To foster transparency, compatibility and the reuse of models, a programming language tailored to the specific needs of functional-structural plant modelling is helpful. The language XL is based on a variant of graph grammars with parallel application, generalizing the well-known Lindenmayer systems (L-systems). At the same time, it is an extension of Java. XL thus unites the paradigms of rule-based, imperative and object-oriented programming. Special features like queries in 3-d structures, instantiation rules and the rate assignment operator are particularly helpful for the connection of processes and architectural development in plant models and will be demonstrated at examples. A model of winter barley, including a representation of genes, a metabolic network and whole-plant morphology, will stand for the potential of XL to provide a consistent framework for linking the metabolic and the individual-plant scale. On the other hand, rule-based models of seed dispersal, competition for several resources among individuals, and plant-herbivore interaction – not yet calibrated at real plant communities – show the potential of the approach to bridge also the gap between the spatial scales of the individual and the population.