Background The spread of invasive alien species (IAS) is widely recognized as one of the main drivers of biodiversity loss in Europe. Damage caused by the expansion of IAS can be significant and therefore requires the implementation of control actions aimed at reducing their population. The horticultural industry in Europe and elsewhere in the world has made a vast array of diverse plant species available to the public. In Europe, some 17 000 taxa (12 000 species plus subspecies, varieties and hybrids) are grown in gardens and new species are constantly sought. Some of these plants have proved to be invasive and to affect native biodiversity. In fact, ornamental horticulture is the main pathway of plant invasion worldwide. In Belgium, almost all black list plants have been introduced as ornamental plants. A survey conducted in 2006 showed that 25 out of the 28 black list species were present in horticultural catalogues. When informed about the risks of biodiversity loss caused by some commercialised species, nursery professionals were found to be responsive to the issue and concerned by the detrimental impacts of these IAS. Objectives The overall objective of the AlterIAS project was to reduce the introduction of invasive plants at source, by raising awareness about their environmental risks amongst the whole ornamental horticulture supply chain in Belgium, from growers to gardeners. The project aimed to promote best practices for preventing the release and spread of invasive alien species through a voluntary Code of Conduct produced with the involvement of the horticultural sector. In a preliminary study, it was found that most horticulture professionals in Belgium were unaware of the risks to biodiversity caused by some commercialised species, but when informed they were willing to contribute to improving the situation. Results Firstly, the AlterIAS project assessed the socio-economic value of invasive plant species for the horticultural sector in Belgium, through an analysis of 146 catalogues in Flanders and Wallonia and a survey involving 67 nurserymen. The resulting report showed that 70% to 90% of the invasive plants listed by the Belgian Platform on Biodiversity were commercially available in nurseries. Of the 55 invasive species available in nurseries, 31 were on the Biodiversity Platform’s black list and 32 were considered of economic value, but 44% of the nurserymen considered that these invasive species represented less than 5% of their total sales. After ten months of negotiations, between the horticulture industry, authorities responsible for the environment and the scientific community, a voluntary Code of Conduct was agreed. This comprised five good practices: keeping informed about invasive plants in Belgium, stopping the selling and planting of certain invasive plants (the consensus list), disseminating information on invasive plants, promoting non-invasive plant alternatives, and participating in the early detection of new invasive plants. The Code of Conduct was proposed to horticulture professionals for subscription. Those subscribing could use a label designed to signify good practice in this area. At the end of the project, 1 027 partners (signatories) were involved in the Code of Conduct, including 243 nurserymen, 28 landscape architects, 52 garden contractors, 151 public green managers, 6 botanical gardens and 478 amateur gardeners. Nearly 200 garden centres and other selling points had adopted the Code of Conduct by 2014. All signatories were committed to withdrawing from sale or not planting the invasive plants on the agreed consensus list. The consensus list led to the withdrawal of 28 species from selling lists (more than 50% of the invasive species previously available in nurseries), including 20 terrestrial plants, 8 aquatic plants, 20 black list plants and 8 watch list plants identified by the Belgian Platform on Biodiversity. This represented around 44% of the total invasive plants listed for Belgium. To help fill the gap in the market left by the withdrawal of invasive plants, the AlterIAS project conducted research and produced a brochure promoting commercially-interesting alternative non-invasive plants. This brochure presented native plants according to similar ornamental function to withdrawn invasive species, such as shrub, border plant, green-screen, groundcover or climbing plants. The brochure proved especially popular with amateur gardeners. Two demonstration gardens were created with the help of landscape architects, including mixed borders with native plants. The extensive general awareness campaign, to promote the Code of Conduct and its good practices, included a 40-minutes film (500 copies on DVD), 60 000 project folders, articles in the press and horticultural magazines, TV and radio features, newsletters, a layman’s report, and stands at various horticultural events. The project organised specialist information sessions for horticulture teachers and two international workshops. The multi-lingual project website (www.alterias.be) has become an important resource for information on invasive plant species. By the start of 2014, the website had recorded around 68 000 visits. At the end of the project, the horticulture professionals surveyed were more aware of the ecological impacts of invasive plants: the proportion of respondents who spontaneously cited negative impact on biodiversity had increased by 17%. There was also a better knowledge of the list of invasive plants in Belgium (28 species correctly cited in 2013, compared to 17 in 2010). This increased awareness should also help nurserymen identify new invasive plants that establish in natural habitats, which are not on existing lists. These can be reported on a factsheet contained in the Code of Conduct. The Code of Conduct represents a stronger and more effective voluntary tool than those adopted in other European countries. A key element of this is the subscription element, which is absent from other codes. This approach may be a first step toward increasing self-control within the horticulture sector. The AlterIAS approach to stopping the introduction of IAS at source is readily transferable, with the project being contacted by interested stakeholders from around Europe and as far afield as Canada. The coordinating beneficiary was invited to the EPPO head office in order to evaluate the feasibility of transferring the Belgian approach to other EU member States. Further information on the project can be found in the project's layman report and After-LIFE Communication Plan (see "Read more" section).