Guided Professional Tour of Ljubljana’s Urban plantings report

On Thursday, October 20^th 2022, representatives from a wide range of professions (planners, municipal service providers, contractors, landscape gardeners) met for the second professional guided tour in Ljubljana.

The site visit was led and moderated by Bernarda Strgar, MSc in Biology, who also completed postgraduate studies in garden design in London and has been engaged in integrated garden and public green space planning for over a decade. In addition, she manages a highly successful perennial nursery. The visit was co-led by Matjaž Mastnak, MSc in Forestry Engineering, a leading expert and dendrologist working at Arboretum Volčji Potok, as well as a highly respected lecturer and the author of numerous professional publications.

Trg Mladinskih Delovnih Brigad (MDB Square) – Existing Trees, New Plantings, and Microclimate

The site visit began at Trg Mladinskih Delovnih Brigad (MDB Square), which was renovated in 2021. The redesigned square combines preserved mature trees with newly planted trees and perennial beds. Retaining existing trees—where appropriate and in alignment with the overall design concept—is of great importance, as established trees are already well adapted to site conditions, offer numerous exosystemic services and generally perform better than newly planted specimens.

At MDB Square, birch trees were selected for the new plantings in order to match the existing tree population of birch (Betula pendula). However, this choice is somewhat problematic, as the square contains extensive paved surfaces that significantly increase ambient temperatures during the summer months. Birch trees are known to tolerate such heat stress poorly. For the newly planted trees, Himalayan birch (Betula utilis) was selected, which represents a more suitable choice given these site conditions.

Along the edges of the tree planting circles, creeping yew (Taxus baccata ˈRepandensˈ) and low-growing Japanese barberry (Berberis thunbergii) are planted surrounding inner areas with a diverse mix of perennials. The planting scheme combines ornamental grasses with flowering perennial species. The use of low-growing woody plants along the edges is intended to discourage visitors from stepping onto planted areas, thereby protecting the vegetation. It is therefore hoped that the planting of these edge species will be completed as soon as possible to ensure the long-term success of the design.

Roadsite avenue trees

We continued our walk along Tržaška Road, where we stopped to observe newly planted field maple (Acer campestre) trees. Field maple is very suitable for urban planting due to its strong tolerance of various stress factors commonly encountered in cities. From this perspective, the selection of maples along Tržaška Road is appropriate. This stop also prompted a discussion on suitable substrates for use in urban environments. Increasingly, engineered substrates are being used instead of existing soils. These substrates are typically based on specific additives designed to improve drainage and structural stability.

Along Tržaška Road, the so-called Stockholm tree planting system has been implemented. This method involves compacting a specially designed aggregate material into the ground, within which soil is incorporated. The soil retains moisture and provides excellent conditions for root growth, significantly improving the growing environment for the planted trees.

We continued our walk along Langus Street, where we encountered a mixed-species avenue planting – an increasingly common practice recently. Compared to traditional avenues composed of a single species—often even a single cultivar—mixed plantings have been shown to be more resilient to external stressors, including diseases and pests, and are therefore often more suitable for urban environments.

We then passed a small roundabout, where Acer platanoides ˈGlobosumˈ trees are planted in the center and surrounded by rose shrubs – an extremely demanding trait for maintenance. It was evident as the area was heavily overgrown with weeds.

Our next stop was a landscaped area adjacent to a private building. Several small planting beds containing only a few low-growing pine shrubs, with white gravel used as mulch. Due to the very limited number of plants, the gravel mulch visually dominates the planting. Such monotonous landscape designs are best avoided. Instead, the aim should be to create more dynamic plantings by incorporating a wider range of plant species, thereby contributing to greater biodiversity. The use of geotextiles should be avoided, both as mulch and as a separating layer between different materials (e.g., between gravel and soil), as was observed in this example. Recent experience shows that different material layers can be effectively separated without the use of textile materials.

Gradaščica Park – Unlocking Biodiversity Potential of urban riverbanks

We concluded the visit at Gradaščica Park along Barjanska Road. The location has significant potential for green space, which is currently underutilized. At present, the park contains a few large trees and several benches. Planting a greater number and variety of plant species would enhance biodiversity, attract a wider range of wildlife, and create a park environment of substantial value for local residents.

The participants of the tour agreed that contemporary urban landscaping should aim for highly functional spaces while prioritizing biodiversity and plant diversity.

Text by prof. dr. Gregor Osterc