The invention relates to a method for producing a stabilized growing medium for cultivation of a plant or seedling. The method comprises the steps of providing a receiving unit for containing the rooting plug to be formed and placing a root structure of a plant or seedling in the receiving unit. Further, the method includes the steps of filling the receiving unit with slurry material forming a stabilized growing medium and curing the slurry material in the receiving unit while holding the root structure of the plant or seedling in the receiving unit.