A method and an assembly of a pre-shaped substrate plug 1 and a container 9 for planting out a plant, of which the plug comprises one or more press sections 5 sideways of an upwardly opening plant insertion recess 3, wherein outer wall dimensions of the press sections 5 are larger than inner wall dimensions of corresponding parts of the container 9, such that during a step of placing the plug 1 in an insertion position into the container 9, a pressing of the plug 1 into the container 9 needs to be performed for the press sections 5 to be able to fit into the corresponding parts of the container 9 while said press sections 5 compress inwardly in a sideways direction and at least partly decrease the plant insertion recess 3 around a lower plant part which had been placed therein preceding the pressing.
