Formal grammars like L-systems have long been used to describe plant growth dynamics. In this article, they are used for a new purpose. The aim is to build a symbolic method derived from computer science that enables the computation of the distribution associated to the number of complex structures in plants whose organogenesis is driven by a multitype branching process. To that purpose, a new combinatorial framework is set in which plant structure is coded by a Dyck word. Moreover, the organogenesis is represented by stochastic F0L-systems. By doing so, the problem is equivalent to determining the distribution of patterns in random words generated by stochastic F0L-system. This method leads directly to numerous applications like parametric identication for plant growth model.