The aim of the study was to assess the biodiversity of segetal flora in winter wheat depending on the date of sowing (four dates, every two weeks: I – 15–16 September, II – 29–30 September, III – 14–15 October, and IV – 28–29 October) and NPK fertilization level (kg ∙ ha –1 ): 1 NPK (310 kg ∙ ha –1 ), including 140 N, 70 P and 100 K, or 1.5 NPK (465 kg ∙ ha –1 ), including 210 N, 105 P and 150 K. On the earliest sowing date (I date), the weed assemblage was the most numerous, both in the tillering (BBCH 22-25) and milk maturity phases (BBCH 77-79). The dominant weeds were: Viola arvensis, Veronica arvensis, Geranium pusillum. The NPK fertilization level significantly differentiated the density of weeds only during the milk maturity stage. Compared to the lower dose, the higher dose of fertilizers contributed to a rise in the number of weeds per 1 m 2 , simultaneously causing a decrease in weed biomass. In the phase of milk maturity (BBCH 77-79), the Shannon-Wiener index increased with the delay in sowing, but at the same time the Simpson dominance index decreased.The different levels of fertilization applied did not have any significant effect on the values of both biological indicators.