Guided Professional Tour of Ljubljana’s Urban plantings report

The professional tour, organized by ZPORS and led by Matjaž Mastnak, was attended by 34 participants. The tour focused on the area around the Technology Park, where a new mixed tree avenue was established this year, featuring sweetgum (Liquidambar styraciflua), ginkgo (Ginkgo biloba), and Turkey oak (Corylus colurna). Sweetgum is increasingly recommended due to its tolerance of hot summers, and ginkgo is also considered a modern and resilient choice. Oaks (Quercus spp.), however, continue to present challenges, as they establish more slowly – especially when large-caliper trees (18/20 cm) are planted. Smaller stock (e.g., 10 cm caliper) would likely achieve better results but are not recommended for urban planting as they can be mechanically damaged or vandalized easily. Historical nursery literature even suggests direct seeding of oaks – it seems that the times were different then! Mixed-species avenues are beneficial, as the failure of one species does not compromise the entire planting. But in regard to the architectural features mixed avenues cannot be as potent as one-species avenues. The tree silhouettes are not uniform.

Participants noted that urban growing conditions for avenue trees are often inadequate. A two-meter-wide green strip, such as the one observed, is already considered generous; typically, planting strips measure only 1–1.2 meters in width, with soil depths of around 50 cm or less.

Proper planting depth is crucial, yet trees are frequently planted too deep. The root collar should be level with the soil surface. Trees are often set 10 cm or more below level, usually because soil subsidence is not anticipated. Planting pits should match the root ball depth (e.g., 60–70 cm for a 60 cm root ball) to ensure adequate aeration, as most tree roots (about 85%) develop within the upper 50 cm of soil where oxygen is available.

Protective tree collars are a simple and cost-effective measure against mowing damage, provided they are properly installed. Newly planted trees, especially larger specimens, require strong staking and flexible ties in a figure-eight form; these should be removed after one to two years once the tree is established. Properly shaped tree watering basins made of a berm of soil up to 15 cm high all around the root ball help direct water to the roots and reduce competition from turf.

Lichens on bark do not harm trees but may indicate reduced vitality. More intense leaf coloration in Norway maple (Acer platanoides) was observed primarily on nutrient-poor sites.

In front of a newly built residential building, trees were planted at least half a meter too deep into construction debris and spaced very densely to create a ‘forest effect’. Whether such plantings will succeed or become neglected thickets remains an open question.

In the inner courtyard formed by surrounding apartment buildings, we examined a dogwood (Cornus) group planting. The color of the shoots harmonized with the façade of the backdrop building; however, from our vantage point, the shrubs partially obscured the view of the children’s playground in the center. This raised the question of whether such visual screening might limit parental supervision. A discussion followed on whether contrasting planting would have been more effective, or whether a simple lawn might have been more appropriate in this setting. Concerns were also expressed about establishing a monoculture next to a playground and about long-term maintenance, as these areas are often maintained only by mowing, while regular pruning of dogwood is necessary.

Elsewhere in the same neighborhood, we observed newly planted tulip (Liriodendron tulipifera) trees positioned just two meters from adjacent buildings. Although this species can reach heights of 20–30 meters or more under favorable conditions, such growth is unlikely here due to limited space. Several newly planted Japanese pagoda trees (Sophora japonica) were also noted. This species is well suited to hot urban sites, can grow tall, and provides a dense shade; however, in this case, the trees were planted in clay and construction debris, making their prospects uncertain.

Boxelder maple (Acer negundo) has been widely used in urban plantings in recent decades due to its tolerance of heat and polluted air, as well as its rapid growth. However, significant drawbacks have become apparent: its branches are prone to breakage, and its vigorous root system can damage surrounding surfaces, regardless of the paving material.

With its large, glossy, deep-green leaves, cherry laurel (Prunus laurocerasus) ranks among the most popular hedge plants. Evergreen and highly tolerant of pruning, it has become a common choice in landscape plantings. However, its frequency and adaptability increasingly indicate invasive potential. It spreads successfully by seed, and branches discarded in forests can quickly develop new roots and form new plants. In poorly maintained areas, it has already begun to suppress native vegetation and shows clear invasive tendencies.

To suppress weeds and retain soil moisture, various mulches are used in public spaces, including organic materials (wood chips, grass clippings, straw, coconut fiber, composted waste) and mineral mulch (gravel, pebbles, plastic, iron filings). In addition to ecological benefits, mulches often serve a design function. During the site visit, we observed several approaches: a thick bark layer (approx. 5 cm), weed-control fabric combined with a thin bark layer, and weed-control fabric alone. The latter is less appropriate as it can lead to soil compaction, poor aeration, and reduced plant performance. Weeds can still penetrate the fabric, and without regular maintenance, they quickly diminish the aesthetics. The combination of fabric covered with gravel is particularly unfavorable, as it further compacts the soil and restricts gas exchange. Fresh wood chips are also not ideal, as they decompose quickly and bind nitrogen from the soil. A better option is bark mulch, or even more effectively, composted wood fibers combined with miscanthus (e.g., Bio Frux), which are decorative year-round, prevent slippage, and are suitable for slopes.

Contractors are typically responsible for maintenance during the first two years after planting. If weeds are consistently removed during this period, plants usually establish well enough to create shade and suppress further weed growth. There are no quick fixes in horticulture; early cultivation and care remain the most reliable approach.

The use of fertilizers was also discussed, particularly the difficulty of fertilizing beneath weed-control fabric. Plants are generally not fertilized during planting, nor is fertilization standard practice in subsequent maintenance. Poor growth cannot simply be corrected by adding fertilizer, as plant performance depends on overall conditions—plant quality, soil, and site suitability.

In residential areas, the Housing Fund as investor typically does not appoint a landscape architect for supervision; oversight is carried out by a construction supervisor, who may lack specific knowledge of plant growth requirements.

Large concrete planters were planted with Japanese maple (Acer japonicum), a species native to mountain forests. A clear contrast was visible between partially shaded specimens and those exposed to full sun, where heat stress had caused decline and even plant loss. In these containers, bark mulch proved to be a good choice, helping to prevent overheating of the root system.

The tour continued in front of the National Institute of Biology building, where ornamental cherry trees – an official gift from Japan to Slovenia and therefore of national significance—are planted. When a new construction site was planned on the location three trees had to be transplanted. Dr. Lena Marion presented the project and shared her experience of relocating mature trees.

She collaborated with an international expert with over 20 years’ experience in transplanting mature trees with bare roots. The team succeeded in postponing construction, and the three cherries were transplanted in mid-September 2021. The exposed root systems measured 6–7 meters in width and about 60 cm in depth. The trees were replanted in a specially prepared substrate supplied by Humko d.o.o. and mulched with fresh wood chips (deciduous tree species only!).

Before concluding, we visited the plaza in front of the Faculty of Chemistry and Chemical Technology and the Faculty of Computer and Information Science, University of Ljubljana. The extensive paved area between the buildings is supplemented by only a few trees of a single species. Their number and layout do not provide adequate shade for such a large concrete surface. Moreover, the selected species is generally in decline, and its growth is further restricted here by heat from surrounding concrete and limited rooting space. The same species planted nearby on lawn areas is also performing poorly, possibly due to poor soil conditions or low-quality planting stock. If decline was evident at planting, immediate corrective action should have been taken.

At the parking structure, Virginia creeper (Parthenocissus quinquefolia) grows unevenly across wooden panels and partly along the surface. From an aesthetic perspective, its use in this setting appears unnecessary or inappropriate. Instead of the predominantly planted ash trees (Fraxinus angustifolia), species naturally occurring nearby – such as willow (Salix sp.)- might have been a more suitable choice.

The professional tour of public green spaces in Ljubljana concluded with the shared understanding that deficiencies in public landscapes must be openly addressed, while examples of good practice should be highlighted, promoted, and implemented.