Can we stop further spread of Crassula helmsii in Europe? Toolkits and guidelines used in Flanders (Belgium)

Australian swamp stonecrop Crassula helmsii, is currently one of the most challenging invasive (semi-)aquatic plants in Flanders. With the UK and the
Netherlands, Flanders is part of the hotspot of this invasive species in Europe yet the species is already expanding towards neighbouring countries.
Despite important impacts on conservation value habitat and the increasing responsibility of the region to limit expansion to other parts of Europe, no
coordinated management is currently in place. We compiled up-to-date information on its distribution, spread, impacts and possible control methods
aiming to provide practical guidelines for practitioners to manage populations. Eradication is no longer considered a realistic goal in the region but
limiting further spread, removal of small and containment of larger populations remain feasible management objectives. We propose a framework to
assess colonization risk which can guide surveillance effort in susceptible sites. We provide a matrix of site-specific methods for different management
objectives as a decision support for managers. To implement a coherent management approach across the region, procedures allowing swift and
decisive action need to be installed at regional as well as local level. Communication to managers and the general public, application of biosecurity
protocols and measures to reduce colonization pressure are essential elements of a successful strategy. Including C. helmsii on the List of Invasive Alien
Species of Union Concern could provide an important incentive for implementation.