Abstract: A method and an assembly of a pre-shaped substrate plug and a container for planting out plant material, of which the plug includes one or more press sections sideways of an upwardly opening plant insertion recess. The outer wall dimensions of the press sections are larger than inner wall dimensions of corresponding parts of the container, such that during a step of placing the plug in an insertion position into the container, a pressing of the plug into the container needs to be performed for the press sections to be able to fit into the corresponding parts of the container while the press sections compress inwardly in a sideways direction and at least partly decrease the plant insertion recess around a seed or lower plant part which had been placed therein preceding the pressing.