining the appropriate ratio is essential.
The development of vegetation for urban greening requires plants with high ornamental value that can adapt to low soil depths of 20 cm or less and to extreme climates, such as extreme differences between hot and cold, and that can adapt to and survive strong solar radiation and wind (Shim et al., 2011). Since nasturtium (Tropaeolum majus) is edible and has a high ornamental value with its beautiful color, scent, and appearance (Barrantes-Martínez et al., 2022), it is very likely to be used as planting material in urban areas. It has been planted and studied mainly as a companion plant to leafy vegetables such as lettuce, Napa cabbage, and celery in arable land (Hong et al., 2022). Cherry tomatoes (Solanum lycopersicum), which are small round fruits with a diameter of 2–3 cm, are one of the most popular vegetable crops in the world, with soft flesh and high sugar content (Kim et al., 2020). Demand for them in the Korean market continues to grow, overtaking that of regular tomatoes (Lee, 2018).
Therefore, in this study, we aimed to propose a combination of companion plants that can be used in building-integrated urban agriculture by comparing and analyzing the growth characteristics of cherry tomatoes (CTs) on rooftops and walls depending on the planting ratio when planted together with nasturtium. Furthermore, we sought to provide basic data for the revitalization of eco-friendly urban agriculture by suggesting appropriate spatial conditions for companion planting of cherry tomato and nasturtium on rooftops and walls.
Research Methods
Conditions and materials
This experiment was conducted on the wall of the greenhouse for related majors and the rooftop of the Design Lab at the Glocal Campus of Konkuk University from May to July, 2023. The environmental conditions of the wall during the experiment period were an average temperature of 23.6 °C, a maximum temperature of 53.9 °C, a minimum temperature of 4.7 °C, and a relative humidity of 75.6%. For the roof, the environmental conditions were an average temperature of 27 °C, a maximum temperature of 58.9 °C, a minimum temperature of 10.0°C, and a relative humidity of 66.6%; showing a higher temperature and a lower relative humidity compared to the wall.
The soil used in the experiment was bed soil (Hanpanseung, Samhwa, Korea), with perlite (New–perls hine, GHC, Korea) as a drainage layer. Plant materials were purchased from a pesticide company and a flower garden in Chungju City in May 2023: cherry tomato cultivar "Dadagi" (Hungnong, Korea) with a pot diameter of about 6 cm and nasturtium with a pot diameter of about 9 cm.
Experimental design
The experimental pots, made of plastic, were 60 cm wide, 20 cm long, and 15 cm high. Non-woven fabric (CG, Korea) was placed on the bottom of the pots to prevent soil loss, followed by 2 cm of perlite as a drainage layer and 12 cm of bedding soil as a planting layer. The experimental plots were divided into five categories according to the planting ratio of CTs and nasturtiums: CC (when only CTs were planted), C2N1 (ratio of CTs to nasturtiums: 2:1), C1N1 (1:1), C1N2 (1:2), NC (when only nasturtiums were planted). A total of 12 plants were planted in each pot, and three pots in each group were replicated and randomly placed. A total of 30 experimental plots were established, 15 in each experimental space (greenhouse wall and rooftop). Irrigation was performed once every three days to prevent the soil from drying out.
Growth Assessment and statistical analysis
To assess the growth of CTs, plant height, leaf length, and leaf width were measured every two weeks starting 14 days after planting in the experimental pots using a 30 cm stainless steel ruler (SSRP-300, SB, Korea) and electronic calipers (Vernier calipers, Mitutoyo, Japan), and the number of leaves was counted visually. Fresh weight was measured using a microelectronic balance (FX-200i, AND, Korea) after the plants were collected after 70 days of growth, the soil was removed and the shoots and roots were divided. Dry weight was measured after the collected plants were dried in a forced convection oven (C-DF, Changshin Sci, Co., Korea) at a temperature of 70 °C for 72 hours. For the internal and external quality assessment of the CT fruits, the first harvest was made about 42 days after planting, when they began to ripen, and the second and third harvests were made at two-week intervals thereafter, and the number, diameter, weight and sugar content of the fruits were measured; a sugar refractometer (Hand Refractometer, Atago, Japan) was used to measure the sugar content. Nasturtium growth was assessed by measuring plant height, and length, width and number of leaves, and number and width of flowers. The number of flowers was counted visually based on fully open flowers.
Statistical analyses in this study were performed using the SPSS Statistics 25.0 (SPSS Inc, Chicago, IL USA) program. One-way ANOVA was performed to confirm the significance of each planting ratio, and Duncan's new multiple range test (MRT) was performed at the p < .05 level as a post hoc test. In addition, an independent samples t-test (p < .05) was performed to compare and analyze the growth of cherry tomato plants and their fruits on the rooftop and wall.
Results and Discussion
Growth and fruit quality of cherry tomatoes (CTs) by planting ratio compared to nasturtiums
Rooftop
Plant height of CTs grown on the rooftop gradually increased with time in all planting ratios, with the highest height observed in CC, where only CTs were planted(Fig. 1). Moreover, when nasturtiums were planted as a companion plant, it was found to be beneficial for the length growth of CTs when the planting ratio of CTs exceeded that of nasturtiums; approx. 70 days after planting, the plant height of CTs in C2N1 was 52.95cm, which was the largest of the companion planting treatment plots. Leaf length and width decreased rapidly starting 42 days after planting, and there was no significant difference in the means by planting ratio thereafter. The number of leaves increased in all treatment plots other than C1N2 until the third measurement, and then gradually decreased. The number of cherry tomato leaves had a positive correlation with plant height (Noh and Lee, 2020). Like the plant height, the number of leaves of CTs planted on the rooftop was highest in CC, followed by in C2N1, C1N2, and C1N1. As for fresh weight, C2N1 was found to be the heaviest with 17.32g and 12.86g for both shoots and roots, respectively, but this finding did not have statistical significance. Dry weight also showed similar results. A previous study in which CTs were planted with basil as a companion plant (Ju et al., 2021) found that the higher the planting ratio of CTs, the better the growth of CTs. This is somewhat consistent with the results of this study. When planting CTs with other plants, it seems to be beneficial for the growth of CTs to plant them in larger numbers than the companion plants.
The results of an internal and external quality analysis of CTs on the rooftop are shown in Fig. 2. Fruit diameter and weight tended to gradually decrease in all planting ratios. When only CTs were planted (CC), the fruit diameter and weight decreased by about 0.63 cm and 2.87 g, respectively, from the second to the third harvest, which was the greatest decrease. As with the growth results, the number of fruits was highest in C2N1, which had a higher CT planting ratio, but common to all planting ratios, it decreased rapidly 56 days after planting. In general, changes in tomato leaf length are known to affect yield (Wi et al., 2021), and it seems that the reduction in leaf length had some effect on the decrease in fruit number of cherry tomatoes planted on the rooftop. Sugar content gradually decreased in the experimental plots of all planting ratios from the first harvest period, but was found to increase from 7.37 Brix to 7.95 Brix in C2N1 at the final harvest. Considering that the sugar content of tomatoes increases as they mature (Lee et al., 2022), it seems likely that CTs will reach a ripeness suitable for harvesting when CTs and nasturtium are planted at a 2:1 ratio.
Wall
The growth characteristics of CTs planted on the wall according to their planting ratios with their companion plant, nasturtium, are shown in Fig. 3. Plant height of CTs increased rapidly between 14 and 24 days after planting, but was maintained at a similar level thereafter. When CTs were planted in a 2:1 ratio with nasturtium (C2N1), they were the longest at 59.33cm around 70 days after planting, a finding which was significant at a 95% confidence level. Leaf length decreased rapidly at 42 days after planting, with no difference between experimental plots shown depending on the planting ratio, and leaf width showed a similar trend to leaf length. It has been reported that tomato leaf area is related to photosynthesis and crop production and affects fruit yield and quality (Kläring and Kumbein, 2013). At 56 days after planting, a decrease in leaf length of 2.25 cm and 1.05 cm was found in C1N1, which had the same ratio of nasturtium to CT, and C1N2, which had a higher ratio of nasturtium, respectively. It seems that this would have a negative effect on the fruit production and quality of CTs. As for the number of leaves, it was advantageous to plant only CTs, and when planted together with nasturtium, C2N1, which has a higher ratio of CTs, showed the greatest number of leaves among the planting ratios. Fresh weight of both shoots and roots of CTs was found to be the heaviest in C1N2, where nasturtiums, a companion plant, were planted at twice the ratio of CTs. Dry weight also showed a similar trend, but there was no significant difference in the means. It was also found that increasing the planting ratio of CTs compared to nasturtiums was beneficial for the growth of CTs in terms of plant height, and leaf length and width.
Fruit diameter of CTs generally tended to decrease from that at the first harvest at all planting ratios, and the largest decrease among the planting ratios was found at C1N1, approx. 1.2 cm(Fig. 4). Fruit weight also showed a similar trend; when nasturtiums were planted at twice the ratio of CTs, the largest and heaviest fruits were harvested. The number of fruits was reduced in the second harvest compared to the first harvest, except in the treatment plot where only CTs were planted. When only CTs were planted, approx. 6.47 fruits were harvested in the first harvest and 8.05 fruits in the second harvest. This was in contrast to the results at other planting ratios, where competition for light and nutrients was inevitable due to the companion planting of nasturtiums (Matsuoka et al., 2020). The sugar content of CTs was similar at most planting ratios, but in C2N1, it was found to increase by approx. 1.17 Brix, from 6.98 Brix at the 2nd harvest to 8.15 Brix at the 3rd harvest as they matured.
Growth of nasturtiums according to planting ratio
When nasturtiums were planted as a companion plant to CTs, the plant height of nasturtiums was highest in the C1N2 planting ratio on both the rooftop and the wall, at 17.50 cm and 24.83 cm, respectively(Table 1). Leaf length of nasturtiums was longest in C2N1, where CTs and nasturtiums were planted in a 2:1 ratio on the roof, and in a 1:1 ratio, leaves did not grow sufficiently long. This trend was also found for nasturtiums planted on the wall. On the rooftop, nasturtium leaf width was greatest in C2N1, as was leaf length, whereas on the wall, planting only nasturtiums was most beneficial for leaf growth, but this finding was not statistically significant. The number of nasturtium leaves was 33.53 and 31.48 on the roof and wall, respectively, and was highest when nasturtiums alone were planted compared to when planted with CTs. The number of flowers was 1.5 times greater or more in NC where only nasturtiums were planted on the roof and wall, whereas the flower width was the greatest at 5.81 cm and 5.12 cm on the rooftop and wall, respectively, when CTs and nasturtiums were planted in a 1:1 ratio. Overall, when only nasturtiums were planted (NC) on both the roof and the wall, it was advantageous for nasturtium growth, but the difference in growth between C2N1 and NC on the roof was found to be minimal. Since nasturtiums show good growth from May to mid-July when high temperatures persist (Hong et al., 2022), it seems desirable to plant them as a companion plant during this period. Notably, when planted together with CTs on rooftops and walls, it appears that keeping the ratio of nasturtiums to CTs at a level that does not exceed the ratio of CTs is good for the growth of nasturtiums so that nasturtiums can fully play their role as a companion plant to CTs. Moreover, with regard to the productivity of edible nasturtium flowers, additional harvest efficiency can be expected if they are planted in an appropriate ratio for each space (Park et al., 2022).
Comparison on rooftop and wall based on growth and fruit quality of cherry tomatoes
The results of an independent samples t-test conducted at a 95% confidence level are shown in Table 2 to compare the growth characteristics and fruit quality of CTs according to building space. As for the height of CTs, it was found that planting them together with nasturtiums on the wall was beneficial for their growth. Leaf length and width of CTs were found to be relatively greater when planted on the wall in all planting ratios, and this finding was statistically significant except for C1N2. As for the number of leaves, it was found that when the CT planting ratio was higher, planting on the roof had a positive effect on increasing the number; when the nasturtium planting ratio was higher, planting on the wall had a positive effect on increasing the number. Both the fresh and dry weight of CTs were greater on the wall than on the roof, and it appeared that planting nasturtiums on the wall at twice the ratio of CTs would help increase the weight of CTs. Fruit diameter of CTs was greater when planted on the wall in all planting ratios, except CC, where only CTs were planted. Fruit weight of CTs grown on the wall was also heavier than that of CTs grown on the rooftop. On the other hand, CT yield was higher on the rooftop than on the wall, but this finding was not statistically significant. In contrast to the number of fruits, the sugar content was higher on the wall, a finding that was found to be significant at the 95% and 99% confidence intervals for C2N1 and C1N1, respectively. Overall, when planted together with nasturtiums, CTs appear to be better grown on walls than on rooftops in terms of growth and fruit quality. Given that, especially in tomato production, low production per cultivation area and high sugar content fruits lead to high profits (Le et al., 2018), it seems that growing cherry tomatoes and nasturtium on the wall in a 2:1 ratio is economically advantageous for growers.
Conclusion
To support the efficient production of crops using buildings in urban areas, this study was conducted to evaluate plant growth and crop quality according to planting ratios of cherry tomatoes (CTs) and nasturtiums by building space, with the aim of providing basic data for promoting environmentally friendly urban agriculture.
It was found that the growth of CTs on the rooftop was generally best when CTs alone were planted, but when planted together with nasturtiums, CTs had the heaviest weight when planted at the ratio of C2N1, where the planting ratio of CTs was greater than that of nasturtiums. However, the fruit size of CTs was largest when the planting ratio of cherry tomatoes to nasturtiums was 1 to 2 (C1N2), while sugar content was highest in C2N1, as was the growth. A similar trend was found on the wall; plant height and leaf length and width, which can affect fruit quality, were found to be greatest in C2N1, where CTs were planted at a higher ratio than nasturtiums. Leaf and flower size of nasturtiums was found to be greatest when CTs and nasturtiums were planted at a 2:1 ratio on the rooftop. On the wall, they also showed the best growth in a C2N1 planting ratio in all aspects other than leaf width. By comparing the growth and fruit quality of CTs between on the rooftop and the wall, it was found that plant height, leaf length, fresh weight, dry weight, fruit diameter, and sugar content were relatively greater when planted together with nasturtiums on the wall than on the rooftop.
Based on a comprehensive analysis of CT growth and crop quality, it seems most desirable to plant CTs and nasturtiums, a companion plant, in a 2:1 ratio on the walls. Moreover, the growth of nasturtiums is also excellent in this ratio, so it is possible to ensure additional productivity of nasturtiums as an edible flower. However, it appears that moisture and nutrients need to be supplemented to improve the external quality of the crop, including the size and weight of the fruits. This study is significant in that it lays the groundwork for the establishment of sustainable, environmentally friendly agricultural technology, which needs to be developed due to the high demand in urban areas. However, the spatial characteristics of urban agriculture using buildings require the presentation of effective management plans for different crops. In addition, further research should be conducted based on the qualitative and quantitative analysis of fruit and seed quality.
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