The existence of vegetation patterns extending along kilometers on the seabed which do not seem to respond to any external cause evidences the fact that the growth of Posidonia oceanica is not completely understood. We present a continuos model for clonal growth plants in order to explain these spatial structures as a result of nonlinearities and long range interaction. Numerical simulations using depth profiles are also presented in comparison with real data of Posidonia oceanica and Cymodocea nodosa.