This study analyzed the structural changes of semicrystalline polylactide (PLA) in the form of spun-bonded mulching nonwoven, during outdoor composting. The investigation was carried out at the microstructural, supramolecular and molecular levels using scanning electron microscopy (SEM), wide-angle X-ray diffraction (WAXD) and the viscosity method, respectively. The results showed the varying degree of influence of the climatic condition, prepared compost mixtures and time on the molecular, supramolecular and micromorphological structure of PLA spun-bonded mulching nonwoven and its degradation rate. The obtained experimental results revealed how the popular outdoor composting method, realized under two different European climatic conditions (in Poland and in Bulgary), affects the degradation of PLA nonwoven, designed for agriculture use.