Root morphology and root system architecture of young Posidonia oceanica plants established on twocontrasting substrate types, sand and rock, were examined to provide insights into the strategy ofadaptation of seagrasses to their environment. After germination, seedlings were planted on sandypatches and on rock within the same area, and survived plants were collected five years later formeasurements of the size of the entire root complex and analysis of individual morphological andarchitectural root traits. Collected plants exhibited up to nine highly intermingled root systems andapprox. 2.5 m of total root length. Maximum horizontal extension, total biomass and total length of rootswere not significantly affected by substrate. However, on sand roots grew vertically reaching up to 13 cm,while on rock they extended more horizontally and did not penetrate deeper than 5e7 cm leading to theformation of a shallow, densely packed root complex. On rock, the number and the length of second orderlaterals on an individual root system were reduced and the topological index higher than onsand (0.8 vs. 0.7) reflecting a more simple (herringbone) branching pattern. Again, root diameter wasgreater than on sand. The results suggest that P. oceanica can adjust root traits early during plantdevelopment according to substrate typology to maximize anchorage and substrate exploration efficiency.This plasticity enables the species to establish and persist also on rocky bottoms which generallyprevent establishment of the majority of seagrasses.