The paper presents results of fertility of common weed species on light soil podzolic in a canopy of winter and spring grain crops and root crops. The research was conducted in the years 1999–2001 on the fields of the experimental farm and the neighbouring, small private farms in Bezek near Chełm in Eastern Lublin Region. The number of diaspors (fruit and seeds) produced by one of the common species of weeds appearing in the crop of various plants was counted. Fertility of the weeds was determined on the basis of 30 specimens. The seeds were counted in each plant. It was established that Matricaria maritima subsp. inodora was the most fertile (on average, over 4000 diaspors), Capsella bursa -pastoris produced about 2500 diaspors, the least fertile (below 50 seeds) were Avena fatua, Polygonum aviculare and Fallopia convolvulus. Consequently, it was much lower than those, noted by other authors on other sites. The studies show that the fertility of the examined weed species was affected by the conditions created by the canopy in which they grew. Generally, the greatest amount of diaspores was produced by the weeds growing in root-crops. But contrary to the common belief, it turned out that the crop of tubers does not create for all the weeds the oportunity to form a greater number of generative propagation units than the crop of grains. Besides, fertility of the examined weeds, independent of the field plant, was characterized by considerable individual changeability, which was related to the given species and the environmental conditions.