The invention relates to a method for multiplying a strain of Trichoderma, which includes the preparation of substantially contaminant-free culture media, containing disinfected wood fibers obtained by means of a twin-screw extruder, the amplification of Trichoderma via a series of steps for manufacturing a primary inoculum and then a secondary inoculum from said primary inoculum, and the multiplication of the microorganisms in each step, wherein the multiplication of the propagules reaches 2×104 to 105.The invention also relates to a plant growing medium containing a secondary inoculum of Trichoderma.