Plant material of invasive alien plant species (IAPS) must be appropriately disposed of to prevent unintended spread. The current guidelines in Slovenia and in several other European countries recommend composting only the parts of the plants from which they cannot sprout and reproduce. At the same time, the vegetative propagules and seeds should be incinerated. We tested whether the seeds and vegetative propagules (rhizomes, stolons, tubers, and branches) of 30 selected IAPS survive industrial composting, which is the method of processing collected organic waste and green cut from parks and gardens. Mature seeds and vegetative propagules were packed in metal boxes, which were filled with compost and included in the hygienisation phase of biowaste processing at the Regional Waste Management Centre, RCERO Ljubljana. After the industrial composting for 17 days, seed germination and viability tests were done and compared with a control group of seeds collected from the same plants but not undergoing the composting process. The composted and fresh vegetative propagules were planted in pots with soil, and the number of rooted parts was counted. None of the seeds and the vegetative propagules survived the industrial composting process, and we can conclude that it is safe to dispose of the IAPS like other organic waste or green cut.

Brown waste, disposal of plant material, germination, green cut, IAPS waste collecting, invasive alien plants, non-native plants, vegetative reproduction, waste management

Invasive alien plant species (IAPS) are causing environmental and ecological problems and are responsible for substantial biodiversity decline in almost all regions of the planet (Vilà et al. 2011; Bellard et al. 2016; Diagne et al. 2021).

In Europe, preventing and minimising the effects of invasive alien species on biodiversity is one of the programmes of the European Commission, resulting in the Invasive Alien Species Regulation (Regulation (EU) 1143/2014; (European Commission 2014). The regulation contains a List of invasive alien species of Union concern, including 41 plant species in 2024. The listed species are subject to restrictions and measures, including restrictions on keeping, importing, selling, breeding, growing and releasing into the environment. However, not all invasive species present in Europe are regulated. For example, the extremely widespread species Reynoutria japonica Houtt. and North American Solidago spp., Phytolacca americana L., which have been spreading very effectively in the last decades, and the highly allergenic Ambrosia artemisiifolia L., are not (yet) included in the EU list. These and several other invasive or potentially invasive alien species and non-native species from alert lists (Essl and Rabitsch 2002; Celesti-Grapow et al. 2009; De Groot et al. 2017; Csiky et al. 2023; EPPO 2024; Nikolić 2024) are at least for now not regulated by the European legislation. The regulation and management of these IAPS is the domain of each European member state.

The economic costs of biological invasions are incredibly high and are still underestimated (Diagne et al. 2021). The prevention and the response in the early phases of plant invasion are much more effective and less expensive comparing interventions in highly invaded areas (Finnoff et al. 2010). However, the early phases are already over in several regions, and in these cases, we have to face the problem of costly plant removal. Usually, the financial and human resources are limited, and in such cases, recognising the high-impact species and prioritising their management is crucial (Blackburn et al. 2014). Actions to remove invasive plants from nature are already a constant practice worldwide.

There is a lot of research on the most effective removal methods for various plant groups, considering the biology and reproduction strategies of different invasive species. Several such scientific publications can be accessed through the websites of organisations that deal with invasive species, e.g., IUCN (https://iucn.org/our-union/commissions/group/iucn-ssc-invasive-species-specialist-group), CABI (https://www.cabi.org/what-we-do/invasive-species/) or GBIF (https://www.gbif.org), national or regional institutions (ministries, agencies and institutes) and different projects dealing with invasive species, as two LIFE projects from Slovenia, Life OrnamentalIAS: https://zrsvn-varstvonarave.si/blog/projekti/life-ornamentalias/ and Life Artemis: https://www.tujerodne-vrste.info/project-life-artemis/).

Despite the worldwide problems with invasive species and numerous studies on their biology, ecology and management, research on disposal methods is still scarce. We found only a few that were dealing with single species (Hassani et al. 2021; Popovic et al. 2021; Wang et al. 2024) or a small group of ecologically connected species (Meier et al. 2014; Strgulc Krajšek et al. 2020). In publications and webpages, primarily written for the general public or different interest groups (for example, Strgulc Krajšek et al. 2016; Dolenc and Papež Kristanc 2020), instructions are based on practical experiences and not on controlled experiments and commonly include the precautionary principle. Robinson et al. (2017), who analysed the websites about the invasive Japanese knotweed (Reynoutria japonica), found conflicting information about the potential socio-economic and ecological problems caused by IAPS and contradictory suggestions about the most appropriate management techniques. Burning the collected material is often suggested if the propagules, such as seeds, tubers, etc., are present. Such an approach is safe but expensive. Still, there was no existing research that would analyse the survival rate of invasive alien plant propagules included in regular industrial biowaste processing. Such research is in the interest of scientists who are frequently asked about the effective and safe methods of IAPS disposal and cannot give complete and reliable answers, and in the interest of the institutions dealing with the biowaste, which strive to simplify the collection and processing procedures.

In Slovenia, biodegradable waste including kitchen and garden waste that residents deposit in designated containers (brown waste) from one-fifth of the households (ca. 470 000 residents) is collected at Ljubljana Regional Waste Management Centre (RCERO Ljubljana) (Sankovič 2017). They also accept yard waste (green cut) from the maintenance of gardens and parks, such as branches, leaves, grass, and water lilies.

The biowaste processing in RCERO Ljubljana has several phases (Fig. 1) (Sankovič 2017). Biowaste from households and yard waste are separately collected in the reception hall. In the first step, all gathered material is ground. The biowaste from households is then sieved, metal particles are removed and then it is transported to the bioreactor, where the fermentation process takes place. After three weeks in the bioreactor, the material is squeezed and transported to the composting tunnels, where it is mixed with previously ground yard waste. The phase of composting in tunnels is 2 to 3 weeks long, and during it, the temperature should be a minimum of 55 °C for at least 4 consecutive days, or above 65 °C for at least 3 consecutive days to meet the standards for hygienisation from the Decree on the treatment of biodegradable waste and the use of compost or digestate (Ministrstvo za kmetijstvo in okolje 2013), which is in accordance to Directive 2008/98/EC of the European Parliament and of the Council (https://eur-lex.europa.eu/eli/dir/2008/98/oj). After this phase, the biomass is transported to the maturation hall for four weeks, where it is turned three times a week. During the maturation, the material partially dries so it can be sifted and cleared of plastic particles and other impurities, like stones and bones. The produced compost is supplied to companies that mix the substrates for flowers and gardens. A portion of the compost is used within the municipality for park maintenance. Residents and companies that work on landscaping around houses, green roofs, and similar projects also purchase a smaller portion.

Figure 1.

The biowaste processing in RCERO Ljubljana.

There are several other biowaste processors in Slovenia. One of them processes the biodegradable kitchen waste using anaerobic digestion (biogas plant) only, and the fermentation takes place at lower dry matter content. Aerobic phases do not follow this phase. The digestate is treated as wastewater, and compost is not produced. Other processors have composting facilities and an aerobic process for treating biodegradable waste without an anaerobic component.

The practice of separate collection of plant material of selected IAPS was established in Ljubljana in 2018 as part of the Applause project, in the frame of which researchers studied the possibilities of using IAPS as a raw material for various products (Berden 2019). The selected plant species used in a project were collected separately; additionally, a collection bin for mixed IAPS material was available. All unused material from the containers was sent to incineration to prevent the potential spreading of IAPS. Such practice is safe but expensive.

The main goal of our study was to test the safety of disposing seeds and vegetative propagules of selected invasive plant species that are common across Europe in industrial composting at Ljubljana Regional Waste Management Centre (RCERO Ljubljana). We included the seeds and vegetative propagules in one phase of industrial composting, namely composting with active air ventilation, which took place in the composting tunnels. After the exposure we tested if the propagules survived. Based on the results obtained, we wrote new recommendations for the disposal of biomass of invasive alien plant species (IAPS).

To our knowledge, this is the first experiment where seeds and vegetative propagules of so many invasive species were included in the industrial composting to inspect their viability after the process.

Of the seeds or vegetative propagules of 30 different plant species that are invasive in Slovenia and Europe, none survived the industrial composting in composting tunnels at RCERO Ljubljana. The propagules of IAPS may occur in two sources of biowaste commonly collected in Ljubljana: brown waste, which is collected in households, and yard waste from the maintenance of gardens and parks. Households usually dispose of smaller amounts of plant biomass removed from gardens and put them in bins for organic waste. However, the material from parks and public areas and the collected material of IAPS during public actions are usually transported directly to the collection centres as the green cut.

The phase of composting in tunnels is common to both sources of biowaste (Fig. 1), and this is the reason that we decided to include the propagules in this phase only. In this phase, the temperatures rise above 70 °C for a few days (Fig. 2). For optimal composting long periods of high temperatures must be avoided to allow the development of eumycetes and actinomycetes, which are the main decomposers of the long-chain polymers, cellulose and lignin (de Bertoldi et al. 1983). The most effective way to control the temperature is forced pressure ventilation and turning the composting material (de Bertoldi et al. 1983).

The biowaste from households in RCERO Ljubljana also goes through the anaerobic fermentation phase, where the temperature is 55(±1)°C, and this phase can additionally reduce the chance of survival of seeds and vegetative propagules. However, in the commercial mesophile bioreactors, the temperatures are lower. In the experiment made by Hahn et al. (2023), where T did not exceed 42 °C, hard-seed species, such as Malva sylvestris L., survived the procedure, but the seeds with softer testa, such as Chenopodium album L., were completely inactivated by the treatments. Johansen et al. (2013) exposed seeds to anaerobic digestion at higher temperatures (55 °C) and complete mortality of all 7 tested plant weeds after two days. They report that the temperature seemed to be the major cause of damage, as the same species germinated after the anaerobic digestion at 37 °C. The survival of seeds after the anaerobic digestion at 37 °C was reported also by Westerman et al. (2012).

The reason that seeds and vegetative propagules did not survive the process of hygienisation must be the combination of high temperatures, humid environment, high concentration of ammonia, pathogen infestations, water-soluble organic phytotoxins, and microorganisms in the substrate, as was already reported by Hassani et al. (2021), who did the composting experiment of Lupinus polyphyllus Lindl. seeds, and several others, for example, Zaller (2007), Eghball and Lesoing (2000) and Johansen et al. (2013). High temperature is most probably not enough to kill the seeds of all species. Seeds of different plant species have different tolerance to high temperatures. So, several of the tested IAPS may have seeds sensitive to temperatures higher than 55 °C. The extreme tolerance was reported by Daws et al. (2007), who discovered that seeds of some desert succulents survive exposure to 103 °C for 17 hours. The seeds of the black locust (Robinia pseudoacacia), which we tested in our experiment, can survive the increased temperatures during the fire. Popovic et al. (2021) exposed the seeds of black locust to 100 °C for 2.5 and 5 minutes and tested their germination. It was significantly lower compared to the control seeds, but several seeds survived the exposure. In our experiment, black locust seeds did not survive the composting process despite lower temperatures during the process (Fig. 2). Even the lower temperature, at least 57.2 °C, was enough to destroy the propagules of aquatic invasive plant species Arundo donax L., Hydrilla verticillata (L.f.) Royle, Eichhornia crassipes (Mart.) Solms, and Pistia stratiotes L. from the Rio Grande River, during the large-scale composting experiment (Meier et al. 2014). In this experiment, the whole plants containing vegetative propagules and seeds were composed, and the result was a valuable compost product without the viable propagules of IAPS.

In our experiment, the seed viability of the control group and both batches of composted seeds was tested by the combination of a germination test and a test of metabolic activity by Tetrazolium staining, which was used when the control seeds did not germinate (most probably because of dormancy or unsuitable germination conditions). None of the methods have shown that the seeds would survive composting. The control seeds of all species, except Rhus typhina and Reynoutria japonica, were viable. We found that all the seeds of Rhus typhina were empty. We checked the seeds from another locality, and they were empty, too. In Slovenia, we have not yet observed the propagation of this species by seeds, but vegetative propagation with stolons is very common. The ripe fruits of Reynoutria japonica were collected at the site, where we had already collected the viable seeds for other experiments. Hence, the result that the seeds were not viable was unexpected. Regarding the results of other tested IAPS, we do not expect that viable Reynoutria japonica seeds will survive composting. Similarly, the branches of Cornus sericea of the control group did not grow roots. Cornus sericea is a species that vegetatively reproduces mainly by ground layering, where the stem is bent down and partly buried in the soil while still attached to the parent plant (Bačič et al. 2015). However, cut branches also can root and serve as vegetative propagules (Strgulc Krajšek et al. 2020). Any other branches or rhizomes that we used in the experiment did not survive the composting. The vital parts of all branches, tubers and rhizomes were almost completely decomposed, only the wooden parts and bark remained.

Based on the fact that 100% of the tested species were inactivated during composting, we believe that we can state with high probability that the industrial composting of IAPS is safe in terms of preventing the spread of IAPS into nature by using the obtained compost, when the composting process meets the requirements of “Decree on the treatment of biodegradable waste and the use of compost or digestate” (Ministrstvo za kmetijstvo in okolje 2013) and Directive 2008/98/EC of the European Parliament and of the Council (https://eur-lex.europa.eu/eli/dir/2008/98/oj).

New recommendations for the disposal of IAPS biomass can be summarised in the following two points:

1. Plant material without seeds or vegetative propagules can be composted in garden compost heaps or disposed of as other household organic waste or green waste from parks and gardens. Disposing of any garden plant material in natural environments, such as riverbanks, forests, or forest edges, must be prohibited.
2. Plant material containing seeds (even if not fully ripe) or vegetative propagules (such as rhizomes, bulbs, tubers, and branches that can regrow) should be disposed of as household organic waste (brown waste) or yard waste from parks and gardens intended for industrial composting. When composting process complies with the standards set by the “Decree on the treatment of biodegradable waste and the use of compost or digestate” (Ministrstvo za kmetijstvo in okolje 2013) and Directive 2008/98/EC of the European Parliament and of the Council (https://eur-lex.europa.eu/eli/dir/2008/98/oj), there is no risk of spreading IAPS through the use of the produced compost.

We used the seeds or vegetative propagules of 30 different IAPS that are invasive in Slovenia. We included trees and shrubs, vines, perennials and annuals, so we covered a variety of life forms. As there were no observed viable propagules after the composting, we proposed the new management recommendation that waste from IAPS can be composted and does not have to be incinerated. This less complicated protocol may simplify and reduce the costs of IAPS disposal and can have another positive effect, namely that more residents will choose to dispose of plant material of IAPS in bins for organic waste or bring it to landfills, and less of the IAPS will end up in compost heaps in the wild, that is still a common practice in Slovenia (Šipek and Šajna 2020).

We thank Ljubljana Municipality for financing the two custom-made metal boxes we used during the experiment and RCERO Ljubljana for their cooperation. We thank Darja Kopitar, Kim Prah and Žan L. Cimerman for their technical help. We thank Jana Kus for the valuable discussion, Mateja Germ and both reviewers for reading the manuscript and giving us several comments that improved the text.