In Belgium, a voluntary code of conduct intended to prevent introduction of invasive plants through the horticultural trade has been implemented since 2009. European Regulation 1143/2014 that imposes legal bans on a number of plant taxa entered into force in 2015. However, studies regarding the presence of invasive plants in the Belgian horticultural trade remained scarce. In 2016–2017 and in 2020, we surveyed a total of 11 garden centres specialized in water gardening and compiled a list of 285 aquatic plant taxa that were being traded. In 2020, four Belgian Consensus List species and one Communication List species were still offered for sale. Also, three species of Union Concern were still being sold in 2020. The plants of the Belgian Lists that were still being sold in 2020 and all except one of the European Union List plants were mislabelled, either because of misidentification or because a taxonomic synonym was used. Mislabelling may explain why some prohibited plants were still in trade. Therefore, retailers and plant producers should be encouraged to correctly identify the plants that are being traded. Our study illustrates that regular surveys are essential to assess the effectiveness of voluntary codes of conduct and legal trade bans.