The E42 project, which started in 1936, would have transformed a part of the barren and deserted countryside located between Rome and the sea into a “dazzling emerald mass” («smagliante massa smeraldina», Esposizione Universale di Roma, 1939): a completely new nature was designed according to the urban spaces and buildings, and was supposed to enhance the metaphysical city of stone architecture through a balanced contrast of volumes and colors. More than thirty years after the only systematic study on the gardens and parks of EUR (de Vico Fallani 1988), it is considered useful to deepen and update the research on this topic. Starting by tracing the development of the project, this paper aims to investigate the features, shape and role of vegetation in relation to the overall program. The goal is to provide new keys to interpret the Eur complex, but also to guide conscious actions of maintenance and restoration.