The aim of the study was to assess the influence of Trichoderma spp. and different fertilization levels on the flowering and nutritional status of Begonia × tuberhybrida Voss. ‘Picotee Sunburst’ plants. Before planting, the tubers were soaked in water or a mixture of spore of Trichoderma spp. (T. viride Schumach–Tv14, T. harzianum Rifai–Thr2, T. hamatum/Bonord/Bainier–Th15) in the form of a suspension for 24 h. The plants were fertilized every 7 days with the multi-component Peters Professional Allrounder fertilizer (20:20:20 + microelements) at a concentration of 0.0%, 0.2%, and 0.3%. Trichoderma spp. accelerated the flowering of the ‘Picotee Sunburst’ cultivar by 2.7–8.7 days, stimulated the development of buds and flowers in the plants and affected their size. The plants bloomed most intensively and had the biggest flowers after the treatment with the 0.3% fertilizer. Trichoderma spp. and the fertilization had no effect on the height of the plants and the number of shoots regardless of the fertilizer concentration, but they stimulated the development of leaves. Trichoderma spp. stimulated the production of chlorophyll. They did not affect the uptake of macroelements, but they stimulated the uptake of microelements (Zn, Fe, and B). The higher the fertilizer concentration was, the higher was the content of microelements in the plants.