A morphological investigation of the alien <i>Ludwigia</i> species in Italy: towards a new key for unambiguous identification

The two freshwater plants <i>Ludwigia hexapetala</i> and <i>L. peploides</i> subsp. <i>montevidensis</i>, invasive in Europe including Italy, can colonise aquatic and bank habitats and possess remarkable morphological plasticity. They can be distinguished mainly by floral characters, while in the vegetative phase they show morphological similarities, making their identification difficult. In some cases, incorrect identification of these species generated confusion in the reference literature. This study aimed to define the morphological differences between <i>L. hexapetala</i> and <i>L. peploides</i> subsp. <i>montevidensis</i> to provide an identification key for these two species even when not flowering. Morphological quantitative and qualitative characters of 138 samples were measured and evaluated. PCA was performed, and each character was compared between the two species in each morphotype. The two species showed significant differences between morphological vegetative characters. <i>Ludwigia hexapetala</i> is associated with longer leaf length/width ratio, matte leaves with no petiole, erect rough stems when flowering, and high frequency of pneumatophore production. Conversely, <i>L. peploides</i> subsp. <i>montevidensis</i> is associated with shorter leaf length/width ratio, shiny leaves with a distinguishable petiole, and creeping, sticky stems. Our morphological study provides a taxonomic identification key for <i>L. hexapetala</i> and <i>L. peploides</i> subsp. <i>montevidensis</i>, allowing accurate identification even in the vegetative phase.