Mendel’s hawkweed experiments failed to confirm his laws because many hawkweed species reproduce through apomixis. Apomixis is a form of asexual seed production, so seeds can develop without the normal genetic contribution of fertilization.
Mendel began studying hawkweed, or Hieracium, after his successful pea experiments. He hoped the plant would provide another test of inheritance, but the results were puzzling: the first generation showed considerable variation, while many offspring resembled the maternal plant almost exactly.
Those patterns did not fit the inheritance ratios Mendel had observed in peas. The difficulty was not simply a poor environment or ordinary sterility. The reproductive biology of hawkweed meant that conventional hybrid crosses were often not taking place in the way Mendel expected.
Mendel corresponded with the botanist Carl Nägeli about the results, but neither could explain them fully. It was not until later in the nineteenth century that scientists recognized apomixis in many hawkweed species, turning Mendel’s failed confirmation into an important historical example of how reproduction affects genetic experiments.