Posidonia oceanica is a clonal growth plant which is crucial to the sustainability of the marine environment. Moreover has been shown that vegetation patterns are present widely in the sea bottom, exhibiting different kinds of patterns according to different environmental conditions. From this perspective the spatial description of these plants is a valuable knowledge from the conservation point of view. However, the modelization represents a computational challenge due to the fact that clonal growth is modeled using two PDE involving 3 dimensions. In order to overcome this problem, we present an approximation which reduces the description to one 2-dimensional field. Since this derivation allow to determine the relation between the parameters of both models, the clonal growth can be reproduced with the simplified version increasing that way the modelization capabilities.