Naturalization of ornamental plant species in public green spaces and private gardens

Ornamental horticulture is the most important pathway for alien plant introductions worldwide, and consequently, invasive spread of introduced plants often begins in urban areas. Although most introduced ornamental garden-plant species are locally not naturalized yet, many of them have shown invasion potential elsewhere in the world, and might naturalize when climate changes. We inventoried the planted flora of 50 public and 61 private gardens in Radolfzell, a small city in southern Germany, to investigate whether local naturalization success of garden plants is associated with their current planting frequency, climatic suitability (as assessed with climatic niche modelling) and known naturalization status somewhere in the world. We identified 954 introduced garden-plant species, of which 48 are already naturalized in Radolfzell and 120 in other parts of Germany. All currently naturalized garden plants in Radolfzell have a climatic suitability probability of ≥ 0.75 and are naturalized in ≥ 13 out of 843 regions globally. These values are significantly higher than those of garden plants that have not become locally naturalized yet. Current planting frequencies, however, were not related to current naturalization success. Using the identified local naturalization thresholds of climatic suitability and global naturalization frequency, and climate projections for the years 2050 and 2070, we identified 45 garden-plant species that are currently not naturalized in Radolfzell but are likely to become so in the future. Although our approach cannot replace a full risk assessment, it is well-suited and applicable as one element of a screening or horizon scanning-type approach.