We studied the germination behaviour of the following types of seeds: weedy Brassica campestris, oilseed rape (Brassica napus), B. campestris ([female]) × B. napus ([male]), B. napus ([female]) × B. campestris ([male]) and, finally, seeds harvested on B. napus ([female]) × B. campestris ([male]) hybrids in open pollination with B. campestris and B. napus. The seeds were germinated in Petri dishes, using three different consecutive treatments and assaying the viability of non-germinated seeds with tetrazolium staining. B. campestris seeds varied in the treatment they required in order to germinate and many seeds were dormant, in contrast to B. napus seeds, which lacked dormancy. B. campestris ([female]) × B. napus ([male]) and B. napus ([female]) × B. campestris ([male]) seeds both resembled B. napus being non-dormant whereas seeds harvested on B. napus ([female]) × B. campestris ([male]) hybrids were more B. campestris-like in germination behaviour. We discuss implications for risk of transgene spread from oilseed rape to weedy B. campestris.