Echium vulgare and Echium plantagineum: A Comparative Study to Evaluate Their Inclusion in Mediterranean Urban Green Roofs

Green roofs (GRs) are proposed to offset against numerous environmental and socio-economic concerns associated with climate change and urban sprawl. In Mediterranean urban areas, to protect and conserve biodiversity through GRs, the use of native plant species from arid environments and with shallow roots is generally recommended. In north Europe, Echium vulgare L. is widely used on GRs for its tolerance to abiotic stresses and its attractiveness for bees; unfortunately, since this species requires cold winters to induce flowering and warm wet summers for vegetative growths, its adaptability to Mediterranean GRs has been questioned. The current study is based on the hypothesis that Echium plantagineum L. can adapt better to the Mediterranean environment than E. vulgare and offer blooms to pollinators, thus providing the important urban ecosystem service (UES) of protecting entomofauna biodiversity. To compare the adaptability of E. plantagineum vs. E. vulgare, both Echium species were grown and studied on the extensive GR installed at ENEA Casaccia Research Center, in the north of Rome, Lazio, Italy. The comparative analysis of the GR performance of the two species was based on several plant-related traits, including seed morphology, rosette stage, inflorescence, flower and root-related traits, and their biological life cycle, most of them showing significant differences (for example, rosette area was 1.42-fold major in E. plantagineum than in E. vulgare). The information provided in this manuscript will be useful to update the herbarium records for conservation biology. A dramatic water stress was purposely applied in the GR before the end of the hot summer season, and while E. plantagineum faced with success the imposed dehydration (88.4% vegetation cover), E. vulgare did not (7.5% vegetation cover), presumably because of its biennial life cycle which did not allow it to complete seed maturation (only 46.9% percentage mature seeds in E. vulgare respect to 89.5% in E. plantagineum). In summary, as the main result, this work shows that in Mediterranean areas, the inclusion of E. plantagineum in seed mixes for flower meadow GRs could represent a valuable alternative to E. vulgare in temperate areas, providing a safeguard for pollinators and allowing water and energy saving.