The paper presents the results of seed production of 21 common segetal weed species on heavy soil in canopy of winter and spring grain crops and root crops. The studies took place in the vegetation period in the years 1999-2001, on the fields of an experimental farm and at neighbouring, small private farms in Bezek near Chełm in Eastern Lublin Region. The number of diaspors (fruit and seeds) produced by an individual belonging to common species of weeds appearing in the crop of various plants was counted. Thirty specimens of each weed species were collected from each crop plant and seeds were counted in each plant. The seed production of plants were marked in the way described by Malicki and Prędyś. Matricaria maritima subsp. inodora was the most fertile species and produced a mean number of 3 291 and a maximum of 15 930 diaspores per one plant, Chamomilla suaveolens (mean 3 188 and maximum 8 064 diaspores per one plant); Apera spica-venti (mean 1 765 and maximum 6 859 diaspores per one plant), Capsella bursa-pastoris produced mean about 822 and maximum 3 848 diaspores per one plant), Galinsoga parviflora (mean 726 and maximum 7 890 diaspores per one plant). The least number of diaspores (about 50 seeds) was produced by Fallopia convolvulus, Erodium cicutarium and Polygonum aviculare. Consequently, it was in many ways much lower than the values noted by other authors on other sites. The studies show that the seed production of the examined weed species was affected by the conditions created by the canopy in which they grew. A majority of weeds growing in root-crops produced the greatest amount of diaspores.