The programming language XL (‘eXtended L-system language’) is an extension of Java, which supports the specification and execution of relational growth grammars, a variant of parallel graph grammars. XL is a powerful generalisation of the well-known L-system approach to functional–structural plant modelling. Some features of XL are discussed that are particularly useful for combining structure and function and for querying plant architectural data, and an exemplary functional–structural plant model of young beech trees is presented that is implemented in XL and includes PAR distribution, assimilate allocation and morphological plasticity. Together with a simpler model of spruce trees, this beech model is included in a virtual landscape with a mixed-species forest stand where competition for light occurs. The open-source platform GroIMP was used for the complete model development process and for visualising the results.