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2025 ZPORS annual conference report
On January 9th 2025, the 15th ZPORS professional conference was held at the Biotechnical Faculty in Ljubljana. The title of the conference was Invasiveness of Alien Plant Species: Species Analysis, Challenges and Limitations of Plant Selection, and the Planning of Public Spaces Using Contemporary Plant Material and Approaches.
The event attracted professionals from the fields of horticulture, landscape architecture, nature conservation, and ornamental plant production, as well as a younger generation of students of horticulture and landscape architecture. This year, about 80 participants attended the conference and, following the lectures, actively contributed their views and comments to the discussion on invasive ornamental plant species in production and their occurrence in the landscape.
Figure 1: The strong relevance of the topic is reflected in the fully packed lecture hall
The lectures focused on the practical challenges encountered in managing invasive alien plants (IAP) and on contemporary approaches to the selection and plantings of new species.
The speakers presented various perspectives on IAP, as well as options for their use, control, and replacement with alternative species. Matjaž Mastnak emphasized that issues related to invasive trees and shrubs in Slovenia are often linked to insufficient maintenance. He stressed the importance of early removal of invasive seedlings, such as the tree of heaven (Ailanthus altissima) and cherry laurel (Prunus laurocerasus), ideally within the first year, when they can still be removed manually. These plants often spread through bird dispersal, while their expansion is also partly driven by irresponsible disposal of plant material near residential areas.
In Switzerland, the import of cherry laurel is already prohibited, and in Bavaria there is growing support for its regulation; it is only a matter of time before these plants are included on invasive species lists. Mastnak also spoke about the LIFE Ornamentalls project and invited representatives of relevant professional fields to participate in the preparing lists of invasive species and their alternatives, such as sterile cultivars of butterfly bush (Buddleia davidii). He highlighted that Slovenia currently lacks a national IAS list and therefore relies solely on Regulation (EU) No 1143/2014, which prohibits planting of (just some) invasive alien species in the entire Union.
Figure 2: Young cherry laurel (Prunus laurocerasus) seedlings observed only days after the conference in forests near Radovljica, highlighting such habitats as potential hotspots for further spread.
Bernarda Strgar emphasized the importance of placing plants in suitable habitats and testing new plants before introducing them to urban or residential plantings. Some fast-growing and resilient species/cultivars may be an excellent choice for certain sites but can become problematic in unsuitable conditions. A good example is fountain grass (Pennisetum alopecuroides), which is commonly planted in public flower beds but requires careful monitoring and control to avoid its potentially aggressive spread.
Cultivar selection is crucial, as some cultivars are sterile and therefore pose no risk of spreading. Many breeding companies have recognized this issue and already offer interesting cultivars of fountain grass. Strgar suggested that Slovenian cities could systematically establish trial beds to test new species and cultivars, thereby gaining valuable data on their adaptability and resilience.
She also spoke in detail about the importance of plants from the Carex, Sesleria, and Satureja genera as adaptable species for the cities of the future.
Figure 3: Ornamental alien grasses are an extremely popular group of plants in mixed plantings. Chinese reed grass (Miscanthus sinensis) spreads rapidly if it is irresponsibly dumped into the wild. A hot spot was observed near a stream by Radomlje.
This year’s international speaker, Thomas Roth, presented examples of best practices from Vienna, Innsbruck, and Graz, where new tree and shrub species adapted to changing climatic conditions and urban environments are being tested. The growth performance of native, non-native, and hybrid species is also being monitored at test sites in Ireland, Finland, Iceland, and the United Kingdom, where 25 different tree species are planted in tailored substrates.
Plants for the urban landscapes of the future must be drought-tolerant, resistant to low winter temperatures, late frosts, diseases, and pests, while also having an appropriate growth habit and form. Many non-native species meet these criteria, even though they are considered invasive in their original habitats. Examples include the Robinia (Robinia pseudoacacia) cultivar ‘Nyirsegi’. Elm cultivars resistant to Dutch elm disease, such as Ulmus × hollandica ‘Fiorente’ and ‘New Horizon’, also perform very well, along with newer species like Zelkova (Zelkova serrata) ‘Green Base’ and the Pannonian oak (Quercus frainetto).
Figure 4: Thomas Roth from Vienna has been researching the growth of trees and shrubs in engineered substrates for more than 15 years.
In her lecture, Urška Kranjc emphasized that when designing public open spaces, legislation requires planners to comply with nature conservation regulations, which also include measures to manage the impacts of invasive alien species (IAP). In practice, this highlights the need to address the diversity of site conditions, including the careful selection of suitable plant species. She outlined several international examples that encourage natural succession, including the Henry Matisse Parks designed by landscape architect Gilles Clément.
Figure 5: The lectures sparked many questions and initiatives, leading to a lively and constructive discussion.
The participants agreed that effective control of invasive plants requires the collaboration of all stakeholders and appropriate awareness-raising among the wider public. One key finding was that plant invasiveness is not only a biological trait of a given species but also a consequence of improper handling, lack of knowledge, and inadequate management.
It was recommended that future efforts, in addition to testing new species, focus primarily on plants from regions with climatic conditions similar to those expected in Slovenia. In this regard, the Slovenian Mediterranean represents an underutilized source of potential ornamental plants.
The conference emphasized that the solution to controlling invasive plants does not lie in new prohibitions but in education, responsible management, and close cooperation among everyone involved in ornamental horticulture. The conference materials are available on the ZPORS website.
Text by prof. dr. Valentina Schmitzer
