Helianthus annuus is an invasive alien species naturalised in the central region of Argentina where it shares an extended area with the sunflower crop. As this species has also invaded several other sunflower crop growing areas in the world, it severely restricts the use of new technologies, for example herbicide tolerance by genetic modification. The natural seed dormancy of the wild Helianthus strains from the centre of origin in North America is well known, but the seed dormancy of the invasive biotypes is still unknown. Dormancy is a fitness trait related to the establishment, dispersion and persistence of invasive weeds. Four experiments were designed to investigate the effect of the pericarp, light, temperature, the after-ripening period and hybridization with the DK3880CL sunflower crop (F1) on the seed dormancy of five invasive H. annuus biotypes. The results showed that pericarp scarification increased imbibition of the whole achene by 19%. Light stimulation only increased germination in the wild biotype without any effect on the domesticated sunflower. A period of 12 months after-ripening at 5°C reduced seed dormancy in the wild biotype and its progeny; the optimal temperature for seed germination at this period was found to be 15°C. Mechanical scarification was the best treatment for overcoming seed dormancy with a differential germination, in the biotypes with the highest response, superior to 63%. Hybridization with domesticated sunflower had a minimal or no effect on seed dormancy but the germination rate was improved in three F1 crosses. Wild biotype dormancy appears to be governed by the maternal pericarp and intrinsic hormone regulation. An increased germination rate of some progenies could constitute an advantage during seedling establishment but only in winters without any frost.