Biological invasions in wetlands by Water Primroses (Ludwigia grandiflora ssp. hexapetala and L. peploides ssp. montevidensis) are an increasing problem especially in wet meadows. The aim of this paper is to quantify differences between species and the adaptation of these amphibious plants in order to establish the consequences for site managers. The hypotheses were: (i) that these species differ in biomass and morphology, and (ii) that terrestrial habitats determine particular adaptations such as reduced biomass, more roots and a bushy form. Biomasses from five sites were collected, following seasonal changes and biological traits were measured on selected plants. Comparing species in aquatic habitats, L. peploides produced less biomass than L. grandiflora. Comparing habitats for L. grandiflora, the biomass in meadows was almost twice that in dykes, particularly due to old stems, the converse of the hypothesis. Terrestrial forms were characterized by a bushy morphology, with shorter internodes and stems, as well as more secondary ramifications, while in aquatic habitats the plant architecture was simpler. Models were built linking morphology to species, sites and water conditions. Adaptations to terrestrial habitats of a particularly resistant form with a significant potential biomass are practical constraints for farmers and managers.