L-systems are widely used for modelling the growth and development of plants. However, until now L-systems did not simulate mechanical collisions between plant parts and entire plants. We show that L-systems can be combined with the position-based collision detection and resolution method, originally devised in computer graphics and subsequently extended to plant modelling. To this end, we extend turtle interpretation by creating a persistent geometric representation of a plant and updating it after each simulated growth step while resolving collisions. We also augment the method to simulate propagation of mechanical collisions between terminal organs represented as surfaces and their supporting branches represented as rods. We illustrate the methods using models of individual plants and fields of plants with all collisions resolved automatically. The presented work is the starting point for faithfully representing different phenotypes in virtual plant breeding trials, and visualizing plants in the field.