Greenhouse and crop models are largely developed within scientific or semi-scientific working groups, with only little commercial interest. The usage of many different modelling tools and the creation of personal models for research and analyses, as well as the protective state of mind of many developers, have led to a large number of simulation models and models coded in software that is not available to public. Although researchers openly present model structures, simulation scenarios and sometimes even printed code, the existence of a common development platform for model IT development is still lacking. The solution is a module-based system, where simulation models are decomposed into coherent functional parts, in order to achieve a flexible modelling system with opportunity for re-use and joint development of model modules. We propose an accessible, module-based, open-source model platform for sharing, application and further development of greenhouse horticultural models. A prototype of a decision-support system with selectable model options in an online toolbox for the greenhouse horticultural sector was created within the Universal Simulator, a software package for collaborative ecological modelling. The code within the Universal Simulator-created toolbox based on C++ code is accessible via XML scripts and can be used as the basis for the creation of greenhouse climate and crop cultivation modelling. The toolbox, which is a further development of the earlier published Virtual Greenhouse, contains detailed model scripts of all major physical greenhouse processes and some crop growth models, and it was created with independent, freely selectable models. This enables the building of simulation models composed of existing model components, e.g., a tomato crop growth model with given inputs and a combination of selected greenhouse set-ups. Using the tomato crop growth simulator as an example, we show the strength of the modular open-source structure for scientific use, education and commercial applications in evaluation study.