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Effective population size for conservation of cultivated plants was studied in Coreopsis grandiflora on populations in variants of 1, 5, 10, 30, 50 and 100 individuals. In five consecutive years these traits were studied on seeds originating from parental populations: seed number and weight per infructescence, thousand seed weight and germination. Plant height and flower size were investigated in a field trial. The influence of population size was expressed mainly in traits of early life-history stages. Inbreeding depression influenced germination already in the first year and was similar for the five years while seed weight was influenced from the second year. In the fifth year there were significant differences among variants of 30 individuals and less and variant of 50 individuals in all evaluated traits. The results document that mostly cross-pollinated cultivars of a population smaller than 50 individuals are exposed to impairment. The population of 10 individuals and less is at risk of extinction. This result will be influenced by the level of cross-pollination and degree of selection.