The population ratio of urban areas in 2016 is 91.82%, which shows that about 92% of the entire population is living in urban areas (residential, commercial, industrial, green areas) in terms of use zoning. It is annually increasing from 90.1% in 2005, and the index by Statistics Korea expects constant growth in the future as well (Statistics Korea, 2017). Development has been continuously made as the population is concentrated into the cities, and natural environments including green spaces are disappearing in this process. Recently there have been active movements to find or create natural environments in the city as their importance is being emphasized as restorative environment. However, it is difficult to find the appropriate space in areas not designated as parks or greens by use zoning. To solve this problem, it has become an alternative to use artificial ground like rooftop and vertical greening or create stereoscopic green space.

In particular, urban street space is a public space used by many people, but it is difficult to secure green spaces that provide amenity, aesthetics and ecological property. Since streets perform a key role in determining urban image by building the frame of the city, it is important to implement greens in street space. To this end, natural elements like street trees, flowerbeds and flower pots are actively implemented.

Container gardens where plants are cultivated in containers can plant a variety of different plants considering aesthetics and functionality because they can provide the planting base in a space that cannot secure planting space (Oh, 2013). Most local governments create container gardens in streets where planting is difficult. Seoul invested 21.256 billion KRW from 2008 to 2020 to create year-round flower streets (hanging flowerpots, street flowerpots, theme flower gardens, etc.) around areas with a lot of traffic in the city and areas that are visually poor (Seoul Metropolitan Government, 2017). Other local governments are also investing human resources and budgets into replacing plants while considering annual changes of the seasons. Most container gardens managed by local governments are annual plants with long flowering duration and distinct colors, with great consideration of the aesthetic aspect. On the other hand, for private spaces that are not containers in public spaces, green areas are formed mostly with evergreens but with minimized maintenance. In this aspect, it is necessary to select adequate plants according to the locational properties and functions of container gardens.

Various factors are to be considered in implementing plants for container gardens in the urban environment, but so far studies on container gardens have been focused on aesthetics such as floral designs to improve the urban environment (Kim, 2008), types and designs of plants on roadside flowerbeds (Byun, 2012), and public urban design applying flowering plants (Kim, 2010). However, it is also important to create an environment for plants to grow well in container gardens. The limited amount of soil in urban trees fails to sufficiently fulfill the needs for evapotranspiration, thereby resulting in poor survival and growth of trees (DeGratano, 2000). In other words, physical properties such as moisture content of soil, density, air permeability and temperature as well as chemical features such as pH, fertility and salinity affect the growth of plants (Thorup, 1969; Adams et al., 1967; Mergel et al., 1974; Gerard et al., 1982). Moreover, unlike plants that are planted in the ground, container gardens are plantd in limited space, thereby requiring constant replacement of containers due to the plant growth. Container gardens in the urban environment provide amenity to the urban environment and visual satisfaction to urban dwellers, and thus they must be maintained so that their aesthetics are not deteriorated. Maintenance must take financial aspects into account, including proper overhead flooding, fertilization management, grading and pruning, as well as selection of plants to grow well in limited containers and constant replacement.

Therefore, this study intends to provide basic data required for maintaining growth health of woody plants while considering aesthetics and economic feasibility in creating container gardens at an initial stage by determining the growth characteristics of plants for soil depth of each container size.

This study is to determine the appropriate soil depth for each species at the initial plantation in containers by examining plant height, plant root collar diameter, biomass, and ornamental value of plants that can be used in container gardens according to soil depth. The minimum soil depth for each species must be decided considering multiple aspects such as growth conditions like plant height and plant root collar diameter, as well as photosynthesis output and visual quality in terms of ornamental value. Pinus densiflora, Pinus parviflora, and Nandina domestica showed poor growth and low ornamental value in 15 cm soil depth, and thus can be planted only in containers with at least 20 cm soil depth. Thuja occidentalis can be grown in 15 cm and has above-average ornamental value, but is desirable to plant in 20 cm. Syringa vulgaris showed almost no difference according to soil depth and thus can be planted in containers with at least 15 cm soil depth, but it would be favorable to grow in at least 20 cm considering ornamental value. Ilex serrata can grow in at least 20 cm soil depth but has overall low ornamental value, thereby not suitable for container gardens.

Since soil depths vary according to container types, it is necessary to choose containers that maintain suitable soil depth for each tree in order to facilitate growth and maintenance. To this end, the plants must be transplanted into a bigger container according to their growth. However, since it is not easy to change the size of containers set up outdoors, the plants must be replaced to younger ones after a certain period of growth. This study has significance in that it presented trees to apply according to soil depth by determining the growth health for each species in minimum soil depth of containers at the initial stage of building container gardens. However, its limitation is that the results come from 2 years of growth. Follow-up research must continue to verify the changes in growth according to soil depth.